renal system

renal system

 in humans (human body), organ system that includes the kidneys, where urine is produced, and the ureters, bladder, and urethra for the passage, storage, and voiding of urine.

      In many respects the human excretory, or urinary, system resembles those of other mammalian species, but it has its own unique structural and functional characteristics. The terms excretory and urinary emphasize the eliminatory function of the system. The kidneys, however, both secrete and actively retain within the body certain substances that are as critical to survival as those that are eliminated.

      The system contains two kidneys, which control the electrolyte composition of the blood and eliminate dissolved waste products and excess amounts of other substances from the blood; the latter substances are excreted in the urine, which passes from the kidneys to the bladder by way of two thin muscular tubes called the ureters. The bladder is a sac that holds the urine until it is eliminated through the urethra.

Human excretory organs

The kidneys (kidney)
General description and location
 The kidneys are bean-shaped, reddish brown paired organs, concave on one long side and convex on the opposite. They are normally located high in the abdominal cavity and against its back wall, lying on either side of the vertebral column between the levels of the 12th thoracic and third lumbar vertebrae, and outside the peritoneum, the membrane that lines the abdomen.

 The long axes of the kidneys are aligned with that of the body, but the upper end of each kidney (pole) is tilted slightly inward toward the backbone (vertebral column). Situated in the middle of the medial concave border is a deep vertical cleft, the hilus, which leads to a cavity within the kidney known as the renal (kidney) sinus. The hilus is the point of entry and exit of the renal arteries (renal artery) and veins, lymphatic vessels, nerves, and the enlarged upper extension of the ureters.

Renal vessels and nerves
 The renal arteries arise, one on each side, from the abdominal aorta at a point opposite the upper border of the second lumbar vertebra (i.e., a little above the small of the back). Close to the renal hilus each artery gives off small branches to the adrenal gland and ureter and then branches into anterior and posterior divisions. The large veins carrying blood from the kidneys usually lie in front of the corresponding arteries and join the inferior vena cava almost at right angles. The left vein is longer than the right vein because the inferior vena cava lies closer to the right kidney.

      The kidneys are supplied with sympathetic and parasympathetic nerves of the autonomic nervous system, and the renal nerves contain both afferent and efferent fibres (afferent fibres carry nerve impulses to the central nervous system; efferent fibres, from it).

Internal configuration
      A cross section of a kidney reveals the renal sinus and two layers of kidney tissue distinguishable by their texture and colour. The innermost tissue, called the renal medulla, forms comparatively dark cones, called renal pyramids (renal pyramid), with bases outward and apexes projecting, either singly or in groups, into the renal sinus. Each projection of one or more pyramid apexes into the sinus is known as a renal papilla. The bases of these pyramids are irregular, with slender striations extending toward the external kidney surface. The paler, more granular tissue external to the medulla is the cortex. It arches over the bases of the pyramids and fills gaps between the pyramids. Each group of pyramids that projects into a papilla, together with the portion of cortex that arches over the group, is called a renal lobe.

      The renal sinus includes the renal pelvis, a funnel-shaped expansion of the upper end of the ureter, and, reaching into the kidney substances from the wide end of the funnel, two or three extensions of the cavity called the major calyxes. The major calyxes are divided in turn into four to 12 smaller cuplike cavities, the minor calyxes, into which the renal papillae project. The renal pelvis serves as the initial reservoir for urine, which flows into the sinus through the urinary collecting tubules, small tubes that open into the sinus at the papillae.

Minute structure
 The structural units of the kidneys that actually produce urine are the nephrons (nephron), of which there are approximately 1,000,000 in each kidney. Each nephron is a long tubule (or extremely fine tube) that is closed, expanded, and folded into a double-walled cuplike structure at one end. This structure, called the renal corpuscular capsule, or Bowman's capsule, encloses a cluster of capillaries (microscopic blood vessels) called the glomerulus. The capsule and glomerulus together constitute a renal corpuscle, also called a malpighian body. Blood flows into and away from the glomerulus through small arteries (arterioles) that enter and exit the glomerulus through the open end of the capsule. This opening is called the vascular pole of the corpuscle.

      The tubules of the nephrons are 30–55 millimetres (1.2–2.2 inches) long. The corpuscle and the initial portion of each tubule, called the proximal convoluted tubule, lie in the renal cortex. The tubule descends into a renal pyramid, makes a U-shaped turn, and returns to the cortex at a point near its point of entry into the medulla. This section of the tubule, consisting of the two parallel lengths and the bend between them, is called the loop of Henle or the nephronic loop. After its reentrance into the cortex, the tubule returns to the vascular pole (the opening in the cuplike structure of the capsule) of its own nephron. The final portion of the tubule, the distal convoluted tubule, leads from the vascular pole of the corpuscle to a collecting tubule, by way of a short junctional tubule. Several of the collecting tubules join together to form a somewhat wider tubule, which carries the urine to a renal papilla and the renal pelvis.

      Although all nephrons in the kidney have the same general disposition, there are regional differences, particularly in the length of the loops of Henle. Glomeruli that lie deep in the renal cortex near the medulla (juxtamedullary glomeruli) possess long loops of Henle that pass deeply into the medulla, whereas more superficial cortical glomeruli have much shorter loops. Among different animal species the length of the loops varies considerably and affects the ability of the species to concentrate urine above the osmotic concentration of plasma.

      The successive sections of the nephron tubule vary in shape and calibre, and these differences, together with differences in the cells that line the sections, are associated with specific functions in the production of urine.

Intrarenal network of blood vessels
      The intrarenal network of blood vessels forms part of the blood-processing apparatus of the kidneys.

Arteries (artery) and arterioles
      The anterior and posterior divisions of each renal artery, mentioned earlier, divide into lobar arteries, each of which enters the kidney substance through or near a renal papilla. Each lobar artery gives off two or three branches, called interlobar arteries, which run outward between adjacent renal pyramids. When these reach the boundary between the cortex and the medulla they split almost at right angles into branches called arcuate arteries that curve along between the cortex and the medulla parallel to the surface of the kidney. Many arteries, called interlobular arteries, branch off from the arcuate arteries and radiate out through the cortex to end in networks of capillaries (capillary) in the region just inside the capsule. En route they give off short branches called the afferent arterioles, which carry blood to the glomeruli where they divide into four to eight loops of capillaries in each glomerulus.

      Near and before the point where the afferent arteriole enters the glomerulus, its lining layer becomes enlarged and contains secretory granules. This composite structure is called the juxtaglomerular apparatus (JGA) and is believed to be involved in the secretion of renin (see below The role of hormones in renal function (renal system)). They are then reconstituted near the point of entry of the afferent arteriole to become the efferent arterioles carrying blood away from the glomeruli. The afferent arterioles are almost twice as thick as the efferent arterioles because they have thicker muscular coats, but the sizes of their channels are almost the same.

      Throughout most of the cortex the efferent arterioles redivide into a second set of capillaries, which supply blood to the proximal and distal renal tubules.

      The efferent glomerular arterioles of juxtaglomerular glomeruli divide into vessels that supply the contiguous tubules and vessels that enter the bases of the renal pyramids. Known as vasa recta, these vessels run toward the apexes of the pyramids in close contact with the loops of Henle. Like the tubules they make hairpin bends, retrace their path, and empty into arcuate veins that parallel the arcuate arteries.

      Normally the blood circulating in the cortex is more abundant than that in the medulla (amounting to over 90 percent of the total), but in certain conditions, such as those associated with severe trauma or blood loss, cortical vessels may become constricted while the juxtamedullary circulation is preserved. Because the cortical glomeruli and tubules are deprived of blood, the flow of urine is diminished, and in extreme cases may cease.

Veins (vein) and venules
      The renal venules (small veins) and veins accompany the arterioles and arteries and are referred to by similar names. The venules that lie just beneath the renal capsule, called stellate venules because of their radial arrangement, drain into interlobular venules. In turn these combine to form the tributaries of the arcuate, interlobar, and lobar veins. Blood from the renal pyramids passes into vessels, called venae rectae, which join the arcuate veins. In the renal sinus the lobar veins unite to form veins corresponding to the main divisions of the renal arteries, and they normally fuse to constitute a single renal vein in or near the renal hilus.

      Lymphatic capillaries form a network just inside the renal capsule and another, deeper network between and around the renal blood vessels. Few lymphatic capillaries appear in the actual renal substance, and those present are evidently associated with the connective tissue framework, while the glomeruli contain no lymphatics. The lymphatic networks inside the capsule and around the renal blood vessels drain into lymphatic channels accompanying the interlobular and arcuate blood vessels. The main lymph channels run alongside the main renal arteries and veins to end in lymph nodes beside the aorta and near the sites of origin of the renal arteries.

The ureters (ureter)
General characteristics
 The ureters are narrow, thick-walled ducts, about 25–30 centimetres (9.8–11.8 inches) in length and from four to five millimetres (0.16 to 0.2 inch) in diameter, that transport the urine from the kidneys to the urinary bladder. Throughout their course they lie behind the peritoneum, the lining of the abdomen and pelvis, and are attached to it by connective tissue.

      In both sexes the ureters enter the bladder wall about five centimetres apart, although this distance is increased when the bladder is distended with urine. The ureters run obliquely through the muscular wall of the bladder for nearly two centimetres before opening into the bladder cavity through narrow apertures. This oblique course provides a kind of valvular mechanism; when the bladder becomes distended it presses against the part of each ureter that is in the muscular wall of the bladder, and this helps to prevent the flow of urine back into the ureters from the bladder.

Structure of the ureteric wall
      The wall of the ureter has three layers, the adventitia, or outer layer; the intermediate, muscular (muscle) layer; and the lining, made up of mucous membrane. The adventitia consists of fibroelastic connective tissue that merges with the connective tissue behind the peritoneum. The muscular coat is composed of smooth (involuntary) muscle fibres and, in the upper two-thirds of the ureter, has two layers—an inner layer of fibres arranged longitudinally and an outer layer disposed circularly. In the lower third of the ureter an additional longitudinal layer appears on the outside of the vessel. As each ureter extends into the bladder wall its circular fibres disappear, but its longitudinal fibres extend almost as far as the mucous membrane lining the bladder.

      The mucous membrane lining increases in thickness from the renal pelvis downward. Thus, in the pelvis and the calyxes of the kidney the lining is two to three cells deep; in the ureter, four to five cells thick; and in the bladder, six to eight cells. The mucous membrane of the ureters is arranged in longitudinal folds, permitting considerable dilation of the channel. There are no true glands in the mucous membrane of the ureter or of the renal pelvis. The chief propelling force for the passage of urine from the kidney to the bladder is produced by peristaltic (wavelike) movements in the ureter muscles.

General description
      The urinary bladder is a hollow muscular organ forming the main urinary reservoir. It rests on the anterior part of the pelvic floor (see below), behind the symphysis pubis and below the peritoneum. (The symphysis pubis is the joint in the hip bones in the front midline of the body.) The shape and size of the bladder vary according to the amount of urine that the organ contains. When empty it is tetrahedral and lies within the pelvis; when distended it becomes ovoid and expands into the lower abdomen. It has a body, with a fundus, or base; a neck; an apex; and a superior (upper) and two inferolateral (below and to the side) surfaces, although these features are not clearly evident except when the bladder is empty or only slightly distended.

      The neck of the bladder is the area immediately surrounding the urethral opening; it is the lowest and most fixed part of the organ. In the male it is firmly attached to the base of the prostate, a gland that encircles the urethra.

      The superior surface of the bladder is triangular and is covered with peritoneum. The bladder is supported on the levator ani muscles, which constitute the major part of the floor of the pelvic cavity. The bladder is covered, and to a certain extent supported, by the visceral layer of the pelvic fascia. This fascial layer is a sheet of connective tissue that sheaths the organs, blood vessels, and nerves of the pelvic cavity. The fascia forms, in front and to the side, ligaments, called pubovesical ligaments, that act as a kind of hammock under the inferolateral surfaces and neck of the bladder.

Blood and nerve supplies
      The blood supply of the bladder is derived from the superior, middle, and inferior vesical (bladder) arteries. The superior vesical artery supplies the dome of the bladder, and one of its branches (in males) gives off the artery to the ductus deferens, a part of the passageway for sperm. The middle vesical artery supplies the base of the bladder. The inferior vesical artery supplies the inferolateral surfaces of the bladder and assists in supplying the base of the bladder, the lower end of the ureter, and other adjacent structures.

      The nerves (nervous system) to the urinary bladder belong to the sympathetic and the parasympathetic divisions of the autonomic nervous system. The sympathetic nerve fibres come from the hypogastric plexus of nerves that lie in front of the fifth lumbar vertebra. Sympathetic nerves carry to the central nervous system the sensations associated with distention of the bladder and are believed to be involved in relaxation of the muscular layer of the vesical wall and with contraction of sphincter mechanism that closes the opening into the urethra. The parasympathetic nerves travel to the bladder with pelvic splanchnic nerves from the second through fifth sacral spinal segment. Parasympathetic nerves are concerned with contraction of the muscular walls of the bladder and with relaxation of its sphincter. Consequently they are actively involved in urination and are sometimes referred to as the emptying, or detrusor, nerves.

Structure of the bladder wall
      The bladder wall has a serous coat over its upper surface. This covering is a continuation of the peritoneum that lines the abdominal cavity; it is called serous because it exudes a slight amount of lubricating fluid called serum. The other layers of the bladder wall are the fascial, muscular, submucous, and mucous coats.

      The fascial coat is a layer of connective tissue, such as that which covers muscles. The muscular coat consists of coarse fascicles, or bundles, of smooth (involuntary) muscle fibres arranged in three strata, with fibres of the outer and inner layers running lengthwise, and with fibres of the intermediate layer running circularly; there is considerable intermingling of fibres between the layers. The smooth muscle coat constitutes the powerful detrusor muscle, which causes the bladder to empty.

      The circular or intermediate muscular stratum of the vesical wall is thicker than the other layers. Its fibres, although running in a generally circular direction, do interlace. The internal muscular stratum is an indefinite layer of fibres that are mostly directed longitudinally. The submucous coat consists of loose connective tissue containing many elastic fibres. It is absent in the trigone, a triangular area whose angles are at the two openings for the ureters and the single internal urethral opening. Slim bands of muscle run between each ureteric opening and the internal urethral orifice; these are thought to maintain the oblique direction of the ureters during contraction of the bladder. Another bundle of muscle fibres connects the two ureteric openings and produces a slightly downwardly curved fold of mucous membrane between the openings.

      The mucous coat, the innermost lining of the bladder, is an elastic layer impervious to urine. Over the trigone it firmly adheres to the muscular coat and is always smooth and pink whether the bladder is contracted or distended. Elsewhere, if the bladder is contracted, the mucous coat has multiple folds and a red, velvety appearance. When the bladder is distended, the folds are obliterated, but the difference in colour between the paler trigonal area and the other areas of the mucous membrane persists. The mucous membrane lining the bladder is continuous with that lining the ureters and the urethra.

General description
      The urethra is the channel that conveys the urine from the bladder to the exterior. In the male it is about 20 centimetres long and carries not only the urine but also the semen and the secretions of the prostate, bulbourethral, and urethral glands. During urination and ejaculation it opens up, and its diameter then varies from 0.5 to 0.8 centimetre along its length, but at other times its walls touch and its lining is raised into longitudinal folds. The male urethra has three distinguishable parts, the prostatic, the membranous, and the spongy, each part being named from the structures through which it passes rather than from any inherent characteristics.

      The prostatic section of the male urethra commences at the internal urethral orifice and descends almost vertically through the prostate, from the base of the gland to the apex, describing a slight curve with its concavity forward. It is about 2.5 to three centimetres long and is spindle-shaped; its middle portion is the widest and most dilatable part of the urethra. The membranous part of the male urethra is in the area between the two layers of a membrane called the urogenital diaphragm. The urethra is narrower in this area than at any other point except at its external opening and is encircled by a muscle, the sphincter urethrae. The two small bulbourethral glands are on either side of it. The membranous urethra is not firmly attached to the layers of the urogenital diaphragm. The spongy part of the male urethra is that part of the urethra that traverses the penis. It passes through the corpus spongiosum of the penis. The ducts of the bulbourethral glands enter the spongy urethra about 2.5 centimetres below the lower layer of the urogenital membrane; except near its outer end, many mucous glands also open into it.

      The female urethra is much shorter (three to 4.5 centimetres) and more distensible than the corresponding channel in males and carries only urine and the secretions of mucous glands. It begins at the internal opening of the urethra into the bladder and curves gently downward and forward through the urogenital diaphragm, where it is surrounded, as in the male, by the sphincter urethrae. It lies behind and below the symphysis pubis. Except for its uppermost part, the urethra is embedded in the anterior wall of the vagina. The external urethral orifice is immediately in front of the vaginal opening, about 2.5 centimetres behind the clitoris, and between the labia minora, the inner folds at the outer opening of the vagina.

Structure of urethral wall
      The urethra of the male is a tube of mucous membrane supported on a submucous layer and an incomplete muscular coat. The membrane forms longitudinal folds when the tube is empty; these folds are more prominent in the membranous and spongy parts. There are many glands in the mucous membrane, and they are more common in the posterior wall of the spongy part. The submucous layer is composed of fibroelastic connective tissue containing numerous small blood vessels, including more venules than arterioles. The thin muscular coat consists of smooth (involuntary) and striated (voluntary) muscle fibres. The smooth muscular layer, longitudinally disposed, is continuous above with the detrusor muscle of the bladder and extends distally as far as the membranous urethra, where it is replaced and partly surrounded by striated muscle of the external sphincter. The somatic nerves to the external sphincter are the efferent and afferent components of the pudendal nerve, arising from the second, third, and fourth sacral segments of the spinal cord.

      The female urethra has mucous, submucous, and muscular coats. As in the male, the lining of the empty channel is raised into longitudinal folds. It also shows mucous glands, mentioned in the preceding paragraphs as existing in the male urethra. The submucous coat resembles that in the male, except that the venules are even more prominent. In both sexes, but especially in females, this layer appears to be a variety of erectile tissue. The muscular coat extends along the entire length of the female urethra and is continuous above with the musculature of the bladder. It consists of inner longitudinal and outer circular layers, and fibres from the latter intermix with those in the anterior wall of the vagina, in which the urethra is embedded.

G.A.G. Mitchell James Scott Robson

Human excretion

General function of the kidney
      The kidney has evolved so as to enable humans to exist on land where water and salts must be conserved, wastes excreted in concentrated form, and the blood and the tissue fluids strictly regulated as to volume, chemical composition, and osmotic pressure. Under the drive of arterial pressure, water and salts are filtered from the blood through the capillaries of the glomerulus into the lumen, or passageway, of the nephron, and then most of the water and the substances that are essential to the body are reabsorbed into the blood. The remaining filtrate is drained off as urine. The kidneys, thus, help maintain a constant internal environment despite a wide range of changes in the external environment.

Regulatory functions
      The kidneys regulate three essential and interrelated properties of the tissues—water content, acid-base balance, and osmotic pressure—in such a way as to maintain electrolyte and water equilibrium; in other words, the kidneys are able to maintain a balance between quantities of water and the quantities of such chemicals as calcium, potassium, sodium, phosphorus, and sulfate in solution. Unless the concentrations of mineral ions such as sodium, crystalloids such as glucose, and wastes such as urea are maintained within narrow normal limits, bodily malfunction rapidly develops leading to sickness or death.

      The removal of both kidneys causes urinary constituents to accumulate in the blood ( uremia), resulting in death in 14–21 days if untreated. (The term uremia does not mean that urea is itself a toxic compound responsible for illness and death.) Whenever the blood contains an abnormal constituent in solution or an excess of normal constituents including water and salts, the kidneys excrete these until normal composition is restored. The kidneys are the only means for eliminating the wastes that are the end products of protein metabolism. They do not themselves modify the waste products that they excrete, but transfer them to the urine in the form in which they are produced in other parts of the body. The only exception to this is their ability to manufacture ammonia. The kidneys also eliminate drugs and toxic agents. Thus, the kidneys eliminate the unwanted end products of metabolism, such as urea, while limiting the loss of valuable substances, such as glucose. In maintaining the acid-base equilibrium, the kidneys remove the excess of hydrogen ions (hydrogen ion) produced from the normally acid-forming diet and manufacture ammonia to remove these ions in the urine as ammonium salts.

      To carry on its functions the kidney is endowed with a relatively huge blood supply. The blood processed in the kidneys amounts to some 1,200 millilitres a minute, or 1,800 litres (about 475 gallons) a day, which is 400 times the total blood volume and roughly one-fourth the volume pumped each day by the heart. Every 24 hours 170 litres (45 gallons) of water are filtered from the bloodstream into the renal tubules; and by far the greater part of this—some 168.5 litres of water together with salts dissolved in it—is reabsorbed by the cells lining the tubules and returned to the blood. The total glomerular filtrate in 24 hours is no less than 50–60 times the volume of blood plasma (the blood minus its cells) in the entire body. In a 24-hour period, an average man eliminates only 1.5 litres of water, containing the waste products of metabolism, but the actual volume varies with fluid intake and occupational and environmental factors. With vigorous sweating it may fall to 500 millilitres (about a pint) a day; with a large water intake it may rise to three litres, or six times as much. The kidney can vary its reabsorption of water to compensate for changes in plasma volume resulting from dehydration or overhydration.

Nonexcretory functions
      The kidneys also perform certain nonexcretory functions. They secrete substances that enter the blood. These are of three kinds: renin, which is concerned indirectly with the control of electrolyte balance and blood pressure; erythropoietin, which is important for the formation of hemoglobin and red blood cells, especially in response to anemia or deficiency of oxygen reaching the body tissues; and 1,25-dihydroxycholecalciferol, which is the metabolically active form of vitamin D. Finally, although the kidneys are subject to both nervous and humoral (hormonal) control, they do possess a considerable degree of autonomy; i.e., function continues in an organ isolated from the nervous system but kept alive with circulating fluid. Indeed, if this were not so kidney transplantation would be impossible.

Renal blood circulation
Intrarenal blood pressures (blood pressure)
      The renal arteries are short and spring directly from the abdominal aorta, so that arterial blood is delivered to the kidneys (kidney) at maximum available pressure. As in other vascular beds, renal perfusion is determined by the renal arterial blood pressure and vascular resistance to blood flow. Evidence indicates that in the kidneys the greater part of the total resistance occurs in the glomerular arterioles. The muscular coats of the arterioles are well supplied with sympathetic vasoconstrictor fibres (nerve fibres that induce narrowing of the blood vessels), and there is also a small parasympathetic supply from the vagus and splanchnic nerves that induces dilation of the vessels. Sympathetic stimulation causes vasoconstriction and reduces urinary output. The vessel walls are also sensitive to circulating epinephrine (epinephrine and norepinephrine) and norepinephrine hormones, small amounts of which constrict the efferent arterioles and large amounts of which constrict all the vessels; and to angiotensin, which is a constrictor agent closely related to renin. Prostaglandins may also have a role.

Factors that affect renal flow
      The kidney is able to regulate its internal circulation regardless of the systemic blood pressure, provided that the latter is not extremely high or extremely low. The forces that are involved in maintaining a circulation of the blood in the kidneys must remain constant if the monitoring of the water and electrolyte composition of the blood is to proceed undisturbed. This regulation is preserved even in the kidney cut off from the nervous system and, to a lesser extent, in an organ removed from the body and kept viable by having salt solutions of physiologically suitable concentrations circulated through it; it is commonly referred to as autoregulation.

      The exact mechanism by which the kidney regulates its own circulation is not known, but various theories have been proposed: (1) Smooth muscle cells in the arterioles may have an intrinsic basal tone (normal degree of contraction) when not affected by nervous or humoral (hormonal) stimuli. The tone responds to alterations in perfusion pressure in such a way that when the pressure falls the degree of contraction is reduced, preglomerular resistance is lowered, and blood flow is preserved. Conversely, when perfusion pressure rises, the degree of contraction is increased and blood flow remains constant. (2) If the renal blood flow rises, more sodium is present in the fluid in the distal tubules because the filtration rate increases. This rise in the sodium level stimulates the secretion of renin from the JGA with the formation of angiotensin, causing the arterioles to constrict and blood flow to be reduced. (3) If systemic blood pressure rises, the renal blood flow remains constant because of the increased viscosity of the blood. Normally, the interlobular arteries have an axial (central) stream of red blood cells with an outer layer of plasma so that the afferent arterioles skim off more plasma than cells. If the arteriolar blood pressure rises, the skimming effect increases, and the more densely packed axial flow of cells in the vessels offers increasing resistance to the pressure, which has to overcome this heightened viscosity. Thus, the overall renal blood flow changes little. Up to a point, similar considerations in reverse apply to the effects of reduced systemic pressure. (4) Changes in the arterial pressure modify the pressure exerted by the interstitial (tissue) fluid of the kidney on capillaries and veins so that increased pressure raises, and decreased pressure lowers, resistance to blood flow.

      The renal blood flow is greater when a person is lying down than when standing; it is higher in fever; and it is reduced by prolonged vigorous exertion, pain, anxiety, and other emotions that constrict the arterioles and divert blood to other organs. It is also reduced by hemorrhage and asphyxia and by depletion of water and salts, which is severe in shock, including operative shock. A large fall in systemic blood pressure, as after severe hemorrhage, may so reduce renal blood flow that no urine at all is formed for a time; death may occur from suppression of glomerular function. Simple fainting causes vasoconstriction and reduced urine output. Urinary secretion is also stopped by obstruction of the ureter when back pressure reaches a critical point.

Glomerular pressure
      The importance of these various vascular factors lies in the fact that the basic process occurring in the glomerulus is one of filtration, the energy for which is furnished by the blood pressure within the glomerular capillaries (capillary). Glomerular pressure is a function of the systemic pressure as modified by the tone (state of constriction or dilation) of the afferent and efferent arterioles, as these open or close spontaneously or in response to nervous or hormonal control.

      In normal circumstances glomerular pressure is believed to be about 45 millimetres of mercury (mmHg), which is a higher pressure than that found in capillaries elsewhere in the body. As is the case in renal blood flow, the glomerular filtration rate is also kept within the limits between which autoregulation of blood flow operates. Outside these limits, however, major changes in blood flow occur. Thus, severe constriction of the afferent vessels reduces blood flow, glomerular pressure, and filtration rate, while efferent constriction causes reduced blood flow but increases glomerular pressure and filtration.

Formation and composition of urine
       Relative composition of plasma and urine in normal menThe urine leaving the kidney differs considerably in composition from the plasma entering it (Table 1 (Relative composition of plasma and urine in normal men)). The study of renal function must account for these differences; e.g., the absence of protein and glucose from the urine, a change in the pH of urine as compared with that of plasma, and the high levels of ammonia and creatinine in the urine, while sodium and calcium remain at similar low levels in both urine and plasma.

      A large volume of ultrafiltrate (i.e., a liquid from which the blood cells and the blood proteins have been filtered out) is produced by the glomerulus into the capsule. As this liquid traverses the proximal convoluted tubule, most of its water and salts are reabsorbed, some of the solutes completely and others partially; i.e., there is a separation of substances that must be retained from those due for rejection. Subsequently the loop of Henle, distal convoluted tubule, and collecting ducts are mainly concerned with the concentration of urine to provide fine control of water and electrolyte balance.

Glomerular filtration
      Urine formation begins as a process of ultrafiltration of a large volume of blood plasma from the glomerular capillaries into the capsular space, colloids (colloid) such as proteins being held back while crystalloids (substances in true solution) pass through. In humans, the average capillary diameter is five to 10 micrometres (a micrometre is 0.001 millimetre). The wall of each loop of capillaries has three layers. The inner layer consists of flat nucleated endothelial cells arranged to form numerous pores, or fenestrae, 50–100 nanometres in diameter (a nanometre is 0.000001 millimetre), which allow the blood to make direct contact with the second layer, a basement membrane. The basement membrane of the capillaries, similar to that which occurs in the lining of many other structures and organs, is a continuous layer of hydrated collagen and glycopeptides. Although once thought to be homogeneous, it appears to consist of three layers that differ in the content of polyanionic glycopeptides. The membrane is negatively charged (anionic), owing to its relatively high content of sialic and aspartic acids. Also present are glycosaminoglycans, such as heparin sulfate. The third, external layer consists of large epithelial cells called podocytes. These cells make contact with the outer surface of the basement membrane by slender cytoplasmic extensions called pedicels (foot processes). These processes are slightly expanded at their point of contact with the basement membrane and are separated from each other by slitlike spaces about 20 to 30 nanometres across. A fine membrane (slit diaphragm) closes the slitlike spaces near the basement membrane.

      There are two physical processes by which glomerular filtrate may pass the barrier of the glomerular wall—simple diffusion and bulk flow. In bulk flow, the solute in the glomerular filtrate with water passes through pores in the basement membrane. In either case the ultimate restriction to the passage of filtrate appears to lie in the hydrated gel structure of the basement membrane. The negative electrostatic charge in the membrane is an additional restrictive force for negatively charged anionic macromolecules, such as albumin (molecular weight 69,000), while larger protein molecules are restricted by size alone. On the other hand, proteins of smaller molecular size—e.g., neutral gelatin (35,000)—pass through freely. It is possible that the endothelial cell layer may also help to exclude very large molecules and blood cells and that a similar effect is exerted by the slit pores and diaphragm.

      The normal process of glomerular filtration depends upon the integrity of the glomerulus, which in turn depends upon its proper nutrition and oxygenation. If glomeruli are damaged through disease or lack of oxygen they become more permeable, allowing plasma proteins to enter the urine. Special cells that may be concerned with the formation and maintenance of the basement membrane of the glomerular walls are called mesangial cells. These lie between loops of the glomerular capillaries and form a stalk or scaffolding for the capillary network. They are themselves embedded in a matrix of glycosaminoglycan similar to that of the glomerular capillary basement membrane and may be responsible for its formation. The mesangial cells are also responsible for ridding the basement membrane of large foreign molecules that may be held there in the course of certain diseases. These cells proliferate and the mesangial matrix enlarges in the course of immunologically induced diseases affecting the glomerulus.

Tubule function
       Effect of tubular reabsorption on urineThe role of the tubules may be assessed by comparing the amounts of various substances in the filtrate and in the urine (Table 2 (Effect of tubular reabsorption on urine)).

      It is apparent that the filtrate must be modified in the tubules to account for the differing compositions of filtrate and final urine; e.g., to allow for the total absence of glucose in the latter, the much smaller volume of urine than filtrate, or for the acidity of urine compared with the neutrality of the filtrate.

      As the filtrate passes along the proximal tubule, most of its water and salts are reabsorbed into the blood of the network of capillaries around the tubules. Of other substances, some are reabsorbed completely, others in part, because this portion of the nephron separates substances that must be retained in the body from those destined for excretion in the urine. The function of the proximal tubule is essentially reabsorption of filtrate in accordance with the needs of homeostasis (equilibrium), whereas the distal part of the nephron and collecting duct are mainly concerned with the detailed regulation of water, electrolyte, and hydrogen-ion balance. All of these processes occur in the tubules through both chemical and physical means, and all are subject to hormonal regulation. Although the urine normally differs markedly from filtrate, if tubule function is progressively reduced in experimental situations by cooling or poisoning, the urine will come increasingly to resemble the filtrate. Also, the more rapidly filtration occurs, the less time there is for the urine to be modified during its passage through the tubules.

Reabsorption from the proximal tubule
      Reabsorption affects all the glucose of the filtrate, up to 70 percent of its water and sodium (the remainder is absorbed in the distal tubule), most of the potassium and chloride ions, some of the uric acid, 40 percent of the urea, and little or none of the sulfate. Of the total solids 75 percent are reabsorbed in the proximal tubule. The first part of the tubule absorbs amino acids, glucose, lactate, and phosphate; the whole convolution absorbs sodium, potassium, calcium, and chloride and, by removing bicarbonate, acidifies the fluid slightly.

      The tubule has only a certain capacity for reabsorption. Thus, normally all the glucose arriving in the filtrate is absorbed; but if plasma glucose is increased to high enough levels, the glucose arrives at the tubule cells faster than it can be absorbed—a condition that occurs in diabetes. In other words, there is a critical rate of delivery determined by plasma concentration and filtration rate, and a maximum reabsorptive capacity for each substance in the filtrate. The rate of tubular reabsorption has an upper maximum value that is constant for any given substance. Consequently, if the plasma level rises sufficiently, all surplus of the substance will pass out in the urine; this is true even for glucose, which is totally reabsorbed under normal conditions. On the other hand, the upper maximum value is much lower for phosphate, so there is normally always some phosphate in the urine. The proximal tubular reabsorption of phosphate is also affected by the phosphate content of the filtrate and is influenced by parathyroid hormone. Phosphate competes with glucose for reabsorption, and its reabsorption is reduced by parathyroid hormone and by vitamin D and is increased, at least for some time, by a high dietary phosphate intake. The amino acids (amino acid) also have their own maximum tubular reabsorption values, but these are high enough to ensure that they are entirely reabsorbed under normal conditions; in certain rare inherited disorders such as cystinuria, in which there is excessive excretion of cystine, their reabsorption is reduced.

      The reabsorption of about 70 percent of the sodium ions in the filtrate means that a similar value of water in the filtrate must accompany these ions as a vehicle to prevent a rising osmotic gradient (i.e., to prevent a rising difference in the concentration of the sodium solution inside and outside the tubule). The energy required for the reabsorption of sodium into the blood uses 80 percent of the oxygen consumed by the kidney and represents one-eighth of the oxygen consumption of a person at rest. There is no evidence for active water transport, and the large volume of water reabsorption occurs passively in response to the movement of sodium. Since sodium is quantitatively the major osmotically active solute, the overall effect is to keep the fluid that remains in the tubular lumen, though much reduced in volume, roughly isosmotic with the original glomerular filtrate.

      The active reabsorption of sodium (a positively charged ion) into the blood leaves the fluid remaining in the proximal tubule electronegative with respect to the peritubular fluids. This provides a driving force for the reabsorptive transport of negatively charged ions such as chloride, bicarbonate, and organic solutes. Reabsorption of neutral molecules such as urea into the blood is also driven by active sodium transport. Because the tubular epithelium is less permeable to urea and creatinine than it is to water or chloride, however, the free passive movement of water out of the tubular lumen leads to a rising luminal concentration of urea (i.e., above the concentration in the original filtrate with plasma). As a result, a smaller proportion of filtered urea or creatinine than of sodium or water is reabsorbed into the blood, resulting in the elimination of a considerable amount in the urine.

Reabsorption from the loop of Henle
      About one-third of the volume of the glomerular filtrate enters the descending limb of the loop of Henle. This fluid is isosmotic with plasma. The reabsorptive characteristics of the descending thin limb and those of the bend of the loop differ greatly from those of the ascending thick limb. The thin epithelium lining the thin limb is permeable to water and solute and has no power of active transport. Accordingly, the fluid entering the limb and the bend of the loop acquires the concentration of the fluid of the surrounding interstitial peritubular fluid. In contrast, the thick ascending limb lined by taller cells has low permeability to water and to urea but actively transports sodium and chloride into the peritubular fluid around both limbs. As a result this fluid in the medullary and deep cortical regions of the kidney becomes highly concentrated, reaching concentrations of up to four times that of the plasma (1,200 mosmoles per litre), mainly owing to the accumulation of sodium and chloride. This accumulation of solute, essential to the formation of a concentrated urine, is discussed in further detail below.

Reabsorption from the distal convoluted tubule
      The active transport of sodium out of the ascending limb renders the fluid entering the distal convoluted tubule less concentrated than plasma. Active sodium reabsorption continues throughout the whole of the distal tubule, and this extends to the early part of the collecting duct. As this part of the nephron is relatively impermeable to water, a large concentration gradient of sodium and chloride between the luminal fluid and the plasma is maintained, the concentration of sodium in the tubule being kept well below that of the plasma. The luminal fluid here is also markedly electronegative to the surrounding tissues. The mechanism of sodium reabsorption appears to be directly linked to the secretion of potassium and of hydrogen ions into the tubule from the blood and is greatly influenced by the hormone aldosterone, which is secreted by the adrenal gland when the body's sodium level is deficient.

The concentration of urine
      As already indicated, the loop of Henle is critical to the ability of the kidney to concentrate urine. The high concentration of salt in the medullary fluid is believed to be achieved in the loop by a process known as countercurrent exchange multiplication. The principle of this process is analogous to the physical principle applied in the conduction of hot exhaust gases past cold incoming gas so as to warm it and conserve heat. That exchange is a passive one; but in the kidney the countercurrent multiplier system uses energy to “pump” sodium and chloride out of the ascending limb of the loop into the medullary fluid. From there it enters (by diffusion) the filtrate (isotonic with plasma) that is entering the descending limb from the proximal tubule, thus raising its concentration a little above that of plasma. As this luminal fluid in turn reaches the ascending limb, and subsequently the distal tubule, it in turn provides more sodium to be pumped out into the surrounding fluid or blood, if necessary, and transported (by diffusion) back into the descending limb; this concentrating process continues until the osmotic pressure of the fluid is sufficient to balance the resorptive power of the collecting ducts in the medulla, through which all of the final urine must pass. This resorptive capacity in the ducts is regulated by antidiuretic hormone (ADH), which is secreted by the hypothalamus and stored in the posterior pituitary gland at the base of the brain. In the presence of ADH the medullary collecting ducts become freely permeable to solute and water. As a consequence the fluid entering the ducts (en route to the renal pelvis and subsequent elimination) acquires the concentration of the interstitial fluid of the medulla; i.e., the urine becomes concentrated. On the other hand in the absence of ADH the collecting ducts are impermeable to solute and water; thus, the fluid in the lumen, from which some solute has been removed, remains less concentrated than plasma; i.e., the urine is dilute.

      The secretion of ADH by the hypothalamus and its release from the posterior pituitary is part of a feedback mechanism responsive to the tonicity of plasma. This interrelation between plasma osmotic pressure and ADH output is mediated by specific and sensitive receptors at the base of the brain. These receptors are particularly sensitive to sodium and chloride ions. At normal blood tonicity there is a steady receptor discharge and a steady secretion of ADH. If the plasma becomes hypertonic (i.e., has a greater osmotic pressure than normal), either from the ingestion of crystalloids such as common salt, or from shortage of water, receptor discharge increases, triggering increased ADH output, and more water leaves the collecting ducts to be absorbed into the blood. If the osmotic pressure of plasma becomes low, the reverse is the case. Thus water ingestion dilutes body fluids and reduces or stops ADH secretion; the urine becomes hypotonic, and the extra water is excreted in the urine.

      The situation is complex because there are also receptors sensitive to changes in blood volume that reflexively inhibit ADH output if there is any tendency to excessive blood volume. Exercise increases ADH output and reduces urinary flow. The same result may follow emotional disturbance, fainting, pain, and injury, or the use of certain drugs such as morphine or nicotine. Diuresis is an increased flow of urine produced as the result of increased fluid intake, absence of hormonal activity, or the taking of certain drugs that reduce sodium and water reabsorption from the tubules. If ADH secretion is inhibited by the drinking of excess water, or by disease or the presence of a tumour affecting the base of the brain, water diuresis results; and the rate of urine formation will approach the rate of 16 millilitres per minute filtered at the glomeruli. In certain disorders of the pituitary in which ADH secretion is diminished or absent—e.g., diabetes insipidus—there may be a fixed and irreversible output of a large quantity of dilute urine.

      The only difference between secretory and reabsorptive tubular mechanisms lies in the direction of transport; secretory mechanisms involve the addition of substances to the filtrate from the plasma in the peritubular capillaries. The small amount of secretion that does occur, except for the secretion of potassium and uric acid, takes place in the proximal tubule. Hydrogen ions are also secreted and ammonia is generated, but they are special cases and are discussed below under Regulation of acid-base balance. As in the case of reabsorption, secretion occurs both passively and actively against an electrochemical gradient.

      Several drugs are actively secreted, and some of these appear to share a common pathway so that they may compete with each other for a limited amount of energy. This may be turned to therapeutic advantage in the case of penicillin, which is eliminated partly by tubular secretion. The drug probenecid, which can be given simultaneously, competes with penicillin at its secretory site and thus helps to raise the level of penicillin in the blood in the treatment of certain infections. Endogenous (originating within the body) compounds that are secreted also include prostaglandins, bile salts, and hippurate. uric acid derived from nucleoproteins freely passes the glomerular barrier and is normally largely reabsorbed in the proximal tubule. In some circumstances, however, it is also secreted by other parts of the same convoluted tubule.

      The secretion of potassium by the distal tubule is one of the most important events in the kidney as its control is fundamental to the maintenance of overall potassium balance. More than 75 percent of the filtered potassium is reabsorbed in the proximal tubule and in the ascending limb of the loop of Henle, and this percentage remains virtually constant, irrespective of how much is filtered. The amount eliminated in the urine, which is ultimately determined by the dietary intake, is controlled by the distal convoluted tubule. In persons consuming a normal diet, probably about 50 percent of the urinary potassium is secreted into the urine by the distal tubules; this amount can be adjusted according to body need. One of the several factors that influence potassium secretion is a hormone secreted by the cortex of the adrenal gland, aldosterone. In the absence of aldosterone and other mineralocorticoids (adrenocortical steroids affecting electrolyte and fluid balance), potassium secretion is impaired, and potentially dangerous amounts can accumulate in the blood. Excess aldosterone promotes potassium excretion.

Regulation of acid-base (acid) balance
      The cells of the body derive energy from oxidative processes that produce acidic waste products. Acids are substances that ionize to yield free protons, or hydrogen ions (hydrogen ion). Those hydrogen ions that derive from nonvolatile acids—such as lactic, pyruvic, sulfuric, and phosphoric acids—are eliminated in the urine. The kidney contains transport mechanisms that are capable of raising the concentration of hydrogen ions in the urine to 2,500 times that in the plasma or, when appropriate, lowering it to one-quarter that of the plasma.

      Theoretically, acidification (acid–base reaction) of urine could be brought about either by the secretion of hydrogen ions into the tubular fluid or by the selective absorption of a buffer base (a substance capable of accepting hydrogen ions; e.g., filtered bicarbonate). Current evidence indicates that both filtration and secretion are essential to hydrogen ion excretion and that both proximal and distal convoluted tubules are involved.

      The bulk of the bicarbonate filtered at the glomerulus is reabsorbed in the proximal tubule, from which it passes back into the peritubular capillaries. This mechanism is designed to keep the normal plasma bicarbonate concentration constant at about 25 millimoles per litre. When the plasma concentration falls below this level, no bicarbonate is excreted and all filtered bicarbonate is reabsorbed into the blood. This level is often referred to as the bicarbonate threshold. When the plasma bicarbonate rises above 27 millimoles per litre, bicarbonate appears in the urine in increasing amounts.

      The brush borders of the cells of the proximal tubules are rich in the enzyme carbonic anhydrase. This enzyme facilitates the formation of carbonic acid (H2CO3) from CO2 and H2O, which then ionizes to hydrogen ions (H+) and bicarbonate ions (HCO3-). The starting point for bicarbonate reabsorption is probably the active secretion of hydrogen ions into the tubular fluid. These ions may be formed under the influence of carbonic anhydrase from CO2 liberated from oxidation of cell nutrients and H2O already in the cells. The filtered base, bicarbonate, accepts the hydrogen ions to form carbonic acid, which is unstable and dissociates to form CO2 and H2O. The partial pressure of CO2 in the filtrate rises, and, as CO2 is highly diffusible, it passes readily from the tubular fluid into the tubular cells and the blood, and the water is either dealt with in the same way or is excreted. In the meantime the proximal tubular cells are actively reabsorbing filtered sodium, which is balanced by the HCO3- formed within the cells from the CO2 generated by the hydrogen ions in the luminal fluid. Thus the bicarbonate actually reabsorbed is not that which was originally the filtrate, but the net effect is the same as if this were the case.

      Other bases besides HCO3- may buffer the hydrogen ions secreted into the distal tubules; in addition, the ions may combine with ammonia also secreted by the tubules. The most important non-bicarbonate base present in the filtrate is dibasic phosphate (Na2HPO4), which accepts hydrogen ions to form monobasic phosphate (NaH2PO4). A measure of the amount of hydrogen ion in the urine that is buffered by bases such as bicarbonate and phosphate is made by the titration of urine with strong base until the pH of the plasma from which the filtrate is derived (7.4) is achieved. This is called the titratable acidity of urine and usually amounts to between 20 and 40 millimoles of H+ per day.

      In normal circumstances about two-thirds of the hydrogen ions to be secreted in the urine is in the form of ammonium salts (e.g., ammonium chloride). ammonia (NH3) is not present in plasma or filtrate but is generated in the distal tubular cells and passes into the lumen probably by passive diffusion down a concentration gradient. In the lumen the NH3 combines with hydrogen ions secreted into the tubule to form ammonium ions (NH4+), which are then trapped in the lumen because the lipid walls of the tubular cells are much less permeable to the charged than to the uncharged molecules.

      It is now known that ammonia is formed from the hydrolysis of glutamine (an amino acid) to form glutamic acid and ammonia by the enzyme glutaminase. A further molecule of ammonia is obtained by the deamination of glutamic acid to form glutaric acid, which is then metabolized. The more acidic the urine is, the greater is its content of ammonium ions; the introduction of hydrogen ions (e.g., from the diet) stimulates production of ammonium by the tubular cells. The ammonium is excreted in the urine as ammonium salts of surplus anions (negative ions) such as chloride, sulfate, and phosphate, thus sparing for retention other cations (positive ions) such as sodium or potassium.

      In summary, hydrogen ion secretion can be considered in three phases. The first occurs in the proximal tubule, where the net result is tubular reabsorption of filtered bicarbonate. The second and third phases take place in the distal tubule, where monobasic phosphate and ammonium salts are formed. The total tubular cell secretion of hydrogen ion is therefore the sum of titratable acidity, the amount of ammonium ion excreted, and the amount of bicarbonate ion reabsorbed. The last may be assessed by calculating the amount of bicarbonate filtered (i.e., plasma concentration of bicarbonate × glomerular filtration rate and subtracting any bicarbonate excreted in the urine). Total hydrogen ion secretion normally amounts to 50–100 millimoles per day but may rise considerably above this in disorders associated with excess acid production, such as diabetes.

Volume and composition
      The volume and composition of normal urine vary widely from day to day, even in healthy individuals, as a result of food and fluid intake and of fluid loss through other channels as affected by environmental conditions and exercise. The daily volume averages 1.5 litres (about 1.6 quarts) with a range of 1–2.5 litres, but after copious sweating it may fall as low as 500 millilitres, and after excess fluid intake it may reach three litres or more. There is also variation within a 24-hour period. Excretion is reduced in the early hours, maximal during the first few hours after rising, with peaks after meals and during the early stages of exertion. The urine produced between morning and evening is two to four times the night volume. The excessive secretion of urine (polyuria) of chronic renal disease is typically nocturnal.

      The volume of urine is regulated to keep plasma osmotic concentration constant, to control the total water content of the tissues, and to provide a vehicle for the daily excretion to the exterior of some 50 grams of solids, mostly urea and sodium chloride (salt). In a man who ingests 100 grams of protein and 10 grams of salt daily, the urine will contain 30 grams of urea and 10 grams of salt; there are many other possible constituents, but they amount to less than 10 grams overall.

       Some urine constituentsSome urinary constituents (Table 3 (Some urine constituents))—the products of metabolism of nitrogenous substances obtained from food—vary widely in relation to the composition of the diet; thus the excretion of urea and sulfate is dependent on the diet-protein content. A high-protein diet may yield a 24-hour output of 17 grams of nitrogen, a low-protein diet of the same calorific value only three to four grams.

      The urine is normally clear. It may be turbid from calcium phosphate, which clears if acetic acid is added. Microscopic deposits include occasional casts, vaguely resembling in form the renal tubules from whose lining they have been shed. An ammoniacal smell is the result of decomposition of urea to ammonia by bacteria and is commonly present on babies' diapers. Certain foods and drugs may cause distinctive odours. The colour of urine depends on its concentration but is normally a bright clear yellow from the pigment urochrome, an end product of protein metabolism. There are also traces of other pigments: urobilin and uroerythrin. The colour may be influenced as well by vitamins, food dyes, beetroot, and certain drugs.

      The specific gravity of urine may vary between 1.001 and 1.04 but is usually 1.01–1.025. Such variation is normal, and a fixed low specific gravity is an indication of chronic renal disease. If fluid intake is stopped for 24 hours, a normal kidney will secrete urine with a specific gravity of at least 1.025. There is a limit to the concentrating powers of the kidney, so that the urine is rarely more than four times as concentrated as plasma. In order to excrete their normal solute load, the kidneys need a minimum water output of 850 millilitres as a vehicle; this volume is often called the minimum obligatory volume of urine. If this is not available from intake it has to be withdrawn from the tissues, causing dehydration; but the usual intake is well above the minimum and the urine is rarely at its maximum possible concentration. The reaction of the urine is usually acid, with an overall range of pH 4 to 8 (lemon pie has a pH of 2.3; the value 8 is slightly alkaline, about equal to the pH of a 1 percent solution of sodium bicarbonate).

      Foreign proteins of molecular weight less than 68,000 are excreted in the urine, while those of the plasma are retained in the body. If, however, the kidneys are damaged by disease or toxins, the glomeruli will transmit some of the normal serum albumin and globulin and the urine will coagulate on warming. Normally, the urine contains only very small amounts of protein (less than 50 milligrams per 24 hours); however, protein content in the urine is increased after exercise, in pregnancy, and in some persons when standing (orthostatic albuminuria). The protein loss may be greatly increased in certain chronic renal diseases; in the nephrotic syndrome it may even reach 50 grams in a 24-hour period. Certain specific and easily identifiable proteins appear in the urine in diseases associated with the overgrowth of cells that make immunoglobulins.

       glucose is found in the urine in diabetes mellitus. In some healthy persons, however, there may also be an abnormal amount of glucose in the urine because of a low threshold for tubular reabsorption, without any disturbance of glucose metabolism. Lactosuria (abnormal amount of lactose in the urine) may occur in nursing mothers. ketone bodies (acetone, acetoacetic acid) are present in traces in normal urine but in quantity in severe untreated diabetes and in relative or actual carbohydrate starvation; e.g., in a person on a high-fat diet.

      The urine may contain hemoglobin or its derivatives after hemolysis (liberation of hemoglobin from red blood cells), after incompatible blood transfusion, and in malignant malaria (blackwater fever). Fresh blood may derive from bleeding in the urinary tract. Bile salts and pigments are increased in jaundice, particularly the obstructive variety; urobilin is greatly increased in certain diseases such as cirrhosis of the liver.

      Porphyrins (porphyrin) are normally present only in minute amounts but may be increased in congenital porphyria, a disease characterized by sensitivity to sunlight or by insanity. The presence of porphyrins also may increase after ingestion of sulfonamides and some other drugs.

      The normally small quantities of amino acids in the urine may be much increased in advanced liver disease, in failure of tubular reabsorption, and in certain diseases due to inborn errors of protein metabolism. phenylketonuria, a disease identified by the presence of phenylpyruvic acid in the urine, is due to lack of the enzyme phenylalanine hydroxylase, so that phenylalanine is converted not to tyrosine but to phenylpyruvic acid. The presence of this acid in blood and tissues causes mental retardation; it may be readily detected if the urine of every newborn infant is tested. Restriction of phenylalanine in the diet in such cases may be beneficial. alkaptonuria, a disease identified by the presence of homogentisic acid in the urine, is due to lack of the enzyme that catalyzes the oxidation of homogentisic acid; deposits of the acid in the tissues may cause chronic arthritis or spinal disease. Other such disorders are cystinuria, the presence of the amino acid cystine in the urine, when the bladder may contain cystine stones; and maple syrup disease (maple syrup urine disease), another disorder involving abnormal levels of amino acid in the urine and blood plasma.

Urine collection and emission
      From the nephrons the urine enters the final 15 or 20 collecting tubules that open on to each papilla of the renal medulla, projecting into a minor calyx. These open into two or three major calyxes, and these in turn open into the renal pelvis, which connects with the upper expanded portion of the ureter.

      Urine is passed down the channel of the renal pelvis and ureter by a succession of peristaltic waves of contraction that begin in the muscle fibres of the minor calyxes, travel out to the major calyxes and then along the ureter every 10–15 seconds. Each wave sends urine through the ureteric orifice into the bladder in discontinuous spurts; these can be seen through a cystoscope if a dye is injected into the bloodstream. Gravity aids this downward flow, which is faster when one is standing erect. Though the overall picture suggests that there is a pacemaker (a set of specialized cells capable of rhythmic contractions) near the pelviureteric junction, this has never been satisfactorily demonstrated in the tissue. The pressure in the renal pelvis is normally low, but the smooth muscle coat of the ureter is a powerful one and the pressure above an obstructed ureter may rise as high as 50 millimetres of mercury. The ureters are doubly innervated from the splanchnic nerves above and the hypogastric network below.

The bladder (urinary bladder)
      The bladder is a hollow organ of variable capacity, with a powerful intermediate muscle coat that empties the organ when it contracts, and two muscular sphincters that keep the exit closed at all other times. This smooth muscle coat constitutes the powerful detrusor muscle. At the base of the bladder the region of the bladder neck, or trigone, is demarcated by the two ureteric orifices and the internal opening of the urethra. Muscle fibres loop around the urethral opening to form the internal sphincter, which is under involuntary control. The external sphincter consists of two layers of striated muscles under voluntary control.

      The mucous membrane lining the bladder is distensible; it is ridged in the empty organ and smoothed out in distension. In micturition (urination) the longitudinal muscle of the bladder shortens to widen the bladder neck and allow urine to enter the urethra. The urethra normally contains no urine except during the act of micturition, its walls remaining apposed by muscle tone. The external sphincter can maintain continence even if the internal sphincter is not functioning.

      The innervation of the bladder and urethra is complex and important. Essentially, there are three groups of nerves: (1) The parasympathetic nerves constitute the main motor supply to the detrusor; they make it contract, raise pressure within the bladder, relax the internal sphincter, and cause emptying. Afferent parasympathetic channels convey impulses from stretch receptors in the bladder wall to higher centres, permitting cognizance of the state of distension of the organ and stimulating the desire to micturate. (2) The sympathetic nerves stimulate closure of the ureteric and internal urethral orifices and contraction of the internal sphincter, and their action on the detrusor is inhibitory; i.e., the effect is to prevent bladder outflow. Thus the sympathetic nerves act to control the situation in the distending bladder up to the point when evacuation can be deferred no longer. Afferent paths in the sympathetic system convey sensations of pain, overdistension, and temperature from the mucosa of the bladder and the urethra. (3) The somatic nerves cause contraction of the external sphincter; their sensory fibres relay information as to the state of distension of the posterior urethra.

      Both the parasympathetic nerves and the somatic nerves (pudendal nerve) to the external sphincter relay impulses to the second through fourth sacral segments of the spinal cord, which constitute a reflex centre for the control of bladder function. This centre connects with higher centres in the brain by ascending and descending fibres in the spinal cord.

Bladder function in micturition
      Certain reflexes combine to ensure both maintenance of a steady holding state for urine and normal progressive micturition with complete emptying. When the internal pressure of the bladder rises, it contracts; and it also contracts when urine enters the urethra.

      Both bladder sphincters are normally closed. As the organ fills with urine, the contractile response of the muscle wall causes a rise in internal pressure. Relaxation then occurs as an active process of adjustment so that the organ may hold its contents at a lower pressure. As urine continues to enter the bladder, this rise and fall of pressure continues in steplike fashion, with the final pressure always gradually rising.

      The repeated transient contraction waves at first are small and are not consciously felt; later, stimuli reach the brain and cause pain and a sharp rise of pressure. These later major contractions can be inhibited voluntarily. The desire to micturate begins at around a content of 400 millilitres, but it can be voluntarily overridden until the content reaches 600–800 millilitres, with a resulting pressure within the bladder of up to 100 millimetres of water. Until this point the sphincters remain contracted to keep the urethral exit closed, but eventually the desire to micturate becomes urgent and irrepressible. Until that time, voluntary inhibition of the detrusor and contraction of the perineal muscles have kept the internal pressure as low as possible and have prevented urine leakage. The threshold is dependent to some extent on the rate of filling and is higher when filling is slow; and training affects the amount the bladder can retain. In young children the situation is less controllable, and even small amounts of urine may excite reflex evacuation. Emotional influences are important. Anxiety inhibits the capacity of the bladder to relax on filling, so that under conditions of stress there may be some involuntary passage of small quantities of urine.

      Micturition is a complex activity, partly reflex and unconscious and mediated by the lower spinal cord centres, and partly under conscious control by the higher centres of the brain. Voluntary micturition begins with willed messages from the brain that reach the bladder via the motor fibres of the pelvic nerves to stimulate the detrusor, at the same time actively relaxing both urethral sphincters. But the reflexes already mentioned ensure that, once the process has begun and urine has entered the urethra, the contraction of the detrusor will continue and the sphincters will remain relaxed until evacuation is complete and the bladder empty. Evacuation is aided by voluntary contraction of a wide range of accessory muscles. The muscles of the abdominal wall contract to increase pressure on the bladder from without; the diaphragm descends and the breath is held; at the same time there is relaxation of the muscles of the perineal floor. Thus voluntary initiation and control of micturition is effected partly by an active process of stimulating parasympathetic sacral nerve outflow, partly by removing the normal inhibition exerted by the higher centres on the reflex centres in the spinal cord. Once begun, micturition is carried through to completion by lower and higher centres acting in concert; sensory messages from the urine-distended urethra also play a part. It follows that even if a bladder is not particularly distended and if reflex emptying is not urgent, the bladder can nevertheless be evacuated by voluntary contraction of the abdominal wall, so initiating the reflex process that, once begun, takes over.

Tests (kidney function test) of renal function
Quantitative tests
      Important quantitative tests of renal function include those of glomerular filtration rate, renal clearance, and renal blood flow. Tests are also made to estimate maximal tubular activity, tubular mass, and tubular function. Radiological and other imaging methods are useful noninvasive diagnostic techniques, and renal biopsy is valuable in detecting pathological changes that affect the kidneys. In both clinical and experimental studies one of the most fundamental measures of renal function is that of the glomerular filtration rate (GFR). The GFR is calculated by measuring the specific clearance from the body of a substance believed to be excreted solely by glomerular filtration. The renal clearance of any substance is the volume of plasma containing that amount of the substance that is removed by the kidney in unit time (e.g., in one minute). Clearance, or the volume of plasma cleared, is an artificial concept since no portion of the plasma is ever really cleared in this fashion.

      It was soon realized, however, that if a substance could be found that was freely filtered by the glomeruli and was neither reabsorbed, metabolized, nor secreted by the renal tubules, its clearance would equal the GFR. This is so in these circumstances because the amount of such a substance excreted in the urine in one minute would equal the amount that has been filtered at the glomeruli in the same time. If the concentration of the substance in the plasma (which is the same as that in the glomerular filtrate) is known, the clearance volume must represent the volume of glomerular filtrate.

      The first substance identified to be excreted in this way was the polysaccharide inulin (molecular weight about 5,000), which is extracted from the roots of dahlias. Although inulin is not naturally found in human plasma it is nontoxic and can be injected or infused into the bloodstream. Its concentration also can be measured readily and accurately. In the adult male the GFR is 125 millilitres per minute per 1.73 square metres of body surface. In the adult female, the values are about 85 percent of those for the same standard area of body surface. inulin clearance is now accepted as the standard for estimation of the GFR.

      Clearance value is not the same as excretion rate. The clearance of inulin and some other compounds is not altered by raising its plasma concentration, because the amount of urine completely cleared of the agent remains the same. But the excretion rate equals total quantity excreted per millilitre of filtrate per minute, and this value is directly proportional to its plasma concentration.

      Substances, such as urea, whose clearance is less than the GFR must be reabsorbed by the renal tubules, while substances whose clearance is greater than the GFR must be secreted by the renal tubules. Since the discovery of inulin, researchers have identified a small number of other substances that are excreted by the kidney in a similar fashion and that have similar clearance values. These include vitamin B12, circulating free in plasma and unbound to protein, and sodium ferrocyanide.

      The clearance of creatinine was used as a measure of renal function before inulin was discovered; because this substance is found naturally in plasma, creatinine clearance is still widely used as an approximate measure of the GFR. Creatinine is produced in the body at virtually a constant rate, and its concentration in the blood changes little; accordingly, creatinine clearance is usually measured over a period of 24 hours. There is evidence that in humans creatinine is secreted into the urine by renal tubules as well; however, the amount is small and constant and has little effect on the measure of the GFR.

      The concept of clearance is also useful in the measurement of renal blood flow. Para-aminohippuric acid (PAH), when introduced into the bloodstream and kept at relatively low plasma concentrations, is rapidly excreted into the urine by both glomerular filtration and tubular secretion. Sampling of blood from the renal vein reveals that 90 percent of PAH is removed by a single circulation of blood through the kidneys. This high degree of PAH extraction by the kidney at a single circulation implies that the clearance of PAH is approximately the same as renal plasma flow (RPF). The 10 percent of PAH that remains in renal venous blood is conveyed in blood that perfuses either nonsecretory tissue, such as fibrous tissue or fat, or parts of the tubule that do not themselves secrete PAH. In practice this small remaining percentage is usually ignored, and the clearance of PAH is referred to as the effective renal plasma flow. In humans PAH clearance is about 600 millilitres per minute, and thus true renal plasma flow is about 700 millilitres per minute.

      Estimation of the GFR and RPF allows the proportion of available plasma perfusing the kidney that is filtered by the glomerulus to be calculated. This is called the filtration fraction and on average in healthy individuals is 125/600, or about 20 percent. Thus about one-fifth of plasma entering the glomeruli leaves as filtrate, the remaining four-fifths continuing into the efferent glomerular arterioles. This fraction changes in a number of clinical disorders, notably hypertension.

      Reference has already been made to the fact that the renal tubules possess a limited capacity to perform certain of their functions. This is the case, for example, in their ability to concentrate and dilute urine and to achieve a gradient of hydrogen ions between urine and blood. Concentrating power can be tested by depriving the individual of water for up to 24 hours, or, more simply, by introducing a synthetic analogue of ADH into each nostril. The water deprivation test assesses the individual's capacity to produce ADH and the sensitivity of the renal concentrating mechanism to circulating ADH. The use of an analogue of ADH assesses only the sensitivity of the renal tubules to the hormone.

      The limits of renal ability to excrete acid and establish a gradient of the concentration of hydrogen ions between plasma and urine has been mentioned above. The power of acidification of urine is best estimated by measuring the pH of urine after the administration of ammonium chloride in divided doses over two or three days. Other specific functions that are tested include the individual's ability to conserve sodium, potassium, and magnesium. In general, these tests are carried out by administering diets that are deficient in these electrolytes and then estimating the minimum rate of excretion after several days.

Radiological and other imaging investigations
      Imaging techniques are used to determine the anatomical site, configuration, and level of functioning of the kidneys (kidney), pelvis, and ureters. A plain X-ray nearly always precedes any other more elaborate investigation, so that the size, outline, and position of the two kidneys, as well as information about the presence or absence of calcium-containing renal stones or zones of calcification can be ascertained. Excretion urography is one of the simplest methods of defining these aspects more precisely, though this radiological method is giving way to noninvasive imaging methods such as ultrasonography and magnetic resonance imaging (MRI). Excretion urography can be used to provide information on both the structure and the function of the renal system. In this test the kidneys are observed in X-rays after intravenous injection of a radiopaque compound that is excreted largely by glomerular filtration within one hour of the injection. A series of X-ray images (nephrograms) then indicates when the contrast substance first appears and reveals the increasing radiographic density of the renal tissue. The X-rays also indicate the position, size, and presence of scarring or tumours in the organs and provide an approximate comparison of function in the two kidneys. Finally the dye collects in the bladder, revealing any rupture or tumour in this organ.

      Obstruction to the flow of urine also may be revealed by distension of the calyceal system above the site of obstruction. This is more clearly detected by urography, in which contrast medium is injected through a fine catheter introduced either directly into the pelvis of the kidney or into the ureteral orifice visualized during cystoscopy. A micturating cystogram (voiding cystourethrogram [VCUG]) involves the injection of contrast substance into the bladder and is of importance in the investigation of urinary tract infection in childhood. It may show the reflux of urine from the bladder upward into the ureters or kidneys on micturition. Because of the risk of radiation to the gonads this test should be conducted only on certain patients.

      A radioactive renogram involves the injection of radioactive compounds that are concentrated and excreted by the kidney. The radiation can be detected by placing gamma scintillation counters externally over the kidneys at the back; the counts, transcribed on moving graph paper, yield characteristic time curves for normal and disordered function.

      A picture of renal circulation can be obtained by introducing a radiopaque substance into the renal arteries via a catheter tube placed through a more peripheral artery in the groin area. The contrast material yields a renal angiogram, showing the renal vascular tree. The technique is especially valuable in demonstrating the presence of localized narrowing or obstructions in the circulation or of localized dilatations (aneurysms). Tumours, which tend to be well vascularized, are also distinguishable from cysts, which are not well supplied with blood. Balloon-tipped catheters can be used to stop active bleeding or to introduce a supportive stent, which is permanently placed inside an artery to stabilize a weakened vessel or to keep a narrowed vessel open.

       ultrasound and MRI (magnetic resonance imaging) have the advantage of being noninvasive and thus pose little risk to the patient. They are useful in detecting tumours of the kidney or adjacent structures and in distinguishing tumours from cysts. Special contrast agents (e.g., gadolinium) may be infused before a MRI examination to evaluate metabolic characteristics of tissues and to facilitate the examination of blood flow to a tumour. This may help to differentiate between benign and cancerous tumours. Ultrasound techniques are comparatively simple and have replaced other methods in detecting the presence of polycystic kidneys, as well as in providing initial screening evaluation of the kidney.

      The visual, usually microscopic, examination of a specimen of kidney tissue removed from a living patient (renal biopsy) is the only investigative method that yields exact histological data on renal structure. The material for examination is usually obtained by inserting a special needle through the skin of the back into the kidney substance and withdrawing a fragment of tissue. A general anesthetic is not usually required, the procedure occupying only a few minutes. Renal biopsy has been valuable in clarifying several renal disorders, notably those affecting the glomeruli, and in revealing their prognosis and natural course. The major, potentially serious complication of biopsy is excessive bleeding, but this is rare. The procedure is not justified, however, if the patient possesses only one kidney or suffers from a bleeding disorder or from severe, uncontrolled high blood pressure.

The role of hormones (hormone) in renal function
      Certain hormones and hormonelike substances are intimately related to renal function. Some of these, such as ADH (or vasopressin), are produced outside the kidney and travel to the kidney via the blood as chemical messengers. Others are produced within the kidney and appear to exert only a local effect. The role of ADH in controlling diuresis has already been discussed. ADH regulates water excretion by increasing the permeability of the collecting ducts to water and salt and by accelerating water and ion transfer in a direction determined by the osmotic gradient. The receptors at the base of the brain form part of the feedback mechanism that (1) stimulates ADH output if the osmotic concentration of extracellular fluid (ECF) is high, so as to concentrate the urine, and (2) reduces ADH output and so dilutes the urine if osmotic concentration of ECF and of plasma falls.

      The hormones of the adrenal cortex are also important in influencing renal function, directly or indirectly. In stress situations, as after an injury or a surgical operation, the output of hydrocortisone (cortisol) and other corticosteroids is increased because the adrenals are stimulated by adrenocorticotropin (ACTH (adrenocorticotropic hormone)), a secretion of the pituitary gland. Hydrocortisone increases protein breakdown, and consequently the output of nitrogen in the urine, and affects water metabolism; lack of hydrocortisone reduces the power of the kidney to deal with normal water loads. The hormone also promotes sodium retention and loss of potassium and hydrogen ions by the kidney. aldosterone influences electrolyte metabolism by facilitating the reabsorption of sodium (sodium-potassium pump) ions at the distal tubules, also at the expense of hydrogen and potassium excretion. The action of aldosterone has been described as priming the sodium reabsorption pump; it is the adrenal hormone most important to tubular function. It also influences the ability of the bowel to absorb sodium, and thus its level of production profoundly influences overall sodium balance. Deficiency of aldosterone allows a steady loss of sodium in the urine, causing a fall in blood pressure that may result in fainting.

      The action of the parathyroid glands (parathyroid gland) is to increase blood calcium by mobilizing calcium from the bones and other sources; if this hormone functions to excess, as in tumours of the glands, the urinary loss of calcium is much increased and calcium stones tend to form in the kidneys and the bladder. Parathyroid hormone also increases the renal excretion of phosphate and accelerates the conversion of hydroxylated vitamin D to the dehydroxylated form in the kidney. The pituitary growth hormone facilitates protein synthesis and decreases the urinary loss of nitrogen. The sex hormones estrogen and progesterone exert an ill-defined activity as regards salt and water metabolism.

      The juxtaglomerular apparatus (JGA), consisting of an asymmetrical cuff of large granular cells in the wall of the afferent arteriole near its entry into the capsule of the nephron, contains renin in the granules in the cells. Renin is a true internal secretion of the kidney. Entering the plasma, it acts as an enzyme that induces one of the plasma globulins to yield angiotensin I, which is inactive, and which gives rise in turn to angiotensin II, the most potent agent for constricting the blood vessels and raising the blood pressure. The formation of renin at the JGA is induced by a fall in blood pressure and inhibited by a rise. When the pressure falls, the output of angiotensin II raises the pressure and also excites the release of aldosterone from the adrenal cortex. This process is another example of a feedback mechanism analogous to that controlling the output of ADH.

      Among the prostaglandins (prostaglandin), a group of hormonelike fatty acids synthesized throughout the body, the ones found in the kidney tissues appear to exert local influence on various aspects of renal function. Unlike true hormones, prostaglandins are not transported away from their site of origin by the blood. The interstitial and collecting duct cells of the kidney produce a characteristic prostaglandin, PGE2, and the renal cortex produces PGI2, or prostacyclin. Renal prostaglandins interact with the renin–angiotensin system in several ways. The renal cortex prostaglandin PGI2 mediates the increased release of renin in response to decreases in renal blood flow. The angiotensin subsequently formed in the plasma stimulates production of the interstitial and duct cell prostaglandin (PGF2), which itself inhibits angiotensin-induced vasoconstriction. For this reason the renal cortex prostaglandin is thought to be an important vasodilator, maintaining renal blood flow when this is threatened (for example, after blood loss). Prostaglandins may also inhibit the action of ADH on the distal tubule and collecting ducts, and the interstitial and duct cell prostaglandin may have a direct effect in inhibiting renal tubular sodium reabsorption; however, the relative importance of these different actions in the healthy human is not known.

      Another substance that causes the dilation of blood vessels, the enzyme kallikrein, may also exert an influence on renal blood flow. Kallikrein is secreted by renal tubules and is added to the urine in the distal tubules. It activates the conversion of kininogen to bradykinin, which is also a powerful vasodilator. Bradykinin is inactivated by a kininase, which also converts angiotensin I to angiotensin II, a substance that causes the constriction of blood vessels. Thus the same enzyme that inactivates the vasodilator bradykinin catalyzes the production of the vasoconstrictor angiotensin II. This relationship again suggests a delicately balanced internal control system.

       dopamine is a putative renal hormone that may affect salt balance. The sympathetic nerves that travel to the kidney, the terminals of which release catecholamines such as norepinephrine, are not believed to be important in controlling tubular salt reabsorption. Transplanted human kidneys function adequately despite the lack of any nerve supply and so renal nerves are not essential. However, because dopamine (also a catecholamine released at sympathetic nerve endings) is present in urine in amounts far in excess of the amount that might be filtered from the blood, it may be deduced that some dopamine is formed within the kidney. It is now believed that dopamine is formed enzymatically within the kidney from its precursor, L-dopa, which freely circulates in the blood, and that only small amounts are released by sympathetic nerve endings. Dopamine is a powerful natriuretic substance (i.e., one capable of increasing urinary salt loss) and renal vasodilator. Its role in salt balance, renal function, and blood pressure control remains speculative.

      The most recently identified hormone that influences renal function is secreted by special “stretch receptor” cells in the atria of the heart in response to a rise in atrial pressure, as during heart failure. This hormone, called atrial natriuretic peptide (ANP), exerts a vasodilator effect on the kidney and also reduces tubular reabsorption of sodium. Both actions result in increased urinary elimination of salt and water and tend to restore atrial pressure toward the normal. It is probably an important hormone controlling the volume of the extracellular fluid.

Biological considerations
      During most of the pregnancy period the glomerular filtration rate (GFR) is increased by as much as 50 percent, corresponding to an increase in renal blood flow of up to 25 percent in the middle three months of pregnancy. Glycosuria is frequent and is due to increased glucose loading of the filtrate; there is some sodium retention with a tendency to abnormal accumulation of serous fluid (edema), and some protein may appear in the urine. Anatomical changes include enlargement and dilation of the pelvis and ureters, caused by both hormonal action and partial ureteric obstruction by the gravid uterus. These changes may be responsible for the increased susceptibility to urinary tract infection during pregnancy.

      The kidneys of the fetus begin to function well before birth, as indicated by a steady rise in the urea and uric acid content of the amniotic fluid in which the fetus exists; the fetus probably swallows fluid and voids it as urine. But even at birth, half the work of excretion is still being carried out via the placental circulation and the maternal kidneys, and this dependence is abruptly curtailed. Kidney function is far from fully developed in the newborn infant (infancy). The glomerular filtration rate is only some 30 millilitres per minute per square metre of body surface, compared to 75 in the adult, and tubular function does not attain adult performance until the end of the first year. The 24-hour output of urine is only some 20 millilitres; the output of water and the renal clearance of sodium, potassium, and phosphate is low; the urine is dilute and often contains protein. Because the kidney has such a poor capacity to excrete solids, the infant is exposed to the dehydrating effect of vomiting and diarrhea, which readily induce renal failure.

      There is an increased urine output at the commencement of muscular exercise, due to the general stimulation of circulation, but a later falling off with the fatigue and sweating caused by severe prolonged exertion. The 24-hour rhythm in output has been mentioned. The small output in the early morning hours is a practical convenience to prevent disturbance of sleep. If the natural sleep rhythm is inverted, as by working on night shift, electrolyte and water output follow suit. The urine is acidic at night and becomes less so, or alkaline, on rising. Output is maximal during the first waking hours and rises after meals. Because of all this variation in water and solute output, any analytic study of urine components must be conducted on specimens obtained over a 24-hour period.

David Le Vay James Scott Robson

Additional Reading

Renal anatomy and function
Don W. Fawcett, William Bloom, and Elio Raviola, A Textbook of Histology, 12th ed. (1994), an illustrated text on histology and fine structure; Henry Gray, Anatomy of the Human Body, 30th American ed., edited by Carmine D. Clemente (1985), a treatise frequently revised over the past century and still regarded as one of the great anatomical texts; and Bruce M. Carlson, Human Embryology and Developmental Biology, 3rd ed. (2004), one of the leading textbooks on human embryology. Michael J. Field, David Harris, and Carol A. Pollock, The Renal System (2001), is a useful introductory textbook covering the basic science of the renal system.

Human excretion
The following works provide detailed and well-written surveys of the field of kidney function in a form suitable for medical students and for others with a good scientific grounding: E.J. Moran Campbell et al. (eds.), Clinical Physiology, 5th ed. (1984); and J.M. Forrester et al. (eds.), A Companion to Medical Studies: Anatomy, Biochemistry, and Physiology, 3rd ed. (1985).

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