Structure and Function of the Kidneys 524

I. The kidney is divided into an outer cortex and inner medulla.

A. The medulla is composed of renal pyramids, separated by renal columns.

B. The renal pyramids empty urine into the calyces that drain into the renal pelvis. From there, urine flows into the ureter and is transported to the bladder to be stored.

II. Each kidney contains more than a million microscopic functional units called nephrons. Nephrons consist of vascular and tubular components.

A. Filtration occurs in the glomerulus, which receives blood from an afferent arteriole.

B. Glomerular blood is drained by an efferent arteriole, which delivers blood to peritubular capillaries that surround the nephron tubules.

C. The glomerular (Bowman's) capsule and the proximal and distal convoluted tubules are located in the cortex.

D. The loop of Henle is located in the medulla.

E. Filtrate from the distal convoluted tubule is drained into collecting ducts, which plunge through the medulla to empty urine into the calyces.

Glomerular Filtration 529

I. A filtrate derived from plasma in the glomerulus must pass through a basement membrane of the glomerular capillaries and through slits in the processes of the podocytes—the cells that compose the inner layer of the glomerular (Bowman's) capsule. A. The glomerular ultrafiltrate, formed under the force of blood pressure, has a low protein concentration.

B. The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys each minute. It ranges from 115 to 125 ml/min.

II. The GFR can be regulated by constriction or dilation of the afferent arterioles.

A. Sympathetic innervation causes constriction of the afferent arterioles.

B. Intrinsic mechanisms help to autoregulate the rate of renal blood flow and the GFR.

Reabsorption of Salt and Water 532

I. Approximately 65% of the filtered salt and water is reabsorbed across the proximal convoluted tubules.

A. Sodium is actively transported, chloride follows passively by electrical attraction, and water follows the salt out of the proximal tubule.

B. Salt transport in the proximal tubules is not under hormonal regulation.

II. The reabsorption of most of the remaining water occurs as a result of the action of the countercurrent multiplier system.

A. Sodium is actively extruded from the ascending limb, followed passively by chloride.

B. Since the ascending limb is impermeable to water, the remaining filtrate becomes hypotonic.

C. Because of this salt transport and because of countercurrent exchange in the vasa recta, the interstitial fluid of the medulla becomes hypertonic.

D. The hypertonicity of the medulla is multiplied by a positive feedback mechanism involving the descending limb, which is passively permeable to water and perhaps to salt.

III. The collecting duct is permeable to water but not to salt.

A. As the collecting ducts pass through the hypertonic renal medulla, water leaves by osmosis and is carried away in surrounding capillaries.

B. The permeability of the collecting ducts to water is stimulated by antidiuretic hormone (ADH).

Renal Plasma Clearance 539

I. Inulin is filtered but neither reabsorbed nor secreted. Its clearance is thus equal to the glomerular filtration rate.

II. Some of the filtered urea is reabsorbed. Its clearance is therefore less than the glomerular filtration rate.

III. Since almost all the PAH in blood going through the kidneys is cleared by filtration and secretion, the PAH clearance is a measure of the total renal blood flow.

IV. Normally all of the filtered glucose is reabsorbed. Glycosuria occurs when the transport carriers for glucose become saturated as a result of hyperglycemia.

Renal Control of Electrolyte and Acid-Base Balance 544

I. Aldosterone stimulates sodium reabsorption and potassium secretion in the distal convoluted tubule.

II. Aldosterone secretion is stimulated directly by a rise in blood potassium and indirectly by a fall in blood volume.

A. Decreased blood flow and pressure through the kidneys stimulates the secretion of the enzyme renin from the juxtaglomerular apparatus.

B. Renin catalyzes the formation of angiotensin I, which is then converted to angiotensin II.

C. Angiotensin II stimulates the adrenal cortex to secrete aldosterone.

Physiology of the Kidneys

III. Aldosterone stimulates the secretion of H+, as well as potassium, into the filtrate in exchange for sodium.

IV. The nephrons filter bicarbonate and reabsorb the amount required to maintain acid-base balance. Reabsorption of bicarbonate, however, is indirect.

A. Filtered bicarbonate combines with H+ to form carbonic acid in the filtrate.

B. Carbonic anhydrase in the membranes of microvilli in the tubules catalyzes the conversion of carbonic acid to carbon dioxide and water.

C. Carbon dioxide is reabsorbed and converted in either the tubule cells or the red blood cells to carbonic acid, which dissociates to bicarbonate and H+. D. In addition to reabsorbing bicarbonate, the nephrons filter and secrete H+, which is excreted in the urine buffered by ammonium and phosphate buffers.

Clinical Applications 549

I. Diuretic drugs are used clinically to increase the urine volume and thus to lower the blood volume and pressure. A. Loop diuretics and the thiazides inhibit active Na+ transport in the ascending limb and early portion of the distal tubule, respectively.

B. Osmotic diuretics are extra solutes in the filtrate that increase the osmotic pressure of the filtrate and inhibit the osmotic reabsorption of water.

C. The potassium-sparing diuretics act on the distal tubule to inhibit the reabsorption of Na+ and secretion of K+.

II. In glomerulonephritis, the glomeruli can permit the leakage of plasma proteins into the urine.

III. The technique of renal dialysis is used to treat people with renal insufficiency.

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