384U R I N A R Y S Y S T E M

Type B cells resorb H+ and

secrete HCO3−

¥The papillary ducts then deliver the urine formed by the uriniferous tubule to the intrarenal passage, namely, the minor calyx, to be drained into a major calyx and then into the pelvis of the ureter.

These excretory passages, lined by transitional epithelium, possess a Þbroelastic subepithelial connective tissue, a smooth muscle tunic composed of inner longitudinal and outer circular layers, as well as a Þbroelastic adventitia.

FORMATION OF URINE FROM ULTRAFILTRATE

Fluid, leaving the glomeruli, enters BowmanÕs space of the renal corpuscle, ßows through the various components of the uriniferous tubule to be modiÞed and concentrated, and leaves the papillary ducts as urine.

Formation of the Ultrafiltrate

Since the renal artery is a direct branch of the abdominal aorta, the two kidneys receive 20% of the total blood volume per minute.

¥Most of this blood enters the glomeruli, where the high arterial pressure expresses approximately 10% of its ßuid volume, 125 mL/min, into BowmanÕs spaces.

¥Vascular pressure is opposed by two forces, the colloid osmotic pressure of the blood and the pressure exerted by the ultraÞltrate present in BowmanÕs space.

¥The average net filtration force, expressing ultraÞltrate from the blood into BowmanÕs space, is relatively high, about 25 mm Hg.

The renal filtration barrier, composed of the fenestrated endothelial cells, the fused basal laminae of the podocyte and capillary, and the diaphragm-bridged Þltration slits between pedicels, permits only the passage of water, ions, and small molecules into BowmanÕs space.

¥The presence of the polyanionic heparan sulfate in the lamina rara of the basal lamina impedes the passage of large and negatively charged proteins through the barrier (see Table 16-2).

¥Type IV collagen of the lamina densa acts as a molecular sieve and traps proteins larger than 69,000 Da or 7-nm diameter.

To maintain the efÞciency of the Þltering system, intraglomerular mesangial cells

¥phagocytose the lamina densa, which then is renewed by the combined actions of the podocytes and endothelial cells.

¥form the mesangial matrix around themselves and release prostaglandins, interleukin-1, and other cytokines.

¥have contractile properties that, by constricting the glomerulus, modulate blood pressure within the glomerular network.

¥form a structural support for the glomerulus.

The modiÞed plasma that enters BowmanÕs space is known as the ultrafiltrate.

Functions of the Proximal Tubule

In a healthy individual, the proximal tubule resorbs approximately

¥As much as eighty percent of the water, sodium, and chloride, as well as

¥Hundred percent of the proteins, amino acids, and glucose from the ultraÞltrate.

The resorbed materials are eventually returned into the peritubular capillary network of the cortical labyrinth for distribution to the remainder of the body.

¥The movement of sodium is via an active transport mechanism utilizing a sodium-potassium-ATPase pump in the basal plasmalemma, with chloride and water following passively.

¥Since salt and water are resorbed in equimolar concentrations, the osmolarity of the ultraÞltrate is not altered in the proximal tubule but remains the same as that of blood.

¥The endocytosed proteins are degraded into amino acids that are also released into the renal interstitium for distribution by the vascular system.

¥The proximal tubule also secretes organic acids, bases, and other substances into the ultraÞltrate.

Functions of the Thin Limbs of Henle’s Loop

¥The descending thin limb of Henle’s loop is completely permeable to water and only somewhat permeable to salts; hence, the ultraÞltrate in the lumen will attempt to equilibrate its osmolarity with the renal interstitium in its vicinity.

¥The ascending thin limb is mostly impermeable to water but is relatively permeable to salts; thus, the movement of water is impeded but that of sodium and chloride is not.

¥The ultraÞltrate will maintain the same osmolarity as the renal interstitium in its immediate surroundings as the concentration gradient decreases, approaching the cortex.

Functions of the Distal Tubule

¥The pars recta of the distal tubule (ascending thick limb of HenleÕs loop) is impermeable to water but

possesses a Na+/K+/2Cl− cotransporter on the luminal surface of the its cells that actively pumps sodium and chloride from the lumen into the cell.

¥The basally located Na+/K+ ATPase pump transfers sodium and chloride out of the cell into the renal interstitium.

¥Since water cannot enter or leave the lumen, the ultraÞltrate is hypoosmotic by the time it reaches the macula densa region.

Cells of the distal convoluted tubule possess aldosterone receptors. In the presence of aldosterone, the distal convoluted tubule resorbs sodium ions from and secretes hydrogen, potassium, and ammonium ions into the ultraÞltrate in its lumen, which it then delivers to the collecting duct.

Functions of the Juxtaglomerular Apparatus

(see Table 16-4)

It is believed that the macula densa cells monitor the osmolarity and volume of the ultraÞltrate.

¥If osmolarity and/or volume of the ultraÞltrate is decreased, the macula densa cells, via gap junctions,

instruct juxtaglomerular cells to release their stored proteolytic enzyme, renin, into the bloodstream and

instruct the smooth muscle cells of the afferent glomerular arterioles to relax thereby increasing blood ßow into the glomerular capillary network

•Renin cleaves two amino acids from the circulating decapeptide angiotensinogen, changing it to angiotensin I, which, in turn, is cleaved by converting enzyme located on the luminal surfaces of capillaries (especially in the lungs), forming angiotensin II.

Angiotensin II is a powerful vasoconstrictor that

increases systemic vascular resistance, including that of the efferent glomerular arteriole, which

U R I N A R Y S Y S T E M 385

increases glomerular hydrostatic pressure thus increasing glomerular Þltration rate,

prompts the release of the mineralocorticoid aldosterone from the suprarenal cortex.

•Aldosterone binds to receptors on cells of the distal convoluted tubules, prompting them to resorb sodium (and chloride) from the ultraÞltrate. The addition of sodium to the extracellular compartment causes the retention of ßuid with the subsequent elevation in blood pressure.

Concentration of Urine in the Nephron (Countercurrent

Multiplier System)

The concentration of urine occurs only in juxtamedullary nephrons, whose long, thin limbs of HenleÕs loop function in the establishment of an osmotic concentration gradient. This gradient gradually increases from about 300 mOsm/L in the interstitium of the outer medulla to as much as 1,200 mOsm/L at the renal papilla.

Ascending Thick Limb

¥The Na+/K+/2Cl− cotransporter of the ascending thick limb of Henle’s loop transfers chloride and sodium ions from the lumen into the renal interstitium.

¥Water is not permitted to leave; hence, the salt concentration of the interstitium increases.

¥Since the supply of sodium and chloride inside the ascending thick limb decreases as the ultraÞltrate proceeds toward the cortex (because it is constantly being removed from the lumen), less and less sodium and chloride is available for transport;consequently,the interstitial salt concentration decreases closer to the cortex.

¥The osmotic concentration gradient of the inner medulla, deep to the junction of the thin and thick ascending limbs of HenleÕs loop, is controlled by urea rather than sodium and chloride.