Mammalian Kidney

1. Mammalian Kidney

2. Filtration • Blood pressure forces fluid from blood in glomerulus into lumen of Bowmen’s capsule • Non-selective process • Filtrate contains salts, urea, glucose, and vitamins in similar concentration as blood plasma

3. Pathway of Filtrate • Proximal tubule • Descending limb loop of Henle • Ascending limb loop of Henle • Distal tubule • Collecting duct • Renal pelvis • Ureter • Bladder • Urethra

4. Nephron – functional unit of Kidney • Two types: • Cortical – in cortex only • Juxtamedullary – loop of Henle descends into medulla • Only mammals and birds have juxtamedullary nephrons, allowing production of hyperosmotic urine • Major function H2O reabsorption • 1100-2000 L of blood/day through kidneys • 180 L filtrate • 99% H2O reabsorbed and most sugars and other organic nutrients

5. Blood Supply • Renal artery • Afferent arteriole • Efferent arteriole • Pertiubular capillaries • Vasa recta • Renal vein

6. Proximal Tubule • Secretion and reabsorption change filtrate volume and composition • Secretion: • H+ and NH3 maintain pH in body fluids • Drugs/poisons secreted from peritubular capillaries into interstitial fluid, then across nephron’s transport epithelium into lumen. • Absorption: • Buffer bicarbonate (HCO3-) • Valuable nutrients: glucose, amino acids, and K+ are actively or passively absorbed from filtrate.

7. Proximal Tubule • Reabsorption of most of the NaCl and water from initial filtrate • Epithelial cells actively transport Na+ into the interstitial fluid. • Positive charge balanced by the passive transport of Cl- out of the tubule. • Water follows salt by osmosis.

8. Descending limb of the loop of Henle • Water reabsorbed from filtrate as it descends loop of Henle. • Transport epithelium is permeable to water but not salt • Water leaves by osmosis because interstitial fluid is hyperosmotic to the filtrate • Osmolarity of the interstitial fluid becomes progressively greater from outer cortex to inner medulla so filtrate continues to loose water.

9. Ascending limb of the loop of Henle • Transport epithelium of the ascending limb is permeable to salt, not water. • Filtrate ascends the thin segment of the and NaCl diffuses out of tubule into interstitial fluid • This increases osmolarity of the medulla. • Salt actively transported from the filtrate in the thick segment of the ascending limb. • By losing salt without giving up water, filtrate becomes progressively more dilute as it moves up to cortex

10. Distal tubule • The distal tubule regulates K+ and NaCl concentrations in body fluids • Varies amount of K+ secreted into the filtrate and amount of NaCl reabsorbed from the filtrate. • Also contributes to pH regulation by controlled secretion of H+ and the reabsorption of bicarbonate (HCO3-).

11. Collecting duct • Epithelium of the collecting helps determine amount salt excreted in the urine • Epithelium permeable to water but not to salt or (in the renal cortex) to urea. • As the collecting duct traverses the gradient of osmolarity in the kidney, filtrate becomes more concentrated as it loses water by osmosis to the hyperosmotic interstitial fluid. • In inner medulla, duct becomes permeable to urea, contributing to hyperosmotic interstitial fluid and • Enables kidney to conserve water by excreting a hyperosmotic urine.