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Lecture 35

BIOL 1119 Lecture 35: Class 35

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BIOL 1119
Christopher Richardson

Class 35 Urine Formation Nephron Loop - Primary function of loop (in juxtamedullary nephrons, vasa recta) o Generates salinity gradient: allows collecting duct to concentrate urine for water conservation as necessary o Electrolyte reabsorption: Na, K, Cl o Tubule epithelial cells in the thick segment of ascending limb of loop have symports on luminal side that simultaneously bind 1 Na, 1 K, and 2 Cl from the tubular fluid and co-transport them into the epithelial cytoplasm ▪ This co-transport occurs by active transport as all 3 electrolytes are pumped into epithelial cell o On the basolateral surfaces, K and Cl leave tubule epithelial cells by diffusion into tissue fluid o Na leaves by active transport which pumps sodium into tissue fluid o In the loop, potassium reenters the tubule epithelial cells from tissue fluid via the Na-K pump ▪ Later K diffuses out of tubule epithelial cell back into tissue fluid o NaCl remains in the tissue fluid of the renal medulla o The thick segment of the loop is impermeably to water o Water cannot follow the reabsorbed electrolytes and stays in tubule ▪ Thick: PERMEABLE to salt, IMMPERMEABLE to water ▪ Thin: PERMEABLE to water, IMMPERMEABLE to salt ▪ Tubule fluid very dilute by time it passes into DCT o At the bottom of loop: tubular fluid is very concentrated DCT and Collecting Duct - Fluid reabsorption in DCT and CD is variable: regulated by hormones - K+ and H+ are the two most important substances secreted by the PCT and DCT as part of regulating their levels in blood or tissue - Only H+, K+, and ammonia are secreted in DCT and cortical collecting duct o In DCT and cortical CD K+ is not reabsorbed – only reabsorbed and not secreted in PCT - Amount of salts and water reabsorbed or secreted are regulated by several hormones: aldosterone, atrial natriuretic peptide, antidiuretic hormone, and parathyroid hormone o Aldosterone: salt retaining hormone ▪ total body Na+ level decrease of blood K+ concentration increase causes adrenal cortex to secrete aldosterone ▪ increased blood K+ (hyperkalemia) directly stimulates adrenal cortex to release aldosterone ▪ if BP decreases or sodium intake decreases: Renin released by JG cells: increase in production of AGII in bloodstream: stimulates adrenal cortex to release aldosterone Effects: ▪ acts on thick segment of the ascending limb of the nephron loop, on the DCT; and on the cortical portion of the collecting duct ▪ stimulates reabsorption of Na and secretion of K ▪ in DCT and cortical CD, increases number of Na-K pumps in basolateral side of tubule epithelial cells ▪ turns on ge
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