The Urinary System Urine formation Urine formation Tubular Reabsorption and Secretion continued Water Conservation System Countercurrent Exchange Mechanism 23 1 DCT and Collecting Duct Fluid reabsorption in DCT and CD is variable regulated by hormonal action K and H are the two most important substances secreted by the tubules as part of regulating their levels in blood or tissue Only H K and ammonia are secreted in DCT and cortical collecting duct Also K is not reabsorbed in DCT and cortical CD while K is only reabsorbed and not secreted in the PCT The amount of salts and water reabsorbed or secreted are regulated by several hormones aldosterone atrial natriuretic peptide antidiuretic hormone and parathyroid hormone ald ANF ADH PTH 23 2 DCT and Collecting Duct Aldosterone total body Na level decrease or blood K concentration increase causes adrenal cortex to secrete aldosterone increased blood K concentration directly stimulates adrenal cortex to release aldosterone If BP drops or if sodium intake decreases renin is released by JG cells which causes increase in angiotensin II levels Angiotensin II stimulates adrena cortex to release ald acts on the 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 it increases number of Na K pumps in basolateral side of tubule epithelial cells 23 3 More Na pumped out of tubule epithelial cells into tissue fluid and more K is pumped into tubule epithelial cells from tissue fluid Thus less Na and more K in cell than in tubule lumen 23 4 DCT and Collecting Duct Aldosterone In DCT and cortical CD aldosterone also increases the number of Na and K channels in luminal side Which allows more Na to diffuse from tubule lumen into tubule epithelial cells and allows more K to diffuse from tubule epithelial cells into tubule lumen K channels are normally not found in luminal membrane in other segments of the renal tubule potassium is mainly secreted into tubule lumen in cortical CD and DCT No Na H pumps in DCT or cortical CD H2O and Cl follows reabsorbed Na so the net effects is retain NaCl and water increase blood vol reduce urine vol which increases concentration of urine Because water is reabsorbed with sodium sodium concentration in tissues and blood in kidney is not increased 23 5 DCT and Collecting Duct Atrial natriuretic peptide ANP atria secrete ANP in response to increased BP has four actions to reduce blood volume and pressure dilates afferent arteriole constricts efferent arteriole which increases GFR Increase GFR which means more Na and water is fultered but reabsorption is the same in PCT thus increase in sodium and water excretion which lowers blood volume and BP inhibits renin angiotensin aldosterone pathway This means less Na reabsorbed which means more is excreted Na never secreted inhibits secretion and action of ADH inhibits NaCl reabsorption in the collecting duct By promoting water and salt excretion overall effect is increase urine vol reduce blood vol and reduce BP 23 6 DCT and Collecting Duct Effect of ADH dehydration and increasing blood osmolarity stimulates hypothalamus hypothalamus stimulates posterior pituitary posterior pituitary releases ADH ADH increases water reabsorption in collecting duct by making it more permeable to water Decrease urine volume 23 7 DCT and Collecting Duct Effect of PTH secreted by the parathyroid glands in response to calcium deficiency hypocalcemia Helps to restore calcium homeostasis Increase phosphate excretion in PCT Increases calcium reabsorption in DCT increases blood Ca2 levels Because phosphate is not retained calcium ions stay in circulation rather than precipitating into bone tissue as calcium phosphate Decrease new bone formation 23 8 Water Conservation The ability to concentrate wastes while controlling water loss was critical to evolution of terrestrial animals Understand how the nephron loop can create a more concentrated urine which allows for control of water loss while excreting out metabolic wastes Different species have evolved different length loops depending on how important water conservation is in their environment Longer loops allow for a greater osmolarity gradient and more concentrated urine to be produced Fish amphibians short to no loops mean urine is same conc as blood Mammals like humans have moderate length loops which can concentrate urine 4x more plasma Desert mammals like kangaroo rats have long loops which can concentrate urine 10 14x more than plasma 23 9 Collecting Duct Concentrates Urine Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Cortex Medulla Collecting duct CD begins in the cortex where it receives tubular fluid from several nephrons As fluid in tubule of CD passes through the medulla medulla can reabsorb water and concentrate urine Collecting duct Nephron loop Tubular fluid 300 mOsm L 300 600 H2O H2O H2O H2O H2O 900 1 200 L m s O m i d u l f e u s s i t f o y t i r a o m s O l Urine up to 1 200 mOsm L 23 10 Collecting Duct Concentrates Urine When urine enters the upper end fo the CD it is isotonic with blood plasma 300 mOsm L When it leaves the lower end of the CD in humans it can be 4x as concentrated or highly hypertonic compared to blood pasma Several factors enable the collecting duct to produce hypertonic urine countercurrent anatomy ascending limb reabsorbing NaCL but not permeable to water trapping urea in medula loops of vasa recta 23 11 Countercurrent Multiplier recaptures NaCl and returns it to renal medulla maintaining an osmolarity or salinity gradient Because the loop multiplies salinity in the deep medulla it is a multiplier fluid is flowing in opposite directions in two adjacent tubules in loop Which is downward in descending limb and upward in ascending limb it is countercurrent flow mechanism Ascending limb thick segment reabsorbs Na K and Cl out of lumen into tissue fluid impermeable to water interstitial fluid surrounding the ascending limb of medulla becomes more hypertonic compared to tubular fluid tubular fluid becomes more dilute No salt pumps in lower thin segment lf ascending limb 23 12 Countercurrent Multiplier Recycling of urea collecting duct medulla some urea in CD leaves the lower end of medullary collecting duct by facilitated diffusion into the tissues The movement of urea back into the deep medulla helps make its tissue more concentrated by adding