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26-39Reabsorption & Secretion in the Collecting Duct•By end of DCT, 95% of solutes & water have been reabsorbed and returned to the bloodstream•Cells in the collecting duct make the final adjustments–principal cells reabsorb Na+ and secrete K+–intercalated cells reabsorb K+ & bicarbonate ions and secrete H+
26-40Actions of the Principal Cells•Na+ enters principal cellsthrough leakage channels•Na+ pumps keep theconcentration of Na+ inthe cytosol low•Cells secrete variableamounts of K+, to adjustfor dietary changes in K+intake–down concentration gradient due to Na+/K+ pump •Aldosterone increases Na+ and water reabsorption & K+ secretion by principal cells by stimulating the synthesis of new pumps and channels.
26-41Secretion of H+ and Absorption of Bicarbonate by Intercalated Cells•Proton pumps (H+ATPases) secrete H+ into tubular fluid–can secrete against a concentration gradient so urine can be 1000 times more acidic than blood•Cl-/HCO3- antiporters move bicarbonate ions into the blood–intercalated cells help regulate pH of body fluids •Urine is buffered by HPO4 2- and ammonia, both of which combine irreversibly with H+ and are excreted
26-42Hormonal Regulation•Hormones that affect Na+, Cl- & water reabsorption and K+ secretion in the tubules–angiotensin II and aldosterone•decreases GFR by vasoconstricting afferent arteriole•enhances absorption of Na+•promotes aldosterone production which causes principal cells to reabsorb more Na+ and Cl- and less water•increases blood volume by increasing water reabsorption–atrial natriuretic peptide•inhibits reabsorption of Na+ and water in PCT & suppresses secretion of aldosterone & ADH•increase excretion of Na+ which increases urine output and decreases blood volume
Antidiuretic Hormone•Increases water permeability of principal cells so regulates facultative water reabsorption•Stimulates the insertion of aquaporin-2 channels into the membrane–water molecules move more rapidly•When osmolarity of plasma & interstitial fluid decreases, more ADH is secreted and facultative water reabsorption increases.
26-44Production of Dilute or Concentrated Urine•Homeostasis of body fluids despite variable fluid intake•Kidneys regulate water loss in urine•ADH controls whether dilute or concentrated urine is formed–if lacking, urine contains high ratio of water to solutes
Formation of Dilute Urine•Dilute = having fewer solutes than plasma (300 mOsm/liter).–diabetes insipidus•Filtrate and blood have equal osmolarity in PCT•Water reabsorbed in thin limb, but ions reabsorbed in thick limb of loop of Henle create a filtrate more dilute than plasma–can be 4x as dilute as plasma–as low as 65 mOsm/liter•Principal cells do not reabsorb water if ADH is low