Lecture 36: Collecting duct, water balance, micturition

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Collecting duct

Reabsorption and Secretion in the --- ---:
-By end of distal convoluted tubule 90-95% of solutes & water have been reabsorbed and returned to the bloodstream
-Cells in the collecting duct make the final adjustments

principal cells

reabsorp Na+, secrete K+ (aldosterone effect) and reabsorp water (ADH effect)

intercalated cells

reabsorb K+ & bicarbonate ions and secrete H+

Principal cells

Actions of the --- ---:
-Na+ enters principal cells through leakage channels
-Na+ pumps keep the concentration of Na+ in the cytosol low
-Cells secrete variable amounts of K+, to adjust for 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

Antidiuretic Hormone

Actions of the Principal Cells:
-Increases water permeability of principal cells in collecting duct
-Stimulates the insertion of aquaporin-2 channels into the membrane
-Water molecules move more rapidly

Intercalated Cells

Secretion of H+ and Absorption of Bicarbonate by --- ---:
-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 irreversible with H+ and are excreted

Diuretics

Substances that slow renal reabsorption of water & cause diuresis (increased urine flow rate)
-Caffeine inhibits Na+ reabsorption (inhibits obligatory water reabsorption)
-Alcohol inhibits secretion of ADH (inhibits facultative water reabsorption)
-Some prescription medicines can act on the PCT, loop of Henle or DCT

Urinary Bladder

Anatomy of --- ---:
-Hollow, distensible muscular organ with capacity off 700-800 mL
-Trigone is smooth flat area bordered by 2 ureteral openings and one urethral opening

Micturition Reflex

--- ---:
-Micturition or urination
-Stretch receptors signal spinal cord and brain when volume exceeds 200-400 mL
-Impulses sent to micturition center in sacral spinal cord (S2 and S3)
-Filling causes a sensation of fullness that initiates a desire to urinate before the reflex actually occurs
-conscious control of external sphincter
-cerebral cortex can initiate micturition or delay its occurrence for a limited period of time

detrusor muscle

parasympathetic fibers cause --- --- to contract, external & internal sphincter muscles to relax

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