regulation of plasma osmolality

how is water input and output regulated

thirst and urine

normal plasma osmolality

\~290 mosmol/kgH2O

variation sufficient to activate compensatory mechanisms

\+/- 3 mosmol/kgH2O

concentrating mechanism

ion transport in the ascending loop of henle - impermeable to water

transport proteins in ascending loop of henle

apical Na+/K+/2Cl- cotransporter

basolateral Na pump

how much of filtered Na, K and Cl is reabsorbed

20%

cotransporter blocked

by loop diuretics - frusemide, bumetamide

producing urine of varying osmolality

isotonic fluid flows from proximal tubule through loop of henle

interstitial fluid also isosmotic

in ascending loop of henle, solutes but not water flows into interstitium

descending loop permeable to water, tubular fluid is concentrated

longer loop of henle

greater differential so more absorption

generating concentrated urine

diluted fluid enters collecting duct - passes through medullary interstitium

water passes from tubule to interstitium - concentrates the tubular fluid

permeability of collecting ducts

relatively permeable to Na but impermeable to water except in presence of ADH

ADH

(arginine vasopressin - distinguish from ADH produced by other species)

produced by posterior pituitary gland

actions of ADH on the kidney

increase water permeability of collecting duct

increase NaCl reabsorption in the thick ascending limb of the loop of Henle

control of ADH secretion

produced in cells in supraoptic and paraventricular nuclei

transported to and released from posterior pituitary

released by rise in plasma osmolality as little as 1%

mode of action of ADH

binds to receptor on basolateral surface

stimulation of adenylyl cyclase,, generate cAMP, activate protein kinase A

increase insertion of aquaporins 2 into apical surface of cell

increase water permeability

obligatory water reabsorption

must occur

\~85% - 65% proximal, 20% distal

facultative water reabsorption

optional, remaining 15% may or may not be reabsorbed in collecting ducts

ADH controls permeability

pneumonic for overall functions of kidney

A WET BED

overall functions of kidney

Acid base balance

water removal

Erythropoiesis

Toxin removal

Blood pressure control

Electrolyte balance

vitamin D activation

erythropoeisis

kidneys produce erythropoietin - carried in blood to bone marrow - stimulates production of red blood cells