The Urinary SystemTubular Reabsorption & Secretion

ObjectivesTo describe routes & mechanism of tubular reabsorption & secretionTo describe reabsorption and secretion processes at specific segments of the renal tubule & collecting duct

proximal tubule- largest contribution in reabsorptiontowards distal tubule- fine tuning of reabsorption process to maintain homeostatic balance of H2O & selected ionssecretion of materials from blood & tubular cells into tubular fluid

Principles of tubule reabsorption & secretion

Reabsorption, Secretion & Excretion in Urine

Routes of reabsorptionTubule Peritubular lumencapillaries (movement of materials b/w or through tubule cells)

Tight junctionsApical membrane of tubular cells in contact with tubular fluidBasolateral membrane of tubular cells in contact with interstitial fluid at base & sides of cells

Routes of reabsorption

Paracellular reabsorptionpassive leakage of fluid b/w tubule cellsaccounts for up to 50% of reabsorption

Transcellular reabsorptionsubstance pass through apical membrane, across cytosol & out into interstitial fluid through basolateral membrane

Routes of reabsorption

Transport mechanism10 active transporte.g. Na+ transport out of tubular fluid via Na+/K+ pumpenergy from hydrolysis of ATP20 active transportuse energy stored in an ions electrochemical gradient to drive another substance across a membraneperformed by membrane proteins known as symporters & antiporters

Water reabsorptionObligatory H2O reabsorption (approx.90%)via osmosis H2O following solutes that are reabsorbedin PCT & descending LOH (presence of aquaporin 1 molecule)

Facultative H2O reabsorption (approx. 10%)adapting to needsregulated by ADH mainly in DCT

Reabsorption & secretion at PCTfluid enters at a rate of 80 ml/min (osmolarity in filtrate & plasma still the same)largest amount of solute & H2O reabsorptionmost involve Na+ reabsorption but also other ions (via Na+ symporter & Na+/H+ antiporter)Na+ symporters also facilitate complete reabsorption of glucose, amino acids, vitamins, nutrients etc. in 1st half of PCT

Reabsorption & secretion at PCTpassive reabsorption of H2O causes passive diffusion of solutes down electrochemical gradient [Cl- (highest) , K+, Ca2+, Mg2+ & urea] into peritubular capillaries via para/transcellular routes in 2nd half of PCT reabsorption of solutes then promotes reabsorption of H2O via osmosis Urea and NH3 are also secreted by PCT tubule cells into tubular fluid

Reabsorption at PCT

Reabsorption & secretion at PCT

Reabsorption & secretion at PCT

Reabsorption in the LOHfluid enters at a rate of 40-45 ml/minno filtered glucose, amino acids & nutrients 15% of H2O reabsorbed at descending LOHlittle or no H2O reabsorption in thick ascending limb hence osmolarity of tubular fluid towards end of LOHreabsorption of H2O via osmosis no longer automatically coupled with reabsorption of filtered solutes due to impermeabilitysolutes reabsorbed include Na+, K+, Ca2+, HCO3-, Cl-

Reabsorption in the LOH

Countercurrent Multiplier System

Reabsorption in the DCTfluid enters at a rate of 25 ml/min80% of H2O has been reabsorbed but 10-15% reabsorption of H2O still occursreabsorption of Na+ & Cl- via Na+/Cl- symportersparathyroid hormone also stimulates reabsorption of Ca2+ at DCT

Reabsorption in the CD90-95% of H2O & solutes have been reabsorbedprincipal cells reabsorbed Na+ & secrete K+intercalated cells reabsorbed K+ /HCO3- & secrete H+presence of Na+ leakage channels allowing passive diffusion of Na+ into peritubular fluid

Reabsorption in the DCT & CD

Hormonal regulationRenin-angiotensin-aldosterone systemrenin secreted by juxtaglomerular cells when blood volume and BP & during sympathetic stimulationresults in production of angiotensin II (active hormone) which will GFR by vasoconstriction of afferent arteriole reabsorption of Na+, CL- & H2O in PCT stimulates release of aldosterone (more Na+ & Cl- being reabsorbed hence also H2O)

Hormonal regulationADH/ Vasopressin H2O permeability of principal cells of last part of DCT and CD (which contains aquaporin 2 molecules)ADH stimulates insertion of aquaporin 2 molecules into apical membrane via exocytosis to facilitate H2O movement from tubular fluid into tubule cells then rapidly into blood

Hormonal regulationAtrial natiuresis peptide (ANP)inhibit reabsorption of Na+ & H2O in PCT & CDsuppress secretion of ADH & aldosterone hence excretion of Na+ in urine causing urine output and blood volume and BP