TUBULAR REABSORPTION Part II DR .IMRANA EHSAN

Solute and water reabsorption inLoop of Henley

Reabsorption in the Loop of Henle Tubular fluid PCT reabsorbed 65% of the filtered water so chemical composition of tubular fluid in the loop of Henle is quite different from plasma since many nutrients were reabsorbed as well, osmolarity of tubular fluid is close to that of blood Sets the stage for independent regulation of both volume & osmolarity of body fluids. THIN DESCENDING LIMB: Highly permeable to water and moderately permeable to solutes. 20% of fluid reabsorbed

Symporters in the Loop of Henle ASCENDING LIMB : THIN ASCENDING LIMB: impermeable to water. Thick ascending limb: has Na+ K- Cl- symporters on liminal side that reabsorb these ions K+ leaks through K+ channels back into the tubular fluid leaving the interstitial fluid and blood with a negative charge Na/Kpump on basolateral membrane Ca, Mg, Na ,K passively move to the vasa recta BY PARACELLULAR ROUTE. info@abl.com.pk

DESCENDING LOOP OF HENLE: No active transport of salt out of the descending loop of Henle. ASCENDING LOOP OF HENLE: Chlorine ions actively transported out of loop into the interstitial space. Oppositely charged sodium ions follow. However, water does not move out of the ascending loop. A concentration gradient IN THE INTERSTITIAL SPACE has been set up by the chlorine ( plus sodium) transport.

Early distal convulated tubule: Reabsorption on the early distal convoluted tubule Na+-Cl- symporters reabsorb Na+ and Cl-, Ca,Mg. Na/K Pump at basolateral membrane Na Cl cotransport on luminal surface Major site where parathyroid hormone stimulates reabsorption of Ca+ depending on body’s needs Reabsorption and secretion in the late distal convoluted tubule and collecting duct 90-95% of filtered solutes and fluid have been returned by now Principal cells reabsorb Na+ and secrete K+ Intercalated cells reabsorb K+ and HCO3- and secrete H+ Amount of water reabsorption and solute reabsorption and secretion depends on body’s needs

Early distal covulated tubule

DISTAL TUBULE AND CORTICAL COLLECTING DUCTS

Medullary collecting ducts

Hormonal regulation of tubular reabsorption and secretion Glomerulotubular balance Increased tubular load increases reabsorption rates Angiotensin II - when blood volume and blood pressure decrease Decreases GFR, enhances reabsorption of Na+, Cl- and water in PCT Aldosterone - when blood volume and blood pressure decrease Stimulates principal cells in collecting duct to reabsorb more Na+ and Cl- and secrete more K+ Parathyroid hormone Stimulates cells in DCT to reabsorb more Ca2+ 2nd line of defence Prevents overloading of distal tubular segments

Fluid flow in pertubular compartment