Tubular Reabsorption (Selective Reaborptlon)

• The glomerular (tubular) filtrate contains some very useful substances like glucose, water and various ions.
• These substances are usetul for the body, hence must not pass out with urine.
• Therefore these are to be reabsorbed from the tubular lumen. This process of reabsorption of selected substances from tubuiar
• lumen to surrounding blood capillaries (of PTCN) is called selective reabsorption.
• pCT, Henle's loop, DCt and even collecting duct, all contribute to reabsorption. But most of the reabsorption takes place in the PCT region.
• Water and some ions are reabsorbed from all parts of the nephron (except ascending limb of Henle, which is impermeable to water).
• Almost all sugars (Glucose etc.), vitamins, amino acids and other organic nutrients are reabsorbed in PCT.
• About 99 % of the filtered water is reabsorbed. Urine thus contains only 1% of the filtered water. Major amount of water is absorbed in PCT. Nephrons (Henle's loop) carries out a`counter current system' to absorb more and more water from tubular filtrate.
• Na+ ions, Cl– ions and bicarbonate ions are absorbed from PCT, Henle's loop and DCT. About 2/3rd of Na+ reabsorption occurs in PCT alone.
• K+ is also reabsorbed from PCT.
  About half of the urea (53%) is also reabsorbed. (thus, only about 47% of the fiitered urea is passed out with urine).Some of this is absorbed in PCT, while rest is absorbed from collecting tubule (when it enters the deep inner medulla region). Other portions of nephron are not permeable to urea. (Students must keep this fact in mind).
• Almost all the uric acid (98%) is also reabsorbed.
     

The reabsorption value of Important subitances (in percentage) from tubuiar Altrate