All Na+ lons thus reabsorbed are ultmately reabsorbed into the blood of the peritubuiar. capillary.

cotransport All Na+ lons thus reabsorbed are ultmately reabsorbed into the blood of the peritubuiar. capillary. The rest of the sodium are absorbed lower down in the nephron. Thus, some 25% of the sodium are reabsorbed in the ascending limb of the loop of Henle and early part of distal tubule. Collecting tubule absorbs about 5%. that is, the remaning part of the sodium Loop diuretics, like furosemide, prevent reatsorption of sodium in the ascending Iimb of the loop of Henle. it Is believed, that the ascending limb of the loop of Henle has enough reserve, ie. if required, it can reabsorb even greater bad of sodium] II. Water reabsorplion Proximal tubules are highly permeable to water. As sodium or other sofutes leave the proximal tubute (due to reabsorption) and appear in the pentubulai space, trie local (peritubular space) osmotic tension rses. This causes flow of the water from inside of the tubules to the pentubular spaces. The m igrating water can follow any one of the two routes, viz.: (i) trans cellular and (ii) para cellular (fig. 8.2.2 ) (trans = through the cells, para = by the side of the cells] Fig. 8.2.2 Water reabsorption ill. Glucose reabsorplion In a healthy man, about 0. 9 mole (= 160 cms) of glucose appears as the filtered load (Table 8.2.1) and 100% of It is reabsorbed. The entire reabsorplion occurs in the proximal tubule The reabsoiption is Tm irnrted and the Tmg is about 2 m rnolemin (360 rngmsmin) (See introductory note of this chapter] When tine renal threshold value is exceeded, alycosuria appears, although glucose continues to be reabsorbed till the Tmg value is reached (see spay phenomenon, in Introductory note of this chapter) The absorption of glucose is active and carrier medi­ated. Along with reabsorption of each molecule of glucose one Ion of Na+ also is reabsorbed, this phenomenon is technically spoken as 'reabsorption of glucose is coupled with that of Na*' (The phenomenon is also called cotransport, or symport) The carrier mediated transport of glucose can be inhibited by the glycoside phloridzin (also spelt, 'phloriziin. resulting n 'phlondzin glycosuria' (Recall, In the small intestine also Just as n proximal convoluted tubule. absorption of glucose caused a concomitant absorption of Na*, rig 3.6.3] IV Ammo add reabsoiption Ammo acids are present n the glomerularfiltrate but the entire filtered load is reabsorbed in the proximal tubule The leabsorption is active and, like glucose, linked up with Na* reabsoiption V. Bicarbonate reabsorption IT has been pointed out above That a pal of Na* is reabsorbed by the mechanism called "antport. where Na+ reabsorption is coupled with concomitant reabsoiption of HCO3 and excretion of H+. The whole process will now be discussed (fig. 3 2 3) in delate Fig 8 2.3. Bicarbonate reabsorption CO2 is produced metabolically by the brush border epahelial cells, or may be obtained from the lumen [fig. 8.2 3). The C02 reacts with H20 in presence of xarbonic anhydrase'to farm H2C03 Details of the enzyme "carbonic anhytfrase' has been given elsewhere in this book (see chloride shift, chap 5, sec IV) In short carbonic anhydrase (CA) is an enzyme That catalyzes the reaction, H20+ CO2 H2C03 The H2C03 then spits into H+ and HC03 ions The H+ is actively eiitruded Into- the lumen In the lunen, there is abundart NaHCO3. (since NaHC03 is freely filtered into the glomerular filtrate). Huge quantities of HC03 ions are thus filtered per day (Table 8.21). The H+ eluded into the lumen produces the blowing reaction. H++NaHC03 Na++ 1-12C03 The Na* enteis the tubular cell by passive diffusion and then actively pumped out Into the pentubular flukd (fig. 8 2.3} The H2C03 within the lumen, sptts into H20 and CO2