The eel Although the HCO3 entering the peritubutar fluid is thus strictly sneaking,

the water goes out as unne and C02 diffuses back into The eel Although the HCO3 entering the peritubutar fluid is thus strictly sneaking, not the very same ion that was in the Tubule, but in effect, it means the same The drug acetazolamide (Diarnox) Inhibits the CA and Thus stops Na reabsorplion that occurs via The antiport system Diamox was once a popular diuretic Unfortunately, the stoppage of HC03 reabsorpTion (which occurs concomitantly) leads fc aeldoss which is a very serious sloe effect of the drug. VI. Phosphate reabsorpTion Phosphate can be reabsorted by the proximal tubute alone The reabsorplion is active. Parathyroid hormone (see chap Vl3) depresses the reateorption vll Potassium About 28 gms of potassium is filtered per day and practically the whole amount is reabsorbed by the proximal tu­bule The potassium which is present in the urine comes form secretion by the distal segment K+ therefore suffer bidirectional transport Vlll. Urea, Uric acid Only about 10% of The filTered load of uric acid is excreted Proximal Tubule is The site of leaoscrption, the reabsorplion being inhibited by The drug probenecid. For further details of uricosurtc drugs and mode of action of probenecid, see 'punne metabolism' (chap 13. sec Vll) [In some persons, unc acid crystallizes in the urinary tract and forms uric acid stones in the kidney Several drugs increase urinary excretion of urit acid and are useful for treatment in gout Such drugs are called uricosuric drugs (see, chap 13 sec Vll)] A, large fraction (60%) of the filtered load of urea s re-absorbed and the reabsorption -s dependent on the water reabsorption as explained below. in the proximal tubule, water reabsoiption occurs In huge amount, so that urea concentration of the tubular fluid becomes very high This high concentration of urea sets up a gradent between the tubular fluid ana the peritubular fluid and, therefore, the urea 6 reabsoibed This means.where reabsorption of water is low, the unnary excretion of urea is high Tubulo glcmerularfeedback (TG feed back) When the OFR increases, by means of the TG feed back, the OFR Is brought Back To normalcy. Conversely, when the GFR decreases the TO feed back works n a direction so that The OFR Increases Therefore, this Is another example of homeosbtic mechanism. The filtered load of Cl•. in the region of macula densa relates The feed nack mechanism The details (to some extent speculative) are as follows : rise of GFR more filtered load more C\- reach the zone of macula densa via the Tubular fluid macuta densa cels 'sense' The evcessiveness of Cl- ions nelicits a signal (unich Is perhaps a chemical agent) the signal reaches the afferent arteriole to the glomerulus constnction and hence fal of GFR and The filtered load Conversely, when there is reduction of the filtered load. The fall of Cl- ions of the region of macula densa produces a signal (chemical) which causes trie efferent artenoie from the giomerulus to constrict, causing rise of the OFR and the filtered