Question

how can we say that human kidney concentrate urine 4 times than of normal osmolality of blood..??

Answer 1

actually when blood comes in glomerular apparatus of kiney it has osmolality of 300...the blood us ultrafiltrated by glomerular apparatus through which all the protein remain outside the apparatus and only protein less plasma entr in further part of nephron for furher process of reabrorbsion and secresn....when this filtarate reaches to PCT about 70-80% of materials lije glucose, H+, Na+, etc are reabsorbed actively or pasdively depending upon nature of reabsorbate. here this filtrate remain in 0smolality of blood thai is isotonic to blood...
Now filtrate reaches to descending limb of loop of henle which is permeable only and only for H2O and no any subtrate can cross from here...water starts duffusong out from descending limb through osmosis and osmolality of filtrate increses as it passes by..bcz only H2O is reabsorbed due to which conc of Na+ has increased now the filrate has reached to lower most part of henles loop...here BLOOD HAVE OSMOLALITY OF 1200 WHICH IS MUCH HYPERTONIC THAN BLOOD...niw the filterate starts rusing up through ascending loop of henle which is permeable for Na+ only nd no water diffuses out...as only Na+ diffuses out fultrate bcm dilute and it bcm HYPOTINIC TO BLOOD...now the fultrate reaches to DCT where there is selective reabsorbtion of Na+, H+ etc tajes place the Na + is reabsorbed in the influence of aldosteron and water is reabsorbed in the influence of vassopressin(ADH)...now filterate moves in collecting duct where H+ and water diffuse out and osmolality of blood reaches to1200 which is HYPERTONIC AND 4 TIMES CINC. THAN BLOOD....now the urine is passed out in renal papillae nd for further removal...hope it helped u....