Changes in the volume and osmolality of tubular fluid along the nephron in diuresis and antidiuresis. The osmolality of the tubular fluid undergoes several changes as it passes through different segments of the tubules. Tubular fluid undergoes marked reduction in its volume in the proximal tubule; however, this occurs iso-osmotically with the glomeru-lar filtrate. In the loop of Henle, because of the aforementioned countercurrent mechanism, the osmolality of the tubular fluid rises sharply but falls again to as low as 100 mOsm/kg as it reaches the thick ascending limb and the distal convoluted tubule. Thereafter, in the late distal tubule and the collecting duct, the osmolality depends on the presence or absence of antidiuretic hormone (ADH). In the absence of ADH, very little water is reabsorbed and dilute urine results. On the other hand, in the presence of ADH, the collecting duct, and in some species, the distal convoluted tubule, become highly permeable to water, causing reabsorption of water into the interstitium, resulting in concentrated urine [5].

Paraventricular neurons


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