Sodium and water absorption in the collecting duct has the largest effect on total body water balance because:O A. The largest change in the transport of these molecules can occur in the CDOB. The largest absorption of water amounts can occur in the CDOC. The largest absorption of Na amount can occur in the CDD. The largest change of osmolarity can occur in the CD

The urinary system (previously known as the excretory system) is one of the 11 principal body organ systems consisting of organs like the kidney, ureters, urinary bladder, and the urethra that lies in the region of the lower abdomen to the pelvis. The kidney, the epicenter of this system facilitates the process of blood filtration, blood pressure and blood volume regulation, excretion of excess salts and water, removal of toxins from circulation, and absorption of essential ions and water in specific situations with the help of its structural and functional units, the nephrons.The process of urine formation and excretion includes stages like glomerular filtration, selective reabsorption (tubular reabsorption), and tubular secretion. The tubular reabsorption phase involves the reabsorption of the content of the filtrate (like ions and water) before it is passed out of the collecting duct in the form of urine. The tubular reabsorption takes place in the tubular regions of the nephron like the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The collecting duct marks the terminal phase of the tubular reabsorption where the reabsorption of the content takes place before it enters the papillary duct which opens into the renal medulla. The absorption is taken place with the help of the hormonal influence of aquaporin channels. By the time the tubular fluid reaches the collecting duct segment (under standard physiological conditions), all amino acids, bicarbonates, glucose, and organic solutes are already removed by the previous structure of the renal tubule. Even the water volume is reduced to around 10% of that of glomerular filtrate when it reaches the collecting duct. Hence, the absorption of water and electrolytes is comparatively lesser in this region when compared with the absorption occurring in the upstream tubular regions. But the variability of the transport rate is higher in the region of the collecting duct which is accompanied by the variability in the ion and osmotic gradient between the tubular lumen and the interstitium. The transportation is majorly influenced by the action of ADH which is released by the posterior pituitary gland under standard physiological conditions. ADH bind with the V2 receptors present on the tubule cells of the CD that activate adenylyl cyclase and increases the production of cyclic AMP. The action of ADH influences the insertion of the increased number of aquaporin channels on the apical membrane of the principal cells of the collecting duct. An increased number of channels increases the permeability of principal cells that facilitate the osmotic pulling of water into the interstitial space and into the peritubular capillaries. As a result, urine is concentrated. If ADH secretion is altered, then fewer aquaporin channels are inserted leading to lesser water recovery. Hence, the collecting duct plays a major role in the maintenance of blood osmolarity, blood volume, and urine osmolarity. The transport of sodium is facilitated under the influence of aldosterone (secreted by the zona glomerulosa of the adrenal cortex) that acts on the principal cells of the collecting duct and this active process is driven by the basolateral Na+, K+ ATPase pump. Increased aldosterone output increased the Na+ reabsorption from the tubular fluid and the water follows the Na+ passively. With the recovery of Na+ from fluid, the pump is balanced by pumping K+ in the opposite direction (in the urine). Thus, the actions of the collecting duct by absorbing the water and sodium content can majorly affect the total body water balance. Hence, option ‘D. The largest change in osmolarity can occur in CD’ is the correct option.Option ‘B. The largest absorption of water amounts can occur in the CD’ is an incorrect option because the largest absorption of water amounts occurs in PCT, even when the ADH is influencing the collecting ducts. Option ‘C. The largest absorption of Na contents can occur in the CD’ is an incorrect option because the largest absorption of sodium content occurs in PCT. Option ‘A. The largest change in the transport of these molecules can occur in CD’ is an incorrect option because the large changes associated with the transport of water and sodium are usually conducted in the upstream segments of the nephron.Hence, it is to be concluded that the collecting duct is crucial in maintaining the total body water balance by influencing the change in osmolarity.