The amount of water in the urine is regulated by what Kidneys Bladder Heart or Stomach?

Answer 1

Kidneys Function One of the most important roles of AVP is to regulate the body's retention of water; it is released when the body is dehydrated and causes the kidneys to conserve water, thus concentrating the urine, and reducing urine volume. In high concentrations, it also raises blood pressure by inducing moderate vasoconstriction. In addition, it has a variety of neurological effects on the brain, having been found, for example, to influence pair-bonding in voles. A very similar substance, lysine vasopressin (LVP) or lypressin, has the same function in pigs and is often used in human therapy. == AVP has three effects by which it contributes to increased urine osmolarity (increased concentration), and decreased urine excretion. These are: 1) It increases the permeability to water of the distal convoluted tubules and collecting tubules in the nephrons of kidneys and thus allows water reabsorption and excretion of a smaller volume of concentrated urine - antidiuresis. This occurs through insertion of additional water channels (Aquaporin-2s) into the apical membrane of the tubules/collecting duct epithelial cells. The aquaporins allow water to pass out of the nephron (at the distal convoluted tubules and the conducting tubules) and into the cells, increasing the amount of water re-absorbed from the filtrate. V2 receptors, G protein-coupled receptors coupled to Gs, on the basolateral membrane of the cells lining the distal convoluted tubules and conducting tubules (in the nephron) have an active site for AVP. The G protein, which is in contact with the V2 receptor inside the cell, move to adenylyl cyclase, triggering adenylyl cyclase to convert ATP into cAMP, plus 2 inorganic phosphates. The cAMP cascade then triggers the insertion of Aquaporin-2 water pores by exocytosis of storage vesicles. The repressor protein that regulates the gene for protein kinase A (PKA) has a binding site for cAMP, causing the repressor protein to change its shape and leave the operator region of the gene. This allows for transcription of the gene for PKA. PKA then signals ATP to dephosphorylate, providing energy for vesicles (which contain aquaporin channel proteins in their membranes) to fuse with the apical membrane of the cell. Calcium ions may also be required in this process, therefore it may be possible that, PLC (phospholipase C- beta) has an associated role. It should be noted that PLC can be activated by a G-protein coupled receptor. 2) AVP's second effect on the kidney is to increase the permeability of the papillary portion of the collecting duct to urea, allowing increased reabsorption of urea into the medullary interstitium, down the concentration gradient created from the removal of water in the cortical collecting duct. 3) The third effect that AVP has on the kidney is that it stimulates sodium reabsorption in the thick-ascending loop of Henle by increasing the activity of the Na+-K+-2Cl--cotransporter.