How does osmoregulation occur in the kidney?


The tubule fluid enters the collecting duct (CD) where the final water reabsorption occurs before the fluid is excreted as urine. Osmoreceptors in the hypothalamus detect the concentration of the blood (water potential) as blood flows through the hypothalamus. If the water potential is low (like in dehydration) then it means there is not a lot of water in the blood, so the osmoreceptors will lose water via osmosis and shrink. This shrinkage stimulates neurosecretory cells to release anti-diuretic hormone (ADH) into the blood from the posterior pituitary. The ADH will bind to collecting duct cells via the ADH-Receptor and causes an enzyme controlled reaction within the cell. This results in vesicles contains aquaporins (specialised protein channels) binding to the cell membranes of the collecting duct cells and inserting the aquaporins into the membrane. This will increase the membrane permeability to water, so more water is reabsorbed and the blood water potential increases back to normal. If water potential of the blood is high then the opposite occurs, where the osmoreceptors gain water and will cause less ADH release so less water is reabsorbed and blood potential decreases to normal.

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