Obviously the amount of water reabsorbed is controlled by the quantity of water in the blood. The less water in the blood, the more that it must be reabsorbed. The hormone ADH (anti-diuretic hormone) controls the extent to which water is reabsorbed. If the blood is concentrated, more ADH is released; making the walls of the collecting duct more permeable to water so more is reabsorbed back into the blood. If the blood is less concentrated, less ADH is released so less water is reabsorbed.
Control of ADH
The concentration of the blood (water potential) is monitored by osmoreceptors in the hypothalamus. The higher the concentration of the blood the less water there is in the blood.
If the concentration is too high impulses are sent to the pituitary gland which then releases more ADH. The water levels will be brought back to normal and the impulses stop.
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