Why do we need to maintain a constant internal environment?The role of ADH in the body - Higher

Water content in the blood is controlled by hormones and a negative feedback mechanism.

ADH

The hypothalamus in the brain has osmoreceptors which detect the level of water in the blood plasma as a stimulus.

If the level of water is too low, this causes the hypothalamus to send a signal to the pituitary gland, to release the anti-diuretic hormone or ADH.

ADH travels in the bloodstream to its target organ, the kidneys.

It causes the kidney tubules to become more permeable. This means more water can leave the kidney tubule to be reabsorbed back into the blood during selective reabsorption.

Different amounts of ADH are released into the bloodstream according to the concentration of water in the blood plasma. The diagrams show what happens when there is either too little or too much water in the blood.

If a person becomes too hot and sweats a lot, but doesn't drink enough water to replace what was lost, too little water might be detected in the blood plasma. More ADH will be released, which results in water being reabsorbed and small volume of concentrated urine will be produced.

If a person has consumed a large volume of water and has not lost much water by sweating, then too much water might be detected in the blood plasma by the hypothalamus. If this occurs, less ADH will be released from the pituitary gland. The kidney tubules become less permeable to water. This means less water is reabsorbed into the bloodstream and a larger volume of dilute urine will be produced.

This type of control is an example of the negative feedback mechanism. It aims to keep the concentration of the blood plasma constant.