Human Nutrition 3(3+0)

Lesson 46 : Water, Oedema and Dehydration

Water Excretion Regulation

The brain and kidneys regulate the amount of water excreted by the body. When the blood volume is low, the concentration of solutes in the blood is high. The brain responds to this situation by stimulating the pituitary gland to release a hormone known as Anti diuretic hormone (ADH), which signals the kidneys to reabsorb and recirculate water. When the individual needs more water, the kidneys will excrete less and even reabsorb some.

When excessive fluid loss occurs, the blood volume, as well as blood pressure decreases. The kidneys respond by secreting an enzyme called rennin. Rennin activates the blood protein angiotensinogen to convert to angiotensin, which causes the blood vessels to constrict and blood pressure to rise. Angiotensin also activates the adrenal glands to release a hormone called aldosterone. Aldosterone causes the kidneys to retain sodium and water. When the body needs water, less is excreted and more is retained.