Human Nutrition 3(3+0)

Lesson 46 : Water, Oedema and Dehydration

Mechanism of water balance

In the body, several mechanisms work together to maintain water balance. One of the most important is thirst. When the body needs water, nerve centers deep within the brain are stimulated, resulting in the sensation of thirst. The sensation becomes stronger as the body's need for water increases, motivating a person to drink the needed fluids. When the body has excess water, thirst is suppressed.

Another mechanism for maintaining water balance involves the pituitary gland (located at the base of the brain) and the kidneys. When the body is low in water, the pituitary gland secretes antidiuretic hormone (also called vasopressin) into the bloodstream. Antidiuretic hormone stimulates the kidneys to conserve water and excrete less urine. When the body has excess water, the pituitary gland secretes little antidiuretic hormone, enabling the kidneys to excrete excess water as urine.

The body can move water from one area to another as needed. When water loss is severe, the amount of water in the bloodstream decreases, so the body moves water from inside the cells to the bloodstream until it can be replaced through increased intake of fluids. When the body has excess water, the amount of water in the bloodstream increases, so the body moves water from the bloodstream into and around the cells. In this way, blood volume and blood pressure can be kept relatively constant.