Water

Water, electrolytes, and an acid-base balance are very important in the proper function of the body, particularly the urinary system. Between 50% and 70% of a person's weight is water: 60% for the adult male and 50% in the adult female. An infant is composed of 75% water. Because adipose tissue contains less water than muscle tissue, the obese have less water than a thin person.

Water and electrolytes are distributed into two major compartments: intracellular and extracellular.

Intracellular compartments hold the most water in the body – 63%. Intracellular fluid contains high concentrations of potassium, phosphate, and magnesium ions.
Extracellular water (37%) is found between cells (interstitial fluid), within blood vessels (plasma), and within lymphatic vessels (lymph).
- Transcellular fluid is also extracellular fluid that includes:
  - cerebrospinal fluid
  - aqueous and vitreous humors in the eyes
  - synovial fluids of joints
  - serous fluids in body cavities
  - glandular secretions
- Interstitial fluids and plasma comprise the largest amounts of extracellular fluids.

Water metabolism consists of a daily intake of approximately 2,500 ml of water with 60% coming from drinking, 30% from water in foods, and 10% from the breakdown of foods.

The thirst center is in the hypothalamus of the brain. Water elimination occurs through several routes:

kidneys (60%),
skin and lungs (28%)
feces (6%)
sweating (6%).

This page was updated in November 2005.