Water turnover and balance

WATER TURNOVER AND BALANCE

The body possesses tremendous capacity to regulate its water content. In a healthy individual, this is achieved by balancing the daily water intake and water output.

Water intake

Water is supplied to the body by exogenous and endogenous sources.

Exogenous water : Ingested water and beverages, water content of solid foods constitute the exogenous source of water. Water intake is highly variable which may range from

0.5-5 litres. It largely depends on the social habits and climate. In general, people living in hot climate drink more water. Ingestion of water is mainly controlled by a thirst centre located in the hypothalamus. Increase in the osmolality of plasma causes increased water intake by stimulating thirst centre.

Endogenous water: The metabolic water produced within the body is the endogenous water. This water (300-350 ml/day) is derived from the oxidation of foodstuffs. It is estimated that 1 each of carbohydrate, protein and fat, respectively, yield 0.6 ml, 0.4 ml and 1.1 ml of water. On an average, about 125 ml of water is generated for 1,000 Cal consumed by the body.

Water output

Water losses from the body are variable. There are four routes for the elimination of water from the body-urine, skin, lungs and feces.

 

Urine: This is the major route for water loss from the body. In a healthy individual, the urine output is about 1-2 1/day. Water loss through a kidneys, although highly variable, is well regu lated to meet the body demands to get rid of water or to retain. It should, however, be remembered that man cannot completely shut down urine production, despite there being no water intake. This is due to the fact that some amount of water (about 500 ml/day) is essential as the medium to eliminate the waste products from the body.

Hormonal regulation of urine production: It is indeed surprising to know that about 180 litres of water is filtered by the glomeruli into the renal tubules everyday. However, most of this is reabsorbed and only 1-2 litres is excreted as urine. Water excretion by the kidney is tightly controlled by vasopressin also known as antidiuretic hormone (ADH) of the posterior pituitary gland. The secretion of ADH is regulated by the osmotic pressure of plasma. An increase in osmolality promotes ADH secretion that leads to an increased water reabsorption from the renal tubules (less urine output). Plasma osmolality is largely dependent on the sodium concentration, hence sodium indirectly controls the amount of water in the body

Diabetes insipidus is a disorder characterized by the deficiency of ADH which results in an increased loss of water from the body.

Aquaporins are membrane proteins that play a key role in water balance of the body, ADH translocates aquaporins from cytoplasm to membrane (of collecting duct cells) to promote reabsorption of water from the renal tubules.

Skin: Loss of water (450 ml/day) occurs through the body surface by perspiration. This is an unregulated process by the body which mostly depends on the atmospheric temperature and humidity. The loss is more in hot climate. Fever causes increased water loss: through the skin. For every 1°C rise in body temperature, about 15% increase is observed in the loss of water (through skin).

Lungs : During respiration, some amount of water (about 400 ml/day) is lost through the expired air. The latter is saturated with water and expelled from the body. In hot climates and/or when the person is suffering from fever, the water loss through lungs is increased.

The loss of water by perspiration (via skin) and respiration (via lungs) is collectively referred to as insensible water loss.

Feces : Most of the water entering the gastrointestinal tract is reabsorbed by the intestine. About 150 ml/day is lost through feces in a healthy individual. Fecal loss of water is tremendously increased in diarrhea.