Water in life processes

The organism possesses tremendous capacity to survive against odds and maintain homeostasis. This is particularly true with regard to water, electrolyte and acid-base status of the body. These three are interrelated, hence they are considered together for the discussion in this chapter. Kidney actively participates in the regulation of water, electrolyte and acid-base balance.

Water and life

Water is the solvent of life. Undoubtedly, water is more important than any other single compound to life. It is involved in several body functions.

Functions of water

1. Water provides the aqueous medium to the organism which is essential for the various biochemical reactions to occur.

2. Water directly participates as a reactant in several metabolic reactions.

3. It serves as a vehicle for transport of solutes.

4. Water is closely associated with the regulation of body temperature.

Distribution of water

Water is the major body constituent. An adult human contains about 60% water (men 55–70%, women 45–60%). The women and obese individuals have relatively less water which is due to the higher content of stored fat in an anhydrous form.

A 70 kg normal man contains about 42 litres of water. This is distributed in intracellular (inside the cells 28l) and extracellular (outside the cells 14l) compartments, respectively known as intracellular fluid (ICF) and extracellular fluid (ECF). The ECF is further divided into interstitial fluid (10.5l) and plasma (3.5l).

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 center located in the hypothalamus. Increase in the osmolality of plasma causes increased water intake by stimulating thirst center.

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 g 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 distinct 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 l/day. Water loss through kidneys although highly variable, is well regulated to meet the body demands—to get rid of water or to retain.

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. It is estimated that 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.