How often do you hear the remark T’m dying of thirst’! Of course, when people say this they do not mean it literally – usually they mean they would like a drink of water or some other liquid. But, nevertheless, the expression does show how important water is.

Water is the most essential constituent of your body. No other substance performs a greater number of vital functions. The protoplasm of every cell is composed predominantly of water and water is also the main constituent of all body fluids – tissue fluid, blood plasma, lymph and cerebrospinal fluid. In fact the body is about twothirds water. An average adult weighing 70 kg contains about 45 litres of water. This is distributed in the different parts of the body, with by far the greatest amount – 30 litres – being present inside the body cells. Water performs a vital function because all the biochemical reactions take place in water. Water also contributes to regulation of body temperature by heat loss through sweating.

Water balance

The amount of water you need depends on a number of factors including age, body size, normal metabolic rate, level of physical activity, and environmental temperature and humidity. Because these factors vary considerably, your daily requirement can be equally variable.

What is important is that you take in enough water, in food and drink, to replace the total amount lost in sweat, urine, faeces and expired air. Water balance, the state of equilibrium between water gained and water lost, is essential to health. Fortunately, the body has a very efficient system for maintaining water balance. When losing too much water, the concentration of minerals in the blood becomes too high. This is sensed by special receptors in the brain, which in their turn – via the thirst centre -evoke the feeling of thirst. By means of a hormone antidiuretic hormone (ADH) or vasopressin – the kidneys reabsorb more water from the filtered blood, resulting in very concentrated, dark urine. Both drinking and reabsorption of water in the kidneys reduces the concentration of minerals in the blood to normal. In certain disorders, the kidneys fail to reabsorb enough water and fluid accumulates in the tissues.

Internal fluid exchange

Apart from the obvious water exchange between the body and its environment, there is also an ‘invisible’ but much larger turnover of water between the different water-containing ‘compartments’ of the body. Nearly 9 litres of water are secreted each day by the glands in the digestive system (salivary glands, sto-mach, pancreas, liver and small intestine) into the gastro-intestinal tract. This water, plus water derived from food and drink, is almost completely reabsorbed into the bloodstream, mainly from the large intestine. Only a very small amount – 0.2 litres – is lost in the faeces.

About 170 litres of water are filtered every day by the kidneys (the water is recycled). 1.5 litres are passed out as urine – the rest is reabsorbed into the bloodstream. In addition, water constantly passes between blood plasma and the interstitial fluids surrounding tissue cells. This exchange allows nutrients in blood to be carried, in watery solution, from small capillaries into the interstitial fluids from where they are absorbed by cells. Waste products from the cells are transferred in solution in the opposite direction and are taken away by the blood. It has been calculated that as much as three-quarters of your total plasma water is exchanged with interstitial fluid water every minute.

Not all the fluid portion of the blood which exudes from the bloodstream into tissues returns to the capillaries and the veins. Some tissue fluids accumulate and are drained away by a separate set of small vessels – the lymphatics. The tissue fluids which enter the lymphatics are known as lymph and consist mainly of protein and water; they finally drain back into the blood in the great veins. The lymphatic system constitutes yet another ‘water compartment’ in the body.

Water losses from the body

Under normal conditions an adult loses more than 2 litres of water daily. At least 1.5 litres of this is urine from the kidneys; approximately 0.2 litres is in the faeces; and about 0.5 litres each from the lungs and the skin.

The water lost in the faeces and from the lungs during normal breathing remains fairly constant. Losses from the kidneys and skin can vary considerably, but tend to balance each other: the greater the loss through the skin, the smaller is the amount excreted by the kidneys.

Water is lost through the skin surface in two separate processes. Perspiration is a continuous loss of almost pure water from the body’s surface. It is a passive process in which water exudes through the skin and immediately evaporates. You are unaware of this process, even though it removes about 0.5 litres of water every day.

The second process, sweating, occurs under hot or humid conditions or when the body is physically exercising. It is an active mechanism in which sweat glands in the skin secrete a solution of salt and water, under the control of the nervous system. This water loss is designed to regulate not the amount of water in the body but the body’s temperature. Evaporation of the water in sweat from the skin surface removes a great deal of body heat and so is an efficient means of cooling. In hot conditions people engaged in hard manual work can lose 10 litres of water a day.


Dehydration is drying-out of the body because more water is being lost than is being taken in. It can be caused by excessive sweating, not taking in the daily requirement of approximately 2.3 litres of water, and certain illnesses – especially those which feature high fever and vomiting or diarrhoea.

Dehydration is a very serious condition, particularly in young children and elderly people. It shows itself in the following symptoms: . thirst (although sometimes this is not the case), . lethargy and listlessness, . the tongue is dry and looks leathery, . the skin loses its normal elasticity, . the amount of urine produced is very small and any urine passed is very concentrated and often dark in colour. In most cases of dehydration salt is lost with the water, so the condition is usually treated by giving a drink of dilute salt solution or, in serious cases, putting the patient on an intravenous saline drip.

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