water balance We often drink for social reasons — a coffee break, or an evening in the pub — and yet in spite of this the body weight of a healthy adult on an adequate diet remains remarkably stable from day to day. This stability indicates that the body fluid volume is staying constant — there is a dynamic steady state, in which the fluid output equals the fluid input (Fig. 1).

Water is the most important dietary constituent. We cannot reduce our water losses from the body to less than about 1200 ml per day (the skin, respiratory, and faecal losses, and a minimum urine volume of about 400 ml per day), so survival with no water intake is only possible for a few days.

What determines how much water we need to ingest? Essentially, the simple answer to this question is that it is the concentration of the solutes in the body — the body fluid osmolality. This normally has a value of about 285 milliosmoles/kg H₂O. If the solutes get too concentrated (if the osmolality increases), this indicates that there is insufficient water to keep them at their correct concentration. Conversely, if the solutes are diluted (decreased osmolality), there is an excessive amount of water relative to solute.

The osmolality of the blood supplying the brain is monitored by ‘osmoreceptors’ in the hypothalamus at the base of the brain, and these play a large part in determining our thirst sensation, and in the release of the hormone ADH (antidiuretic hormone, or vasopressin) from its storage site in the posterior pituitary gland. Water deficit leads to thirst, and to ADH release into the circulating blood. The ADH acts on the kidneys to increase renal water reabsorption (see kidneys). Water excess suppresses thirst and decreases ADH release.

Water deprivation

What happens when our water intake is inadequate? The continuing obligatory water loss from the skin and from the lungs (Fig. 1) causes a rise in the extracellular fluid osmolality, and this causes water to move from the cells to the extracellular fluid, so that there is water deficiency, and an increased osmolality, in all of the body fluid compartments. The increased osmolality increases ADH release (Fig. 2), and the osmolality and cellular dehydration causes the sensation of thirst.

The normal body fluid osmolality of 285 milliosmoles/kg H₂O is between the osmotic threshold for ADH release (280 milliosmoles/kg H₂O), and that for thirst (290 milliosmoles/kg H₂O), as shown in Fig. 2.

When we are deprived of water, renal mechanisms are activated to conserve water, but, in practice, the situations in which water intake is low are often those in which losses from the lungs and skin are high (e.g. hot, dry environments). The skin loss shown in Fig. 1 (400 ml) is ‘insensible perspiration’, and occurs because the skin is not completely waterproof. Sweating is an additional loss (at up to 5 litres per hour), and is adjusted not to the needs of water balance, but to the needs of temperature regulation. There is no convincing evidence that we humans can ‘train’ to manage with less water by alterations of our physiological mechanisms, although our behaviour can certainly be modified to conserve water. For example, hard physical work produces metabolic heat, so avoiding such work reduces the need to sweat.

What are the physiological effects of water deprivation? The first sign is the sensation of thirst. This begins when the body fluids have decreased by 2% from the normal volume of about 40–45 litres in a 70 kg person. When the deficit reaches 4%, the mouth and throat feel dry, and functional derangements develop — apathy, sleepiness, impatience. At 8% deficit, there is no longer any salivary secretion; the tongue feels swollen, and speech is difficult. By the time the deficit reaches 12% (4.5–5 litres less water in the body than there should be), the victim is unable to recover without assistance, and is unable to swallow. The lethal limit of water deprivation is about 20%.

Osmoregulation

It is important to appreciate that people can have a water deficit even if they are drinking. This is because there is a maximum possible urine concentration — about 1400 millosmoles per kg H₂O in adults, but only about half of that value in children. So if we ingest hypertonic solution — solutions with a higher solute concentration (osmolality) than the plasma — we may need to excrete more water to remove the solute than we took in with it. For example, if we ingest 1 kg of sea water, of osmolality 2000 millosmoles/kg H₂O, and we can produce urine with a maximum osmolality of 1400 millosmoles/kg H₂O, we need 2000/1400 kg urine (i.e. 1.5 litres) to excrete the solute and we end up dehydrated in spite of the fluid ingested. This is particularly important in infants. Newborn infants have a maximum urine osmolality of only about 600 milliosmoles per kg H₂O, so it is very easy to dehydrate them by giving them excessively concentrated drinks.

Volume regulation

The body fluid solute concentration (osmolality) is not the only regulator of our water intake — the body fluid volume is also important. Indeed, sometimes volume regulation and osmoregulation may be in ‘conflict’. For example, during prolonged physical activity, sweating leads to a reduction in body fluid volume by loss of salt (NaCl) and water. Drinking water in response to this lowers the body fluid osmotic concentration, which limits thirst. Full restoration of the body fluid volume therefore requires replacement of the lost salt, as well as water.

Chris Lote

Bibliography

Lote, C. (1993). The kidneys' balancing act. Biological Sciences Review, 5, 20–3.

See also body fluids; kidneys; survival at sea; sweating.