The body's water balance and dehydration

In adults, water makes up roughly half to about two thirds of body weight, and in babies even more. Part of it is inside the cells, the rest outside – between the cells and in blood plasma, the liquid part of the blood. Water moves back and forth between these spaces, depending on where there are more dissolved substances such as salts.

When the body loses water, the blood becomes more “concentrated”: there are more dissolved substances in the same volume. Special sensors in the brain notice this. They trigger thirst and cause the hormone ADH (also called vasopressin) to be released. ADH is like an order to the kidneys: “Hold on to water!” The kidneys then take more water back from the forming urine into the blood, and urine becomes scarcer and darker.

If the amount of water falls further, blood volume also shrinks. The heart has to beat faster to pump enough blood. You become tired, less focused and less capable, and the body finds it harder to cool itself, because there is less water for sweat and less blood available for the skin.

How much water someone needs varies greatly: body size, heat, exertion, clothing, altitude and health all play a part. That is why this entry deliberately gives no fixed drinking amounts. It makes more sense to drink early and regularly rather than waiting until thirst is very strong.

Sweat carries away not only water but also salt. Someone who drinks only very large amounts of plain water during long exertion while eating hardly anything can develop a dangerous sodium deficiency (hyponatraemia). Normal meals usually provide enough salt. The rule of thumb: drink according to thirst and need, not “as much as possible”.

Alcohol does not count as a fluid source, because it suppresses the release of ADH so that the kidneys excrete more water. Food, on the other hand, also contains water: soups, fruit and vegetables make a noticeable contribution to the water balance.

Heat and exertion are the best-known water thieves, because the body sweats a lot. Shade, breaks during the midday heat and an adjusted pace often reduce losses more than any water bottle can make up for.

Less well known are cold and altitude. Cold air is dry; with every breath out you release moisture – visible as a cloud of breath. At the same time, thirst is often weaker in the cold, and people dislike drinking cold water. Warm drinks help with drinking regularly. At altitude you also breathe faster and deeper, which increases losses further.

Diarrhoea and vomiting can cost a great deal of water and salt in a short time. That is one reason why clean or properly treated drinking water is so important: contaminated water causes exactly the illnesses that make water loss worse.

This entry explains basics and is not a treatment guide. Warning signs that mean you should call the local emergency number (112 in the EU) or get medical help include confusion, reduced consciousness, fainting, no urine for many hours, and persistent vomiting or diarrhoea.

Small children, older people and the chronically ill are at particular risk. Children lose fluid quickly relative to their size, and older people often sense thirst less well. Actively offer them drinks and watch for changes in behaviour. Anyone who has been given a fluid restriction by a doctor, for example because of a heart or kidney condition, follows that instruction.

Urine colour is a simple but rough indicator: pale and plentiful usually suggests enough water, dark and scant too little. Some foods, vitamins or medicines change the colour, however, so it is only one clue among several.

For household emergency stocks, authorities plan with fixed reference values per person per day, because planning needs a number. That is different from the actual need of a particular person on a hot day – which can be considerably higher. More on this is in the entries on water needs and water storage.