2.12.1 Fluid loss
During an average day, a person in a temperate climate such as the UK, loses about 2.5 litres of water.
How is water lost from the body?
The main loss is through the production of urine. Some water is lost through the skin as sweat and some in the faeces. Water is also lost from the lungs in breathing; you know that breathing out onto a cold glass surface produces condensation, and in cold weather the water vapour in the breath condenses into visible droplets in the air.
Urine output is controlled by the kidneys and even in cases of quite severe dehydration, urine production continues, since it is needed to rid the body of the nitrogen-containing compound, urea, which is produced as a result of the breakdown of amino acids from proteins. In a dehydrated person, the output of urine can be as low as 0.5 litres per day and it will be a dark brown colour. A more normal output is 1.5 litres and a light yellow colour indicates that the body is well hydrated. Sweating is part of the system that regulates the body temperature. Heat from the body is used to evaporate sweat from the surface of the skin and so the evaporation has a cooling effect. A typical loss of water through sweating of about 0.5 litres per day can increase in hot weather and during exercise to up to 2 litres per hour. The losses from the lungs (0.4 litres daily) and in faeces (0.1 litres daily) are normally fairly constant. However, diarrhoea increases the loss from the digestive tract hugely and can quickly result in dangerous levels of dehydration if the fluid is not replaced. Diarrhoeal diseases are common where people live in overcrowded conditions without a clean water supply, and there are an estimated 10 million cases and 5000 deaths each day throughout the world. Since both water and ions are lost, the best treatment for diarrhoea is oral rehydration, using sachets of commercially prepared rehydration mixture or a home-made solution containing eight teaspoons of sugar (to provide energy and to mask the taste of the salt) and one teaspoon of salt in a litre of water, together with some mashed banana or orange juice if available.
In addition to water, what are the main mineral ions present in this home-made rehydration solution?
Salt is sodium chloride, so the solution will contain sodium ions and chloride ions. Bananas and orange juice are good sources of potassium ions. All three ions are vital for the normal functioning of the body.