4.5.3.3a - Water, ions and urea balance

Osmotic changes in body fluids

Body fluids include the blood plasma and tissue fluid around cells. These fluids contain water and dissolved substances such as ions, glucose and urea. If the water concentration of body fluids changes too much, water can move into or out of cells by osmosis.

Cell membranes are partially permeable, so water can move across them. If body fluids become too concentrated, water leaves body cells by osmosis. If body fluids become too dilute, water enters body cells by osmosis. In both cases, the cells may not function efficiently because their internal water balance has changed.

For this specification point, the important effect is not a detailed named disease. The key idea is that body cells lose or gain too much water by osmosis and therefore do not work efficiently.

Routes for water, ions and urea loss

Water leaves the body in several ways. Water vapour leaves through the lungs during exhalation. Water, ions and urea are also lost from the skin in sweat.

There is no control over water loss by the lungs or over water, ion or urea loss by the skin. For example, if you are hot you may sweat more because that helps cooling, but the body cannot use the skin as a precise way of choosing exactly how much water, ion or urea to remove for balance.

The kidneys are the controlled route in this lesson. Excess water, excess ions and urea are removed by the kidneys in urine. This matters because the body needs to keep enough water and ions for cells to work, while also getting rid of waste products.

A careful answer separates the routes:

Deamination and urea

This section is Higher Tier only. Proteins in the diet are digested into amino acids. The body cannot store excess amino acids in the same simple way it can store glucose as glycogen, so excess amino acids need to be excreted safely.

In the liver, excess amino acids are deaminated. Deamination removes the nitrogen-containing part of an amino acid and forms ammonia. Ammonia is toxic, so it is immediately converted to urea for safer excretion.

The sequence is:

protein in diet -> amino acids after digestion -> excess amino acids -> deamination in the liver -> ammonia -> urea -> excretion in urine

The kidneys do not make the urea in this GCSE point. The liver forms urea from toxic ammonia; the kidneys remove urea from the blood and excrete it in urine.

Kidney filtration and selective reabsorption

The kidneys maintain water balance by producing urine. They do this by filtration of the blood followed by selective reabsorption.

During filtration, small substances such as glucose, ions, water and urea are filtered from the blood. The body should not lose all of these substances. Glucose is useful, some ions are needed, and water must be kept within a suitable range.

Selective reabsorption means useful substances are taken back into the blood. In this lesson, the key examples are glucose, some ions and water. The substances left to be removed, including urea and excess water or ions, form urine.