Control of water and ion content of the bodyOxford AQA IGCSE Biology: Revision notes

The body constantly loses water and ions, and these losses must be balanced by what we take in. There are three main routes of water loss:

Only the kidneys can finely adjust how much water and how many ions leave the body. The lungs and skin lose water as a side-effect of breathing and cooling, not as a controlled response, so the kidneys make up the difference to keep the body's water and ion content balanced overall.

The liver carries out three important jobs relevant to this topic:

• Deamination: the body cannot store excess amino acids, so the liver removes the nitrogen-containing part of the amino acid. This produces ammonia, which is toxic. The liver immediately converts the ammonia into urea, a much less toxic substance, ready for excretion by the kidneys.
• Detoxification: the liver breaks down poisonous substances (including alcohol and other drugs); the breakdown products are excreted in urine via the kidneys.
• Breakdown of old red blood cells: the liver breaks down old blood cells and stores the iron released from them, for reuse later.

Urea then travels in the blood plasma from the liver to the kidneys, where it is removed from the body.

Each kidney filters blood and processes it in several steps:

1. Filtration: blood is filtered, removing small molecules (including glucose, ions, urea and water) from the blood, while large molecules like proteins and blood cells are kept in the blood.
2. Reabsorption of glucose: all the glucose that was filtered out is reabsorbed back into the blood — none should appear in urine.
3. Reabsorption of ions: the dissolved ions needed by the body are reabsorbed back into the blood.
4. Reabsorption of water: as much water as the body needs is reabsorbed back into the blood.
5. Release as urine: what is left — urea, excess ions and excess water — is released from the body as urine.

This means urine composition changes depending on what the body needs to keep or get rid of, but a healthy kidney should never let glucose appear in urine.

The concentration of the blood is monitored constantly, and the kidneys adjust water reabsorption using a hormone called ADH (antidiuretic hormone), released by the pituitary gland.

If the water content of the blood is too low (the blood is too concentrated):

• The pituitary gland releases more ADH into the blood.
• More ADH causes the kidneys to reabsorb more water.
• This produces a small volume of concentrated urine.

If the water content of the blood is too high (the blood is too dilute):

• Less ADH is released into the blood.
• Less water is reabsorbed in the kidneys.
• This produces a large volume of dilute urine.

This is a good example of the body responding in the opposite direction to the original change, so that the water content of the blood is brought back towards normal.

Negative feedback is a control mechanism where a change in a condition triggers a response that reverses the change, bringing the condition back towards a normal (set) level.

For ADH:

The system works continuously, monitoring blood water content and adjusting ADH release moment to moment, which is why the water content of the blood normally stays within a narrow, stable range even though our intake of fluid and food varies a great deal through the day.