Osmoregulation

Osmoregulation is the control of the water potential of the blood: keeping the balance of water and salts in body fluids steady. In humans the kidneys do it, controlled by antidiuretic hormone (ADH). When the blood is too concentrated, more ADH is released and the kidneys put more water back into the blood. When it is too dilute, ADH release drops and the extra water is passed out in urine. It is an example of homeostasis by negative feedback.

What is osmoregulation?

AQA's A-level specification defines osmoregulation as the control of the water potential of the blood. OpenStax puts it more widely: the maintenance of salt and water balance. Both mean the same job, keeping body fluids at the right concentration so that water doesn't rush into or out of cells by osmosis.

AQA's GCSE specification gives the reason: if body cells lose or gain too much water by osmosis, they do not function efficiently. Blood that is too dilute makes water move into cells, so they swell; blood that is too concentrated draws water out, so they shrink.

How the body gains and loses water

This is AQA's GCSE list: there is no control over loss through the lungs or the skin, so the kidneys are where the balance is adjusted. Water comes in through food and drink, and some is made in the body by aerobic respiration.

How the kidneys make urine

At GCSE, AQA describes two steps. The kidneys produce urine by filtration of the blood, which pushes small molecules out of the blood, then selective reabsorption, which takes back the useful substances: glucose, some ions and as much water as the body needs. What is left, with the urea, is urine.

ADH and negative feedback

1. Change: the water potential of the blood falls, for example after sweating a lot or eating salty food. The blood is too concentrated.
2. Detection: receptors in the hypothalamus, in the brain, detect the change.
3. Response: more ADH is released into the blood from the pituitary gland. AQA's A-level specification names the posterior pituitary, and OpenStax notes ADH is made in the hypothalamus and stored in the posterior pituitary until it is released.
4. Effect: ADH makes the walls of the collecting ducts and distal tubules more permeable to water. OpenStax explains that it does this by inserting water channels called aquaporins. More water is reabsorbed into the blood.
5. Result: a small volume of concentrated urine, and the water potential of the blood rises back to normal. As it does, ADH release falls.

The opposite happens when the blood is too dilute: ADH release drops, the tubule walls let through less water, more of it stays in the tubule, and you produce plenty of dilute urine. Whichever way the change goes, the response reverses it. That is negative feedback.

The nephron (A-level)

Each kidney is made of many tiny tubes called nephrons. GCSE doesn't need their structure, but AQA's A-level specification asks for the role of each part:

The loop of Henle is the key to concentrated urine. By keeping the tissue fluid in the medulla salty, it gives the medulla a low water potential. As the collecting duct runs down through the medulla, water leaves it by osmosis, as long as ADH has made its walls permeable.

Worked example: a salty meal

Question: explain how the volume and concentration of urine change after someone eats a very salty meal.

1. Salt is absorbed into the blood, so the water potential of the blood falls.
2. The hypothalamus detects this, and more ADH is released from the posterior pituitary.
3. ADH travels in the blood to the kidneys and makes the collecting ducts more permeable to water.
4. More water is reabsorbed by osmosis into the blood, into the salty medulla around the ducts.
5. So a smaller volume of more concentrated urine is produced, and the water potential of the blood returns towards normal.

Osmoregulation in other animals

Every organism has to balance water and salts. OpenStax describes fish as osmoregulators that solve opposite problems in fresh and salt water:

Most marine invertebrates don't regulate at all. OpenStax calls them osmoconformers: their body fluids simply match the seawater around them.

Common mistakes

• Saying ADH is made in the kidneys. It is released from the pituitary gland and acts on the kidneys.
• Getting the direction backwards. More ADH means more water reabsorbed and less, more concentrated urine.
• Saying ADH "absorbs water". It changes the permeability of the tubules; water moves by osmosis.
• Using "water content" loosely at A-level. Use water potential: concentrated blood has a lower water potential.
• Forgetting the negative feedback. Once the blood returns to normal, ADH release falls again.

ADH is Higher tier in GCSE Biology, and the nephron is A-level only. Practise on today's GCSE Biology Higher or A-level Biology: Year 2 short timed paper. The homeostasis section of the human physiology course shows negative feedback in action.

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