Osmoregulation

Where it happens

Osmoregulation involves the hypothalamus in the brain, the pituitary gland, and the kidney, working together to keep the water potential of the blood within a narrow range regardless of how much a person drinks or sweats.

Inside the kidney, the parts that respond to ADH are the distal convoluted tubule and the collecting duct of each nephron. These tubules run through the medulla, where the surrounding tissue fluid is very concentrated, so once ADH makes their walls permeable, water leaves the filtrate by osmosis and enters the blood in the surrounding capillaries.

The skin, lungs and gut also lose water, but only the kidney can vary its water loss quickly and precisely, which is why it is the main organ of osmoregulation.

Inputs and outputs

• Input: water gained from drinking, food and the water produced by respiration.
• Input: solutes such as salt from the diet, which lower the water potential of the blood.
• Signal: ADH released from the pituitary gland, the amount rising when blood water potential falls.
• Output: urine, whose volume and concentration change with the level of ADH.
• Output: water lost through sweat, breath and faeces, which the kidney compensates for.
• Result: blood water potential held within a narrow range around the set point.

The steps

1. Water is lost through sweating, breathing or too little drinking, or salt is taken in, so the water potential of the blood falls.
2. Osmoreceptors in the hypothalamus detect the drop in water potential of the blood flowing past them.
3. The hypothalamus signals the pituitary gland to release more antidiuretic hormone (ADH) into the bloodstream.
4. ADH travels in the blood to the kidney and increases the permeability of the distal convoluted tubule and collecting duct to water.
5. More water is reabsorbed from the filtrate into the blood by osmosis, so a small volume of concentrated urine is produced and the thirst centre also prompts drinking.
6. As the blood's water potential rises back to normal, the osmoreceptors detect the change and ADH release falls.
7. With less ADH the tubules become less permeable, less water is reabsorbed and urine becomes more dilute, the opposite correction, completing the negative feedback loop.

Why it matters and how it is controlled

Osmoregulation keeps the concentration of blood and body fluids constant, which is essential for cells to work normally; too little or too much water in the blood would disturb the movement of water into and out of cells by osmosis.

Control is by negative feedback: the hypothalamus is the receptor and coordinator, the pituitary gland is the effector that releases ADH, and the kidney tubules are the target. The response always opposes the original change, so after drinking a large volume of water the sequence runs in reverse, water potential rises, ADH falls, tubules become less permeable, and a large volume of dilute urine is produced.

If ADH cannot be produced, the tubules stay impermeable, several litres of dilute urine are lost each day and the person must drink constantly to replace the loss.

How it is examined

You may be asked to explain what happens to ADH secretion and urine after drinking a large volume of water, to describe osmoregulation as a negative feedback mechanism, or to name the gland that secretes ADH and the tubule it acts on.

Structured questions often show a graph of urine volume against time after a person drinks a litre of water or eats a salty meal, and ask you to explain the shape using ADH. Flow-chart questions give blank boxes for stimulus, receptor, hormone, effector and response, and expect the exact terms: hypothalamus, pituitary gland, ADH, collecting duct, water reabsorption.

A frequent essay theme compares osmoregulation with blood glucose control as two examples of negative feedback, so be ready to point out that both use a hormone, a detector and a response that reverses the change.

Common misconceptions

Worked exam-style question

Question. Two students, P and Q, each produced urine samples over four hours. Student P drank one litre of plain water at the start; student Q ate a packet of salted crackers and drank nothing.

Student P produced a large volume of pale urine in the first two hours, while student Q produced a small volume of dark yellow urine throughout. (a) Name the hormone responsible for the difference and the gland that releases it.

(b) Explain the results for student Q. (c) Explain why this control mechanism is described as negative feedback.

Model answer. (a) Antidiuretic hormone (ADH), released by the pituitary gland. (b) The salt absorbed from the crackers lowers the water potential of the blood.

Osmoreceptors in the hypothalamus detect this and the pituitary gland releases more ADH. ADH increases the permeability of the distal convoluted tubule and collecting duct to water, so more water is reabsorbed by osmosis into the blood.

A small volume of concentrated urine is produced, which is why it is dark. (c) The change in ADH secretion produces a response that opposes the original change: when water potential falls, more water is retained and water potential rises back towards the set point; when water potential rises, ADH falls and excess water is removed.

Because the response reverses the stimulus, it is negative feedback.

Source:SRC-DSKP-EN

Frequently asked questions