Osmoregulation is the control of the levels of water and mineral salts in the blood. It is a homeostatic mechanism. There are three important homeostatic mechanisms: osmoregulation, thermoregulation and regulation of blood sugar levels. Homeostasis is important because it results in our cells being bathed in tissue fluid which has the correct amount of water, mineral salts, glucose and temperature.


  1. Dehydration
  2. Waterlogging
  3. What happens if you drink too much beer?
  4. Water gain
  5. Water loss


Homeostasis is the maintenance of constant internal conditions. My cells are bathed in a liquid (tissue fluid) which has constant conditions. My body temperature is always 36 ºC, I am a mammal and I am virtually never ill. We call this thermoregulation. The amount of glucose in my blood is constant; my body achieves this using hormones such as insulin, adrenalin and glucocorticoids; there is no special name for this mechanism, it is referred to as control or regulation of blood sugar level. The amounts of water and various mineral salts is also held constant; this is osmoregulation.

Osmoregulation is very important: our tissues do not lose or gain water by osmosis because the concentrations of water and salts is the same inside and outside the cells. The osmotic strength of our blood obviously depends upon how much glucose and mineral salts it contains as well as how much water is present; however if we just think about the water and assume that the amounts of sugar and salts are correct, we will be able to understand how the brain and kidneys osmoregulate.

1. Dehydration

The hypothalamus detects changes in the amount of water present in the blood. If there is too little water (the blood is too concentrated) it tells the pituitary gland to secrete ADH. This hormone has an effect on the kidney; ADH makes the kidney re-absorb water from the ultra-filtrate. Higher levels of ADH make the kidney work harder to reabsorb more and more water. This results in the production of very small quantities of very concentrated urine. The result of reabsorbing water is to reduce the concentration of the blood. By negative feedback the pituitary makes less ADH.


Too little water in the blood, detected by the hypothalamus.
More ADH produced by the pituitary gland.
More water reabsorbed by the kidneys, caused by ADH.
Blood becomes less concentrated.
Negative feedback; hypothalamus detects change in blood concentration.
Pituitary produces less ADH.
Blood returns to correct osmotic concentration.

2. Waterlogging

The hypothalamus detects that there is too much water in the blood. If the blood is too dilute, our cells will absorb water by osmosis and become “waterlogged”. Animal cells are in danger of swelling and bursting if they are placed in a solution which is too dilute. It is very important that the blood does not become so dilute that our cells are stressed by waterlogging.

When the blood becomes too dilute, our pituitary glands stop making ADH. The kidney stops reabsorbing water. Large volumes of very dilute urine are formed. So you just sit or stand there for a long time when you need to urinate!!! This is just the opposite of what happens when your blood is too concentrated. When the concentration of the blood starts to rise, the pituitary gland starts to make ADH again. This is negative feedback again. Eventually the concentration of the blood will return to normal.


Too much water in the blood, detected by the hypothalamus.
Less ADH produced by the pituitary gland.
Less water reabsorbed by the kidneys, caused by ADH.
Blood becomes more concentrated.
Negative feedback; hypothalamus detects change in blood concentration.
Pituitary produces more ADH.
Blood returns to correct osmotic concentration.

3. What happens if you drink too much beer?

Beer contains alcohol which is a diuretic. Tea also contains a diuretic. Diuretics are the opposite of anti-diuretics. The alcohol in your beer makes you produce lots of urine and this results in your blood becoming very concentrated. You are likely to end up with a “hangover” if you drink too much alcohol. The nasty feeling in your head is caused by the effect of the very concentrated blood on your brain cells. You can stop yourself from becoming too dehydrated by drinking lots of water. If you know that you have had too much beer, drink lots of water before you go to bed. You will have to get up in the middle of the night to get rid of all this extra water; after you have been to the “loo”, drink some more water. Yes, you will have to get up again but you are less likely to end up dehydrated and might not feel so bad in the morning.

If you are really clever, you will not drink too much beer in the first place

4. Water gain:

There are three ways in which your body gets water:

  1. Drinking,
  2. Water content of food,
  3. Tissue respiration.

If your body is dehydrated you will feel thirsty. Your hypothalamus has detected that your blood is too concentrated: as well as stimulating the pituitary gland to make ADH, it will stimulate you to drink. However, you will not be able to drink exactly the right amount of water to get you blood back to the exact concentration.

How much water you get out of your food depends upon what it is that you eat and how much you eat. You might end up very fat if you ate something every time that you felt thirsty. So the water content of you food does not help you to control your water content. Marine mammals cannot drink seawater to replace the water lost in their urine. They rely on the water in the food they eat; even so they do not eat more to get their water balance right.

When your cells respire glucose they produce water. How much water is produced depends upon how active you are. This does not help you to balance you water content. Camels can get water out of the fat in their humps by tissue respiration; however this is only temporary. A dehydrated camel will have a floppy hump. It will replace its water by drinking as soon as it can.

5. Water loss:

There are several ways in which your body can lose water:

  1. In exhaled air,
  2. By evaporation through moist surfaces like the cornea (eye),
  3. In sweat,
  4. In faeces,
  5. By lactation,
  6. In vomit,
  7. Spitting,
  8. Urination.

Mammals are terrestrial animals and they must conserve water. Unfortunately we lose water in some rather uncontrolled ways. We cannot stop breathing even in the desert when it may kill us. Every time we breathe out we lose water. In a cold climate you can see this happening as the water vapour in our breath condenses. Water also evaporates through our skin, we are not 100% waterproof. When we get too hot we start to sweat, as the water in the sweat evaporates, we cool down. We do not do this to get rid of excess water.

The colon does not remove all the water from our faeces, so when we defaecate we lose some water. This is a problem when the colon is infected by some nasty bacterium. Dysentery is a killer disease, so much water is lost in the faeces that the body becomes dehydrated.

Some diseases make us vomit. Even if you drink water, because it all come up again none is absorbed and the body becomes dehydrated. Water is also lost if you spit.

Where do babies get all their water from? Well, they drink milk. If a woman breast feeds her baby, she must replace all the water she loses in her milk, but this does not help her to control her water content. A lactating woman cannot give her baby more or less milk to help her control how much water is in her body.

So it all comes down to how much water is lost in the urine. This can be controlled exactly. You never have to think about it. The hypothalamus detects the amount of water in the blood, it controls how much ADH is secreted by the pituitary gland. ADH controls how much water is excreted by the kidney.

At the beginning we decided to forget about what happens to the sugar and mineral salts. Other hormones control how much sugar and salts remain in the blood. If there is too much glucose it can be converted into glycogen by the liver; if there is too little glucose the liver can convert glycogen back into glucose. If there are excess salts they can be excreted by the kidney. The control mechanisms work in just the same way but usinfg different hormones.

6. Glossary:

Hypothalamus: a region of the brain which monitors the conditions of the blood; i.e. how much glucose, mineral salts and water are present. It controls the pituitary gland.
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Pituitary Gland: an endocrine gland at the base of the brain, just underneath the hypothalamus. This gland is sometimes called the master endocrine gland because it controls all the other endocrine organs in the body.
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Anti-Diuretic Hormone: a hormone (chemical messenger) produced by the pituitary gland. ADH stimulates the kidney to reabsorb water. The more ADH there is in the blood the harder the kidney works to reabsorb water.
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Negative feedback: a control process. When a hormone has had an effect on its target organ the process of negative feedback can switch the endocrine organ off. Here is a simple analogy to help you understand negative feedback.

Positive feedback is the reverse of negative feedback. When we are having an arguement, we do not really listen to each other. If I raise my voice to make you listen to me, you react by raising your voice. eventually we are both shouting at each other. We still don’t listen. We start to get angry. I throw a pot at you and you throw one back. If we are unlucky positive feedback results in a death. One of us has a heart attack, or we start hitting each other and one gets killed. Positive feedback always makes matters worse. Suppose one of us starts to cry, this is a signal to stop. The other one realises that matters have gone too far and stops shouting. Then we start to talk instead of shouting. Now it is possible to listen properly. Instead of killing each other, we listen and things get back to normal. The crying had a negative feedback effect.