2.69 understand how the kidney carries out its roles of excretion and osmoregulation

Excretion
The kidneys remove urea from the blood. This is done in the nephron....

- Blood (from the renal artery) flows through the glomerulus, increasing pressure as it 'bunches up'.
- This high pressure causes small molecules (water, urea, glucose and salts) too squeeze through the membrane in-between the blood vessels (in the glomerulus) and the Bowman's capsule.
- This membrane acts as a filter so the bigger molecules (proteins and blood cells) do not leave the blood.
- As the filtrate (the small molecules that have been filtered out) flows through the nephron, the useful stuff (such as glucose) is selectively reabsorbed back into the bloodstream.
- The remaining substances are waste as they are of no use to the body, these substances include water, salts and urea.
- These remaining substances combine to form urine, which flows out of the nephron, through the ureter and down to the bladder where it is stored.

Osmoregulation
Osmoregulation is the adjustment of water content of the blood (it is a form of homeostasis). Basically, the kidney makes sure the blood has the right level of water in it. To do this, it has to constantly balance the amount of water that enters and leaves the blood. This is done by adjusting the amount of water the blood reabsorbed when flowing through the nephron. If the person is dehydrates (or very sweaty) the kidneys will reabsorb lots of water (this means less water is lost in urine). Similarly, if someone is very hydrated, the kidney can lower the level of water reabsorbed into the bloodstream, this means the person will lose more water through urine (this is why you wee loads if you drink loads). This is controlled by ADH (produced in the pituitary gland) which changes the permeability of the kidney tubules accordingly (to allow more/less water to be reabsorbed).

There is a little more on how osmoregulation takes place in post 2.75