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The kidneys
The kidneys do three main things
- They remove urea from the blood
- They control the amount of ions (salts) in the blood
- They control the water content of the blood
Urea is produced in the liver where proteins (which can’t be stored) are broken down into fats and carbohydrates, with the waste product as urea it is filtered out of the blood by the kidneys, because urea is poisonous.
Salts are eaten, and while the body needs some salts, a salty meal (for example) however, will have far too much salt, so kidneys will filter out the excess salts.
Water which is taken in can be lost from the body in three ways; in the breath, in the sweat and in the urine. Because the water lost from breath is constant, the water content has to be balanced between the amount you sweat and the amount dumped from the kidneys. Therefore on a cold day, if you don’t sweat then you will produce more urine which will be pale and dilute, while on a hot day if you sweat a lot, you will produce less, but it will be concentrated. The name of this process is osmo-regulation and is an example of homeostasis
The kidneys are made up of the medulla and the cortex. The cortex is the lighter exterior, while the medulla is an area made up of feathery like structures closer to the centre which are attached to the ureter.
Inside the kidneys there are millions of nephrons which filter the blood. They do this by having blood build up a high pressure in the glomerulus (a ball of capillaries) so that the small molecules (e.g. water, glucose urea and ions) are forced through into the Bowman’s capsule while the blood cells and proteins stay. This mixture is no called glomerial filtrate and has been filtered by ultra filtration. This flows down the first coiled tubule where all the glucose is reabsorbed (using active uptake), as are the required salts. Water is then reabsorbed at the loop of henle, and also at the second coiled tubule. It then heads down the collecting duct into the ureter. This flows down into the bladder. (Note the urea and excess ions are not absorbed.
Also See
Here are the other comprehensive GCSE Biology notes by Prometheus:
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