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gcse AQA Unit B3.3 Removal waste water control Temperature Sugar separate triple award science Unit 3 BIOLOGY 3

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revision notes practice exam questions past papers

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The kidneys maintain our body's water balance by controlling the water concentration of blood plasma. The kidneys also control salt levels and the excretion of urea. Water that is not put back into the blood is excreted in our urine.

Our bodies take in water from food and drinks. We even get some water when we respire by burning glucose to release energy.

We lose water in sweat, faeces, urine and when we breathe out (on a cold day you can see this water as it condenses into vapour).

For the cells of our body to work properly, it is important that their water content is maintained at the correct level. This means our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys.

The kidneys

Humans have two kidneys. They are bean-shaped organs, approximately 11.5cm long, that are situated in the abdominal cavity, just below the ribcage, one on either side of the spine.

Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body.

What the kidneys do

As the blood passes through the kidneys, all the small molecules are filtered out of the blood.

This includes molecules of:

water

salt

glucose

urea (a waste product from the breakdown of proteins)

The kidneys then reabsorb all of the glucose and as much water and salt as the body needs, putting them back into the blood. This leaves some water and salt, and all of the urea, which is now called urine. The urine passes from the kidneys to the bladder, where it is stored prior to being excreted from the body.

The kidneys do more than just control the body's water balance. They also control:

1. The level of salts salts: class of chemical compounds, mostly metallic oxides. Examples are sodium chloride, potassium chloride, and magnesium sulphate in the blood.

2. The excretion of urea urea: a nitrogenous waste product resulting from the break down of proteins. It is excreted in urine and other metabolic waste.

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The kidneys maintain our body's water balance by controlling the water concentration of blood plasma. The kidneys also control salt levels and the excretion of urea. Water that is not put back into the blood is excreted in our urine. Our bodies take in water from food and drinks.