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The kidneys are excretion organs,they perform three main roles:

1. The removal of urea from the blood. urea is produced in the liver form the breakdown of excess amino acids.

2. Adjustments of ion levels in the blood.

3. Adjustments of water content in the blood.

They do this by filtering stuff out of the blood under high pressure, and then reabsorbing the useful things. the end product is urine.

Nephrons are the filtration units of the kidneys.

1. Ultra-filtration:

1.1 A high pressure is built up which squeezes water, urea, ions and glucose out of the blood and into the bowman's capsule.

1.2 The glomerulus and the bowman's capsule act like filters, so big molecules like proteins and blood cells are not squeezed out and stay in the blood.

2. Re-absorption

As the liquid flows along the nephron the liquid is reabsorbed:

2.1 All the glucose is selectively reabsorbed - its moved out of the nephron back into the blood against the concentration gradient.

2.2 Sufficient water is reabsorbed, according to the level of the hormone ADH the process of maintaining the right water content in the body is called osmoregulation.

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