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Unit 2: Excretion
Excretion is the removal of metabolic waste from the body.
Metabolic waste is any waste substance that may be toxic or produced in excess by
reactions in the body.

Substances to be excreted: carbon dioxide + nitrogen containing compounds (urea)

Why they need to be removed

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Liver cells are called hepatocytes, they have many microvilli on the surface and have
many metabolic functions including: protein synthesis, transformation/storage of
carbohydrates, synthesis of cholesterol and bile salts and detoxification. This means
it has a dense cytoplasm and specialised amounts of organelles.

Kupffer cells are specialised macrophages that move…

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ammonia needs to be converted very quickly because it is toxic, it is combined
with CO2 to make urea. This happens in the ornithine cycle. Urea is moved in
the blood to the kidneys where it is filtered out and excreted.
Ornithine cycle: 2NH3+ CO2 CO(NH2)2 + H20


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Each kidney contains nephrons, each nephron starts in the cortex, a nephron consists of a
glomerulus, bowmans capsule, proximal convoluted tubule, loop of henle, distal convoluted
tubule and a collecting duct.

The glomerulus is surrounded by the bowmans capsule, fluid is pushed into the bowmans
capsule by ultrafiltration.
As fluid…

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Blood flows into the glomerulus from the afferent arteriole,which is wider than the efferent
arteriole causing a increase in blood pressure. The pressure in the glomerulus is higher than
in the bowmans capsule so it pushes fluid from the blood in the glomerulus into the bowmans

There are…

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Therefore the deeper into the medulla the lower the water potential this is due to loss of
water by osmosis and diffusion of sodium and chloride, in the ascending part salts are
diffused out, and further up actively transported out, the walls are impermeable to water.

This is known as…

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Kidney failure
The most common causes of kidney failure are diabetes mellitus,hypertension or

Treatment of kidney failure

Haemodialysis blood is passed into a machine containing a dialysis membrane.
Heparin is added to the blood to prevent clotting and bubbles are removed and the
clean blood is passé back into…


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