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  • Created on: 21-11-12 22:19

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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
Excess carbon dioxide is toxic, it has 3 effects:
CO2 is carried by hydrogencarbonate ions, which dissociates forming H+ ions
which compete with oxygen for space in the haemoglobin, reducing oxygen
CO2 also directly combines with haemoglobin making carbaminohaemoglobin,
which has a lower affinity for oxygen
Excess CO2 can cause respiratory acidosis, once CO2 is dissolved in blood plasma
it can make carbonic acid: CO2 + H2O H2CO3 which dissociates: H2CO3
H+ + HCO3
The hydrogen ions lower blood PH and make it more acidic. PH affects how proteins
The body cannot store amino acids, but they contain lots of energy so it would be
wasteful to excrete them. They are sent to the liver where deamination occurs to remove
the NH2 group. It initially forms a highly toxic NH3 compound (ammonia), which is
converted to urea. The remaining keto acid is either used in respiration or stored as fat.
Deamination: amino acid + oxygen keto acid +ammonia
Formation of urea: ammonia + carbon dioxide urea + water
2NH3 + CO2 CO(NH2)2 + H20
the liver
Oxygenated blood from the heart travels to the liver by the hepatic artery
Deoxygenated blood from the digestive travels to the liver by the hepatic portal
Blood leaves the liver by the hepatic vein
A bile duct carries bile from the liver to the gall bladder

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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 around in the sinusoids and
break down old blood cells. One of the products of haemoglobin breakdown is
bilirubin which is the brown in faeces.…read more

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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
the liver is able to detoxify many compounds such as hydrogen peroxide or alcohol. Toxins
can be rendered harmless by oxidation, reduction, methylation or combined with other
Liver cells contain catalase which converts hydrogen peroxide to oxygen and water.…read more

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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 flows along the nephron substances are reabsorbed in the proximal/distal
convoluted tubules, loop of henle and in the collecting duct by selective reabsorption.…read more

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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
capsule.…read more

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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 a hairpin countercurrent multiplier. The low water potential in the
surrounding tissue means water diffuses by osmosis out the descending loop of henle.…read more

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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 the body. It is usually performed at a clinic 3 times a
week for 7 hours.…read more


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