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Excretion
Excess amino acids are broken down by the liver
 First, the nitrogen containing amino groups are removed from any excess amino
acids, forming ammonia and organic acids – this process is called deamination
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 Ammonia is too toxic for mammals to excrete directly, so its combined with carbon
dioxide in the ornithine cycle to form urea
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Kidneys filter the blood and remove urea as
urine which can then be excreted
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 Alcohol – ethanol is broken down into ethanal by the liver, which is then broken
down into acetic acid so it can be excreted
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The liver converts excess glucose into glycogen and
stores it in cells until it is needed for energy
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 The hepatic vein takes deoxygenated blood away from the liver
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 The bile duct takes bile to the gall bladder to be stored
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 Each lobule has a central vein in the middle that connects the hepatic vein
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 Blood runs through the sinusoids, past the hepatocytes that break down remove
harmful substances and oxygen from the blood
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 The blood runs to the central vein, the central vein from all the liver lobules form the
hepatic vein
 Cells called kupffer cells are attached to the walls of the sinusoids
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 The bile duct is connected to the central vein by tubes called canaliculi
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They also regulate
water potential of the blood
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 As the blood passes through the capillaries, substances are filtered out of the blood
and into long tubules that surround the capillaries
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 Useful substances are reabsorbed into the blood from the tubules in the medulla
and cortex – this is called selective reabsorption
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 The filtered blood leaves the kidney through the renal vein

Blood is filtered at the start of the nephrons
The long tubules and bunch of capillaries where the blood is filtered are called nephrons –
there are around one million nephrons in each kidney
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 Each arteriole splits into a structure called glomerulus – a bundle of capillaries
looped in a hollow ball called the bowman’s capsule
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 The arteriole that takes blood to the glomerulus is called the afferent arteriole, and
the arteriole that takes blood away from the glomerulus is the efferent arteriole
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This high pressure forces liquid and small
molecules into the bowman’s capsule
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Larger
molecules like proteins and red blood cells stay in the blood
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They pass along the rest of the
nephron tubule and useful substances can be re absorbed along the way
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Useful substances are reabsorbed along the nephron tubules
 Selective reabsorption takes place as the filtrate flows along the proximal convoluted
tubule, through the loop of Henle and along the distal convoluted tubule
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 The epithelium of the wall of the proximal convoluted tubule has microvilli to
provide a larger surface area for the reabsorption from the filtrate into the blood
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 Some urea is also reabsorbed by diffusion
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Water is reabsorbed in the loop of Henle, DCT and collecting duct
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Kidneys regulate water potential of the blood
If the water potential of blood is too low, more water is reabsorbed by osmosis into the
blood from the tubules of the nephrons
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If the water potential of the blood is too high, less water is reabsorbed by osmosis into the
blood from the tubules of nephrons
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The loop of Henle has a counter current multiplier mechanism
 Near the top of the ascending limb, sodium and calcium ions are actively pumped
out into the medulla
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This creates low water potential in the medulla as there is a high
concentration of ions
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This makes the
filtrate more concentrated
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 Near the bottom of the ascending limb sodium and calcium ions diffuse out and into
the medulla, lowering the water potential in the medulla
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This causes water to move out of the collecting duct by
osmosis
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Hormones control water reabsorption
 The water potential of the blood is monitored by cells called osmoreceptors in a part
of the brain called the hypothalamus
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A small amount of concentrated urine is produced, which
means less water is excreted from the body



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