Liver & Excretion

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Liver Structure

  • Hepatic Artery - supplies the liver with blood
  • Hepatic Vein - carries blood away
  • Hepatic Portal Vein - brings blood from the duodenum (small intestine) which is rich in products of digestion so harmful substances can be removed or broken down
  • Bile Duct - takes bile that is produced by the liver to the gall bladder, Bile is released into the duodenum via the bile duct to emulsify fats during difestion

The liver is made of lobules (cylinders of cells called hepatocytes). Each lobule has a central vein that connets to the hepatic vein as well as branches of the hepatic artery, hepatic portal vein and bile duct. The hepatic artery and hepatic portal vein are connected to the central vein by capillaryies called sinusoids.

THe liver contains thousands of sinusoids, designed to mximise the surface area of blood in contact with liver cells. Blood runs througgh the sinusiods past the hepatocytes that remove harmful substances and oxgyen.

  • Bile Canculi - connects to the central vein from the bile duct
  • Kupfer cells  - remove bacteris and breakdown RBCS
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Formation of Urea

When amino acis are absorbed by liver cells, they are oxidised in the presence of an enzyme. The amine grroup and hydrogen atom are removed in deamination and ammonia is produced

Ammonium ions exist in aqueous in dynamic equilibrium with ammonia molecules. IT is highly toic and can not be allowed to accumulate, with the help fo specific catalysts in the liver cells, Cos reacts chemically with NH3. Urea (less toxic) is produced with water

Thi series of reactions is called the ornithine cycle. The urea and water are released from the liver cells into the bloodstream and transport to the kidneys where the blood is filtered.

Ornithine Cycle:

  • Ornithine reacts with NH3 and CO2 to produce Citrulline and water
  • Citrulline reacts with NH3 with produce water and Arginine
  • Arginine reacts with water to produce urea and Ornithine
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Roles in detoxification

Hepatocytes break down and chemically changes many harmful substances and drugs (paracetamol, antibitotics, alcohol) as wel as some hormones such as insulin, testosterone and oestrogen

Ethanol is converted to ethanol by the enzyme alchol dehydrogenase, and then to ethanoate by aldehyde dehydrogenanse. This can enter the Krebs cycle and be metabolised to produce ATP

Types of Nitrogenous Waste

Ammonia - high solubility, high oxicity and is found in an aqueous environment ->fish

Urea - high solubility, medium toxicity, fouund in terrestrial environments->Mammals

Uric Acis - low solubility, low toxicity in the air ->birds

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