Excretion, homeostasis
and the Liver
Excretion
- Excretion is the removal of waste products of metabolism from the body.
- The main metabolic waste products are:
- Carbon Dioxide from cellular respiration
- Bile Pigments from the breakdown of haemoglobin, excreted in bile from the liver into the small intestine.
- Nitrogenous Waste (urea) from the breakdown of excess amino acids in the liver.
- The liver is the main organ involved in homeostasis, makes up 5% of your body mass, and is a multi-lobed structure just below the diaphragm. It is fast growing, and damaged area heal quickly.
- The liver has a large oxygenated blood supply provided by the hepatic artery.
- The hepatic portal vein supplies the liver with products of digestion, and is the starting point for many metabolic processes of the liver.
- Hepatocytes: The main liver cells which are highly metabolically active which divide and replicate.
- Sinusoids: Spaces between hepatocytes where blood from the hepatic artery and portal vein mixes to increase the oxygen content of the substrate-rich blood, allowing hepatocytes to remain active.
- Kupffer Cells: Like macrophages, ingest foreign particles to protect the liver from disease. Found in sinusoids.
- Canaliculi: Spaces in the liver where bile is secreted, from the canaliculi, the bile drains into ductules which transport it to the gall bladder.
- Hepatic Artery: Artery which supplies oxygenated blood.
- Hepatic Portal Vein: Vein which supplies blood rich with digestive products for metabolism.
- Hepatic Vein: Vein which removes deoxygenated blood.
- Bile Duct: Channel which transports bile produced from haemoglobin to the gall bladder.
- Gall Bladder: A sac in which bile is stored before being released into the small intestine.
- Falciform Ligament: Ligament which separates the left and right lobes of the liver.
- Portal Triad: Between each hepatocyte is a triad of portal veins, arteries and bile ducts which drain into or out of the central vein of a hepatocyte to transfer substances via diffusion.
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Functions of the Liver
- Carbohydrate Metabolism
- Hepatocytes are closely involved in homeostatic control of blood glucose levels. When blood glucose levels rise, insulin levels rise and stimulate hepatocytes to convert glucose to glycogen for storage.
- When blood suagr levels drop, glucagon levels rise and stimulate hepatocytes to convert glycogen into glucose for use in reactions.
- Deaminination of Excess Amino Acids
- Most plasma proteins are synthesised in the liver.
- Transamination is the conversion of one amino acid into a different amino acid. This is important as our diet does not always contain the required balance of amino acids.
- Demaination is the removal of an amino group from an amino acid. As the body cannot store excess proteins/aminos, excess would be wasted unless hepatocytes removed and detoxified the amine group, then allowing for the amino to be broken down for energy.
- Detoxification
- Many metabolic processes produce harmful substances, and the liver is the place where these substances are made harmless.
- For example, hepatocytes contain the enxyme catalase to break down hydrogen peroxide into harmless hydrogen and water.
- Similarly, hepatocytes contain alcohol dehydrogenase to break ethanol down into ethanal, and then into ethanoate for synthesis of fatty acids.
Deamination and the Ornithine Cycle
- The body breaks down unstorable amino acids and proteins to metabolise them for energy.
- In doing so, the amino group is removed creating the toxic ammonia.
- Ammonia is then converted into the less toxic urea, which is not harmful in the concentrations it is usually found in the blood.