15.4 excretion, homeostasis and the liver

main metabolic waste products in mammals

carbon dioxide - one of the waste products of cellular respiration which is excreted from the lungs

bile pigments - formed from the breakdown on haemoglobin from old red blood cells in the liver. They are excreted in the bile from the liver into the small intestine via the gall bladder and the bile duct

nitrogenous waste products (urea) - formed from the breakdown of excess amino acids by the liver. All mammals produce urea as their nitrogenous waste. Fish produce ammonia while birds and insects produce uric acid

what vessel supplies blood to the liver

hepatic portal vein (blood loaded with products of digestion)

hepatic artery (oxygenated blood)

parts of liver cells (hepatocytes)

• large nuclei

• prominent golgi apparatus

• lots of mitochondria

sinusoids

the spaces where blood from the hepatic artery and hepatic portal vein are mixed

Kupffer cells

act as macrophages in the liver cells - help to protect against disease

secretion of bile

• the hepatocytes secrete bile from the breakdown of the blood into spaces called canaliculi

• from these, blood drains into the bile bile ductules which take it to the gall bladder 

carbohydrate metabolism

• when blood glucose levels rise, insulin levels rise and stimulate hepatocytes to convert glucose to the storage carbohydrate glycogen

• when blood sugar levels fall, the hepatocytes convert the glycogen back to glucose under the influence of the hormone glucagon

deaminaton of excess amino acids

• the removal of an amine group group from a molecule

• the body cannot store proteins/ amino acids

• the amino group of an amino acid is removed and converted into ammonia

• the remainder of the amino acid can then be fed into cellular respiration or converted into lipids for storage

transamination

the conversion of one amino acid into another

the ornithine cycle

detoxification in the liver

• eg, breakdown of hydrogen peroxide using the enzyme catalyse

• the liver detoxifies ethanol using the enzyme alcohol dehydrogenase to break the ethanol into ethanal. Ethanal is converted to ethanoate which may be used to build up fatty acids or in cellular respiration

diagram of the structure of the liver