5.1.2 Excretion and the Liver

Removal of metabolic waste

carbon dioxide

nitrogenous waste

Liver structure

Functions of liver

control of blood glucose levels; amino acid levels; lipid levels

synthesis of bile, plasma proteins, cholesterol

Formation of urea

Excretion: removal of metabolic waste from the body

Metabolic waste: a substance that is produced in excess by the metabolic processes in the cells; may become toxic and harmful

Excretory organs:

lungs: carbon dioxide passes from cells to bloodstream to lungs and out of the body; increase breathing rate to excrete more carbon dioxide if pH of blood is too low

liver: some substances pass into bile for excretion with faeces; convert excess amino acids to urea to be excreted

skin: sweat contains salts, water, urea, uric acid, ammonia; important for osmoregulation and homeostasis

transported in blood as hydrogencarbonate ions

form carbonic acid with lowers pH of blood as it dissociated into hydrogen ions and hydrogencarbonate ions

low pH affects bonds in haemoglobin structure resulting in lower affinity for oxygen; and proteins in plasma become denatured

haemoglobinic acid and carboaminohaemoglobin formed meaning oxygen can't bind to haemoglobin

body cannot store excess amino acids due to nitrogen

amino acids must not be excreted as they contain as much energy as carbohydrates

deamination, then ammonia then urea which is excreted; keto acid used in respiration

hepatocytes: liver cells; cuboidal shape with many microvilli to increase surface area for exchange; protein synthesis; transform and store carbs; synthesis of cholesterol and bile salts; detoxification; dense cytoplasm

hepatic artery: oxygenated blood from heart travels from aorta into liver; supplies oxygen for aerobic respiration to release ATP for metabolic processes

hepatic portal vein: deoxygenated blood from digestive system travels from aorta into liver; contains digestive products and toxic products

hepatic vein: deoxygenated blood travels via vena cava to heart

bile duct: carries bile from liver to gallbladder where it is stored; also contains excretory products from liver

liver divided into lobes, divided into lobules (cylindrical)

inter-lobular vessels: branches of hepatic artery and hepatic portal vein run smaller until they are parallel to the lobules

sinusoids: chamber where blood from h.artery and h.portal vein is mixed; lined with hepatocytes; blood is in close contact with liver cells for exchange

Kupffer cells: specialised macrophages move around in sinusoids; breakdown and recycle old blood cells; products of haemoglobin breakdown is bilirubin (bile pigment)

bile canaliculi: where bile is released into from hepatocytes; join together to form bile duct

intra-lobular vessel: branch of hepatic vein at centre of lobule; sinusoids join together and empty blood into vessel

synthesis of red blood cells in fetus

storage of vitamins A, D and B12

storage of iron

storage of glycogen: forms granules in cytoplasm of hepatocytes which can be broken down to release glucose in blood; inert and insoluble so water potential of cytoplasm is not altered and it doesn't interfere with any chemical reactions

detoxification of alcohol, drugs

breakdown of hormones

destruction of red blood cells

Detoxification

toxins from drugs, alcohol, body processes etc

catalase converts hydrogen peroxide into oxygen and water

cytochrome P450 is a group of enzymes which break down drugs

Alcohol:

depressed nerve activity and contains chemical potential energy which can be used for respiration

ethanol broken down into acetyl CoA which enters aerobic respiration

NAD is required to oxidise and break down fatty acids; if too much alcohol, then NAD is used up detoxifying alcohol instead of breaking down fatty acids; fatty acids converted back into lipids and stored in liver; fatty liver leads to cirrhosis or hepatitis

amino groups make amino acids toxic so cannot be stored

Deamination: remove amino group to produce ammonia (very toxic and soluble) and keto acid (enter respiration to release energy)

Ornithine cycle: ammonia combined with carbon dioxide to produce urea (less soluble and toxic) which can be excreted

Sammer Sheikh