• Urea is formed in the liver which is eliminated through kidneys. Liver cells also degrade the haemoglobin of worn out red blood corpuscles into bile pigments (bilirubin and biliverdin). Liver cells also excrete cholesterol, certain products of steroid hormones, some vitamins and many drugs. Liver secretes these substances in the bile. The bile carries these substances to the intestine and are passed out with faeces.
• Skin plays a major role in excretion. It helps the body to get rid of excess of water, salts and waste such as ammonia in aquatic animals.
• The mammalian skin possesses sweat glands and sebaceous glands that play excretory roles.
• Epithelial cells of colon excrete excess salts of calcium, magnesium and iron along with faeces.
• It helps in removing gaseous form of excretory wastes like CO2 and a little amount of water vapour.