Excretion is the process by which an organism gets rid of waste metabolic products such as nitrogenous waste from amino acid metabolism and carbon dioxide from respiration. These waste products are harmful or toxic, and would damage cells if allowed to accumulate it.
Carbon dioxide from respiration combines reversibly with water to give carbonic acid, which in turn dissociates to give hydrogen ions and hydrogencarbonate ions.
CO2 + H2O <> H2CO3 <> H+ + HCO3-
Carbon dioxide is excreted via the lungs. The respiratory centre in the medulla oblangata increases the rate of breathing when the carbon dioxide concentration in the blood rises. Hydrogen ions are removed by the kidneys to maintain blood pH.
Mammals cannot store proteins or amino acids, so any excess is converted into fats or carbohydrates for storage, or for use in respiration. The amino group (-NH2) of each amino acid is removed in the process of deamination in the liver, forming the very soluble but very toxic compound ammonia. This is combined with carbon dioxide from respiration, in the ornithine cycle in the liver, to give the much less toxic but adequately soluble compound urea. This is removed from the blood by the kidneys.
The liver is made up of several lobes. The hepatic artery brings oxygenated blood from the aorta and the hepatic portal vein brings blood from the gut. The hepatic vein takes blood to the vena cava. Liver cells, or hepatocytes, are arranged…