Nature of Excretory Wastes:
The wastes formed as a result of various metabolic activities are:
i. Respiratory waste products are formed as a result of oxidation of glucose (food) (through cellular respiration). These mainly constitute carbon dioxide and water. The carbon dioxide is eliminated from the body through the lungs (during expiration), while water becomes a part of the rest of the water contained in the body.
ii. Nitrogenous waste products are formed as a result of deamination of unwanted amino acids, body’s own proteins and nucleic acids. The three main nitrogenous waste products excreted by animals are ammonia, urea and uric acid. Urea is a highly poisonous waste product and its accumulation beyond a certain concentration in the body may cause death. Urea is eliminated through the kidneys.
iii. Other waste materials like salts (NaCl), vitamins and water that we take directly through food, which need to be excreted out of the body.
Excess salts are mainly excreted by kidneys. The excess water is removed by kidneys as urine and some of it is excreted as sweat by the sweat glands on our skin. In addition, bile pigments are formed due to break down of haemoglobin in dead RBCs in the liver. A large amount of these pigments is given out in faeces while some of it is excreted in urine.