Excretion

Excretion is the removal of waste substances from the body of a living organism. Waste substances include

 • by products formed during metabolic processes in the body; • excess food substances which cannot be stored in the body; and

 • unwanted materials present in food which are absorbed by the body.

  The main waste substances come from metabolic reactions which digested food substances undergo in the cells of the body.

    In Cellular respiration, glucose is oxidalized to produce energy. At the same time, carbon dioxide, water and heat are formed as by products. Carbon dioxide is an unwanted substance in animals. If it is allowed to accumulate, it becomes harmful since it forms carbonic acid which, when in excess, can upset the delicate acid base balance of the body fluids. Carbon dioxide is, therefore , an excretory product. Water is needed by the body. However, excess water has to be excreted as it will dilute the body fluids. Excess heat too is harmful as it raises the temperature of the body, adversely affecting enzymes , which only operate within a narrow temperature range. Therefore, excess heat too has to be dissipated to the external environment. 

     Amino acids are metabolized in cells to produce protoplasmic materials for growth and repairs of tissues. excess amino acids in the diet, however, cannot be stored. They are deaminated to produce

 • carbon fragments which can be converted to carbohydrates and stored , and

 • nitrogenous fragments which give rise to ammonia, the most poisonous of the metabolic by products.

      In aquatic unicellular organisms, ammonia cannot be got rid of rapidly by diffusion. In multicellular animals this is not possible. So ammonia is converted to less toxic substances such as urea and uric acid. Urea is more easily formed than uric acid. It is water and can afford to loose water. Uric acid is practically insoluble and is usually formed by animals that need to conserve water and cannot afford 'water costly' excretion processes. Many animals have special excretory systems to deal with the excretion of nitrogenous wastes. In mammals highly specialized organs extract nitrogenous wastes from the blood and get rid of them from the body.

Note : Egestion and secretion should not be confused with excretion. Egestion is the removal of solid undigested substances in food, not by products of metabolism. secretion is the production of useful substances such as enzymes and hormones by metabolic processes.