Excretion – In this chapter you will learn:

Definition and need of excretion.
Waste products in plants.
Methods of excretion, adopted by plants.
Types of waste products in animals.
Excretory organs found in certain animals.
Excretory system in man, including the structure and function of kidney.
Brief description of osmoregulatory function of kidney.
Brief introduction of kidney stone, kidney failure, diagnosis and its treatment.

13.1 Introductions

All organisms carry out hundreds of reactions in their bodies which altogether are known as metabolism. The metabolism can be generally divided into two types: the catabolism and the anabolism.

The catabolism is the breaking down of complex organic molecules into simpler compounds in order to obtain energy which is utilized for the performance of various activities of life. In catabolic reaction carbohydrates, proteins and lipids are broken down or (oxidized) through enzymatic reactions step by step to generate chemical energy in the form of ATP molecules.



By looking at the above reactions you can know that carbon dioxide, water and ammonia are formed in these reactions. They are regarded as waste and surplus products because they are not utilized in the body of organisms (with the exception of plants which can use CO2 for photosynthesis).

These waste products must be removed from the body as soon as possible because these are toxic (poisonous) for the cells if they accumulate in the bodies of the organisms. The CO2 if not removed quickly can lead to lowering of pH which can be dangerous for life.

Ammonia is even more toxic than CO2, it should be quickly removed from the body or converted into less toxic material but even then the less toxic materials like urea, uric acid have to be eliminated from the bodies of organisms.

The concentration of water in the tissue fluid of mammals is maintained within the range 97-99% or a mean of 98%. Similarly the amount of water in the blood is regulated at about 92%. If the water exceeds this level then it collects in the tissues causing dropsy.

If the concentration of H2O falls below the normal line, it can cause dehydration of body. These waste products which are actually metabolic by-products have therefore to be eliminated from the bodies of living organisms. The process of removing the waste products from the body is known as excretion.