The term respiration has several different usages. The Latin word, from which it is derived, means “to breathe” or “exhale” and, in this sense, respiration was originally applied to the exchange of gases between an organism and its environment.
It referred to the obvious activities of breathing or their equivalent. As the years went by, it became apparent that the really fundamental exchanges were occurring at the cellular level, and the term “internal respiration” was often applied to this phase of gaseous exchange.
At present, the adjective “internal” has been dropped, and respiration is frequently applied to cellular processes of energy production.
So in its true physiological sense respiration is a chemical activity taking place within the protoplasm of the cell and results in the liberation of energy.
The salient features of respiration are the intake of oxygen and output of the carbon-dioxide.
The oxygen is used in the oxidation of digested food in the cell to liberate energy. Carbon dioxide is produced as a result of the oxidation of food materials.
Its presence in the body is harmful; therefore, it is removed from the body during this activity.
In small animal like protozoans and sponges, the oxygen is taken directly from the air or from the watery medium surrounding them into all parts of their structures and carbon dioxide is given out from all their parts directly into the surrounding medium.
It is because the cells of all parts are in direct contact with the environment and an exchange of gases between cells and their unrounding environment occurs directly.
In insects air is delivered directly to the through the tracheae but in larger forms the cells are deprived of direct contact with the external environment, it is because of their complex structure.
They, therefore, require the aid of respiratory and circulatory systems to permit satisfactory gaseous exchange and distribution of oxygen to all parts of the body.
In these animals the process of respiration involves the following steps, according to G.S. Carter.
1. External respiration:
External respiration is usually defined as “breathing”. It refers to those mechanisms by which oxygen is brought into the body from the environment and carbon dioxide is expelled from the body into the environment.
The exchange of gases takes place at the respiratory surface which may be integument, gills, tracheae or lungs.
2. Transport of respiratory gases:
This phase of respiration involves the transportation of oxygen from the respiratory surface to the body tissues and carbon dioxide from the tissues to the respiratory surface.
In higher animals the transportation of respiratory gases is effected through blood, the components of which are very much sensitive to the respiratory gases.
3. Internal or tissue respiration:
This phase of respiration includes all forms of oxygen consumption by the cells and production of carbon dioxide in the cells as results of oxidative processes which lead to liberate energy for biological work.
In other words it refers to the sum of enzymatic reactions both oxidative and non-oxidative by which energy is made available to maintain the other vital activities.
Robert Hook was the first who gave the real understanding of respiration in the living organisms. No doubt Priestley discovered oxygen but it was Lavoisier who established the idea that during respiration, oxygen is used and carbon dioxide is released by some life processes which now collectively called “respiration”.