Exchange of Gases by Animal Respiratory Organs!
In small organisms (e.g.. Amoeba, Paramecium) the exchange of gases occurs through the general surface of the body or the cell membrane.
However, larger animals (e.g., birds, mammals) have a much greater requirement of energy.
Hence they need much more oxygen than can be met through diffusion across the general surface of the body. Therefore, they have special respiratory organs that have a greatly enlarged surface area through which oxygen can diffuse.
For example, the human lungs have millions of air sacs whose surface area is many times that of the body. These air sacs are involved in the exchange of gases.
In animals, there are three types of respiratory organs—tracheae, gills and lungs. Insects have a fine system of air tubes reaching all parts of the body. Such a tube is called trachea.
Oxygen reaches the tissues through the tracheae. Gills are respiratory organs found in aquatic animals. You may have seen the gills of a fish. Reptiles, birds and mammals have lungs for the exchange of gases.
Aquatic animals have to use the oxygen dissolved in water. Hence, they have some special organs such as gills to absorb it. In fish, as water enters the mouth, it passes through the chambers where the gills are present. The exchange of gases takes place through the gills. The blood vessels in the gills absorb the oxygen dissolved in water.
Terrestrial animals use the abundant oxygen of the atmosphere for respiration. Since the solubility of oxygen in water is low, there is not much oxygen available for aquatic organisms.
Therefore, to make up for the low availability of oxygen, the rate of breathing in aquatic organisms is much higher than that in terrestrial organisms.