Respiratory Gases -Respiration
Physiologically the most important gases are oxygen, carbon dioxide and nitrogen. Molecular oxygen comprises 21% of the atmosphere; carbon dioxide is only 0.3% while nitrogen forms 78.0%. All the gases present in the air exert a combined pressure which is referred to as one atmosphere (760 mm Hg) or 101.3 k Pa. Most of the oxygen is in the air but some of it is also dissolved in the bodies of water and in soil water. An animal's immediate source of oxygen or respiratory medium therefore, is either water or air. Comparison of the gaseous composition and physical characteristics of air and water emphasise the adaptations needed by terrestrial and aquatic animals to overcome the problems of respiration. For example, the oxygen content of air is about 20 times that of water saturated with air. The diffusion rate for oxygen in air is much more than the diffusion rate in water. In addition there is another hazard. Carbon dioxide diffuses rapidly from air into water. Therefore, elimination of carbon dioxide in water and air are different.
Extraction of oxygen from different media thus presents special problems as a result of the physical characteristics of the environment. Therefore, to understand the physiology of respiration, we must have a basic knowledge of some of the physical properties of gases. The exchange of oxygen and carbon dioxide either in the respiratory organ or at the tissue level is dependent on the partial pressure, concentration and diffusion of gases. Let us consider them one by one.