Active body cells require lots of oxygen for energy production in aerobic respiration as we learnt in Lecture IV. A major by product of aerobic respiration is carbon dioxide. Carbon dioxide dissolves in water to form carbonic acid and hence disturbs acid base balance in the body. Accumulation of carbon dioxide is therefore toxic to the cells as it may cause acidosis. Living cells have mechanisms of intake of oxygen and elimination of carbon dioxide. The two gases are known as respiratory gases. In small organisms such as protozoans the body surface provides an adequate surface area for the exchange of respiratory gases by diffusion. In mammals and other higher animals there are specialized systems for exchange of respiratory gases.