Characteristic Feature of Respiration
From our opinion we may say that the characteristic feature of respiration is the intake of oxygen and the output of the carbon dioxide. The oxygen taken in is employed in the oxidation of digested food in the cell to liberate energy. Carbon dioxide is generated as a consequence of oxidation of food materials. Its presence in the body is harmful, and therefore it is removed from the body during this process. In lower organisms such as protozoans, sponges, and platyhelminths, oxygen is taken directly from the surrounding aquatic medium and carbon dioxide is given out directly into the surrounding medium. In larger animals, several cells are deprived of direct contact with the external environment because of their complex structure and larger size. Therefore they require the help of respiratory and circulatory systems so as to facilitate exchange of gases and distribution of oxygen in all parts of the body.