Respiration can be defined simply as breathing, or more specifically, as the physical and chemical processes by which oxygen is assimilated into a living organism and carbon dioxide and water are given off. The mechanics of breathing and its function have interested scientists since ancient times. The Greeks generally thought respiration was intended to cool the heart, although the followers of Hippocrates also thought that air provided some sort of nourishment to the body.

William Harvey, in his revolutionary 1628 description of the circulation of the blood, established that blood flowed through the lungs and then back into the heart for dispersion throughout the body. Harvey's discovery prompted new investigations into the nature of respiration. The British physiologist Richard Lower (1631-1691) showed clearly in 1669 that exposure to air in the lungs caused dark venous blood to take on a bright, scarlet color. From this, Lower concluded that the blood...