The respiratory exchange is very much smaller in cold-blooded animals than in animals which maintain the temperature of the body at a fixed level. In warm-blooded animals it rises as the temperature falls, falls as it rises, the increased oxidation warming the body, the diminished oxidation allowing it to cool; whereas in cold-blooded animals it increases as the temperature rises, owing to the greater activity induced by warmth, and falls as the temperature falls. The respiratory exchange is increased by muscular activity. If the amounts of oxygen absorbed and carbonic acid given out are measured while a man is at rest, and again while he is doing hard physical work, it is found that during work the respiratory exchange is twice as great as during rest. During periods of starvation the respiratory exchange remains unaltered, since heat has to be constantly produced if the temperature of the body is to be kept from falling. Since the purpose of respiration is to give to the blood the opportunity of renewing its supply of oxygen, and of getting rid of the carbonic acid with which it is charged, it might be supposed that the respiratory exchange would be increased, so far as the intake of oxygen is concerned, by breathing oxygen gas instead of air; but it appears that under normal conditions nothing is gained. When an animal is breathing air, its blood takes up all the oxygen that it wants—all the oxygen, that is to say, for which its tissues are asking. Offering it pure oxygen in place of mixed oxygen and nitrogen does not induce it to take up more. The hæmoglobin is almost saturated with oxygen when the blood leaves the lungs under ordinary conditions. In certain diseases of the lungs, however, in which the blood becomes unduly venous, the respiration of oxygen may be beneficial; but even in these cases the results are disappointing, because the system is suffering much less from deficiency of oxygen than from accumulation of carbonic acid. Substituting oxygen for air does not facilitate the escape of carbonic acid.