The active cells of the body continually call for oxygen. This element is supplied by the arterial blood, which, in turn, depends on the respiratory processes to replenish its supply. The oxygen pressure in the blood must be sufficiently high to satisfy the bodily needs, if physiologic disaster is to be averted. The physician has long realized that anoxemia may attend hemorrhage and anemias of extreme type, because the oxygen carriers, the red blood cells, have become depleted in numbers or are deprived of their normal content of the respiratory pigment hemoglobin. Diseases of the circulation and of the lungs may bring about anoxemia in other ways. The severe symptoms and fatal effects of extreme deprivation of oxygen are quite familiar; the manifestations of mild oxygen hunger are not so well known, particularly because the body will often react so as to compensate for a diminution in the oxygen supply.