INFLUENCE OF VARIOUS CONDITIONS ON RESPIRATION. 703 



symptoms of anoxemia begin to appear at different pressures of 

 oxygen in different individuals depending upon the health, phys- 

 ical fitness, age, etc. It may be said in general that for a person 

 who is physically fit and is at rest no evidence of an anoxemia 

 appears until the pressure of oxygen falls to about 15 per cent. 

 Below this point the supply of oxygen becomes deficient and 

 symptoms of anoxemia begin to develop. These symptoms con- 

 sist in compensatory reactions exhibited by the organs of circula- 

 tion and respiration, the effect of which is to increase the amount 

 of air inspired and to accelerate the circulation. The specific 

 reactions that occur are an increase in the depth and the rate of 

 the respirations, an increase in the rate of the heart-beat, a small 

 increase in systolic pressure, and a decrease in venous pressure. 

 We assume that these reactions are due primarily to a direct effect 

 of a deficient supply of oxygen to the respiratory, cardiac, and 

 vasomotor centers in the medulla, and that the end-result is to 

 luring more oxygen to the blood and to augment the flow of blood 

 through the tissues. The compensation is more effective in some 

 individuals than in others. In those who respond best the oxygen 

 pressure may be dropped to 6 to 7 per cent., until the individual 

 in fact loses consciousness from lack of oxygen to the cerebral 

 centers. In others the heart and blood-vessels compensate through 

 a smaller range, the anoxemia at a certain pressure of oxygen caus- 

 ing a sudden collapse of vascular tone, manifested by a rapid fall 

 in diastohc pressure, or affecting the heart so that it becomes in- 

 efficient and suffers dilatation. This reaction to anoxemia as de- 

 veloped by the method of rebreathing gives a physiological method 

 of testing the efficiency of the cardiovascular system and the 

 state of physical fitness or unfitness in anj- given individual. 



Increased Percentages of Carbon Dioxid. — It was pointed out 

 by the researches of Friedliinder and Herter* and later observers 

 that death from increased percentages of CO2 is accompanied by 

 symptoms quite different from those due to lack of oxygen. As the 

 CO2 is increased a noticeable hj^perpnea may be observed (Zuntz) at 

 a concentration of about 2 per cent. When the concentration of 

 CO2 reaches 8 per cent, to 10 or 15 per cent, there is distinct dysp- 

 nea; but beyond this point further concentration, instead of aug- 

 menting the respirations, decreases them, and the animal dies, at 

 concentrations of 40 to 50 per cent., without convulsions, but with 

 the appearance, rather, of a fatal narcosis. It is probable that in 

 these concentrations the CO2 exercises a direct toxic action on the 

 nerve cells. 



* Friedlander and Herter, "Zeitschrift f. ph3'Siol. Chemie," 2, 99, 1878, 

 and 3, 19, 1S79; Haldane, loc. cit. 



