RESPIRATION: THE GASEOUS INTERCHANGES 381 



at all to deficiency of oxygen. So long as the partial pressure 

 of that gas in the lung air-cells is above 25 mm. of mercury, 

 the amount of it taken up by the blood depends on how much 

 hemoglobin there is in that liquid and not on how much oxygen 

 there is in the air. So, too, breathing pure oxygen under a pres- 

 sure of one atmosphere, or air compressed to one-half or a fourth 

 its normal bulk, does not increase the quantity of oxygen ab- 

 sorbed by the blood, apart from the small extra quantity dissolved 

 by the plasma. The widespread statements as to the exhilaration 

 caused by breathing pure oxygen are erroneous, being founded on 

 experiments made with impure gas. 



The General Oxygen Interchanges in the Blood. Suppose we 

 have a quantity of arterial blood in the aorta. This, fresh from the 

 lungs, will have its hemoglobin practically saturated with oxygen 

 and in the state of oxyhemoglobin. In the blood-plasma some 

 more oxygen will be dissolved, viz., so much as answers to a pres- 

 sure of that gas equal to 100 mm. of mercury, which is the partial 

 pressure of oxygen in the pulmonary air-cells. This tension of the 

 gas in the plasma will be more than sufficient to keep the hemo- 

 globin from giving off its oxygen. Suppose the blood now enters 

 the capillaries of a muscle. In- the liquid moistening this organ the 

 oxygen tension is practically nil, since the tissue elements are 

 steadily taking the gas up from the lymph around them. Conse- 

 quently, through the capillary walls, the plasma will give off 

 oxygen until the tension of that gas in it falls below 25 mm. of 

 mercury. Immediately some of the oxyhemoglobin is decom- 

 posed, and the oxygen liberated is dissolved in the plasma, and 

 from there next passed on to the lymph outside; and so the tension 

 in the plasma is once more lowered and more oxyhemoglobin 

 decomposed. This goes on so long as the blood is in the capillaries 

 of the muscle, but on account of the shortness of this interval, 

 about one second, not all the oxyhemoglobin has time to decom- 

 pose before the blood has passed on into the veins. Here further 

 decomposition is quickly brought to an end by the rising tension of 

 the oxygen dissolved in the plasma, the last oxygen given off from 

 the corpuscles not being taken up by the lymph because of the 

 passage of the blood on out of the capillaries. The blood will now 

 go on as ordinary venous blood into the veins of the muscle and 

 so back to the lungs. ' It will consist of (1) plasma with oxy- 



