422 THE HUMAN BODY 



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 oxygen 

 dissolved in it at a tension of about 25 mm. (1 inch) of mercury,. 



(2) A number of red corpuscles containing reduced hemoglobirt. 



(3) A number of red corpuscles containing oxyhemoglobin. Or 

 perhaps all of the red corpuscles will contain some reduced and 

 some oxidized hemoglobin. This venous blood, returning to 

 the heart, is sent on to the pulmonary capillaries. Here, the 

 partial pressure of oxygen in the air-cells being 100 mm. and 

 that in the blood-plasma much less, oxygen will be taken up 

 by the latter, and the tension of that gas in the plasma tend to 

 be raised above the limit at which hemoglobin combines with it. 

 Hence, as far as the plasma gets oxygen thoL3 red corpuscles 

 which contain any reduced hemoglobin rob it, and so its oxygen 

 tension is kept down below that in the air-cells until all the hemo- 

 globin is saturated. Then the oxygen tension of the plasma rises 

 to that of the gas in the air-cells; no more oxygen is absorbed, 



