410 THE HUMAN BODY. 



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

 in the plasma will be more than sufficient to keep the haemo- 

 globin from giving off its oxygen. Suppose the blood now 

 enters the capillaries of a muscle. In the liquid ioistening 

 this organ the oxygen tension is almost nil, since the tissue 

 elements are steadily taking -the gas up from the lymph 

 around them. Consequently, through the capillary walls, 

 the plasma will give off oxygen until the tension of that gas 

 in it falls below 25 millimeters of mercury. Immediately 

 some of the oxyhaemoglobin is decomposed, 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 oxyhaemoglobin decom- 

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

 of the muscle, or at any rate so long as the muscular fibres 

 keep on taking oxygen from the lymph bathing them; if 

 they cease to do so of course the tension of that gas in the 

 lymph will soon come to equal that in the plasma: the latter 

 will therefore cease to yield oxygen to the former; and so 

 maintain its tension (by the oxygen received from the last 

 decomposed oxyhaemoglobin) at a point which will prevent 

 the liberation of any more oxygen from such red corpuscles 

 as have not yet given all of theirs up. 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 millimeters 

 (1 inch) of mercury. (2) A number of red corpuscles con- 

 taining reduced haemoglobin. (3) A number of red corpus- 

 cles containing oxyhaemoglobin. Or perhaps all of the- red 

 corpuscles will contain some reduced and some oxidized 

 haemoglobin. The relative proportion of reduced and un- 

 reduced haemoglobin will depend on how active the muscle 

 had been; if it worked while the blood flowed through it, it will 

 have used up more oxygen, and the blood leaving it will con- 

 sequently be more venous, than if it rested. 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 130 mm. (5.2 inches) 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 haemoglobin combines with it. Hence, as 

 fast as the plasma gets oxygen those red corpuscles which 



