THE BLOOD GASES. 385 



and in the state of oxyhaemoglobin. In. the blood plasma 

 -some more oxygen will be dissolved, viz: so much as answers 

 to a pressure of that gas equal to 130 mm. (5.2 inches) 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 haemoglobin 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 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 oxy- 

 liaemoglobin is decomposed, and the oxygen liberated is dis- 

 solved in the plasma, and from there again passed on to the 

 lymph outside; and so the tension in the plasma is once 

 more lowered and more oxyhaemogiobin decomposed. 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 main- 

 tain its tension (by the oxygen received from the last de- 

 composed oxyhaemogiobin) at a point which will prevent 

 the liberation of any more oxygen from such red corpuscles 

 is have not yet given all 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 milli- 

 meters (1 inch) of mercury. (2) A number of red cor- 

 puscles containing reduced haemoglobin. (3) A number of 

 red corpuscles containing oxyhsemoglobin. Or perhaps all 

 of the red corpuscles will contain some reduced and some 

 oxidized haemoglobin. The relative proportion of reduced 

 and unreduced haemoglobin will depend on how active the 

 muscle was; if it worked while the blood flowed through it 

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

 will consequently be more venous, than if it rested. This 



