382 THE HUMAN BODY 



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

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

 hemoglobin. (3) A number of red corpuscles containing oxy- 

 hemoglobin. 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 fast as the plasma gets oxygen those red cor- 

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

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

 hemoglobin is saturated. Then the oxygen tension of the plasma 

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

 and the blood returns to the left auricle of the heart in the same 

 condition, so far as oxygen is concerned, as when we commenced 

 to follow it. 



The Carbon Dioxid of the Blood. The same general laws apply 

 to this as to the blood oxygen. The gas is partly merely dissolved 

 and partly in a loose chemical combination with some one or more 

 of the constituents of blood. Carbon dioxid is about twenty times 

 as soluble in blood-plasma as is oxygen under equivalent conditions 

 of temperature and pressure. We can therefore account for more 

 of it than of oxygen in the state of simple solution. Not more 

 than 6 per cent of the total amount present in venous blood can be 

 accounted for, however, in this way. The remainder must be in 

 some form of easily dissociable chemical compound. The nature 

 of this combination is not certainly known, but it is thought that 

 it may be for the most part simple sodium bicarbonate. The 

 large amount of sodium in the blood, and the ease with which it 

 combines with carbon dioxid, favor such a view. Sodium bicar- 

 bonate under ordinary circumstances is not easily enough dissoci- 

 ated to serve as a carbon dioxid carrier for the Body, but there 

 is reason to believe that in the presence of the blood proteins it 

 breaks down whenever the carbon dioxid tension of the plasma 

 falls below a certain point. 



We may summarize the carbon dioxid interchanges as follows: 

 1. The tissues constantly produce and give off to the lymph 



