RESPIRATION. THE GASEOUS INTERCHANGES 423 



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 easily dissociable chemical combination. Two such combina- 

 tions are known to exist in blood. The first is a combination of 

 carbon dioxid with sodium, forming sodium carbonate; the second 

 of carbon dioxid with the blood proteins, including hemoglobin, 

 forming a compound somewhat analogous with oxyhemoglobin. 

 This latter compound is more readily dissociable than sodium 

 carbonate, and since, as we have seen, there is always, even in 

 arterial blood, a considerable percentage of carbon dioxid, we 

 may suppose that under ordinary circumstances the sodium car- 

 bonate circulates as such, and the protein compound serves as 

 the carrier of carbon dioxid from tissues to lungs. 



We may summarize the carbon dioxid interchanges as follows: 



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

 carbon dioxid. It is present in lymph, therefore, at all times in 

 considerable quantity, probably amounting to a carbon dioxid 

 tension of 70 mm. of mercury. 



2. The blood entering the capillaries contains carbon dioxid 

 under much less tension than this (about 35 mm.), there is there- 

 fore a movement of carbon dioxid from lymph to blood. This 

 movement, by raising the tension of carbon dioxid in the blood 

 brings about conditions under which chemical combination may 

 take place, chiefly with the blood proteins. 



3. The venous blood as it enters the lungs contains carbon 

 dioxid under a higher tension than that of alveolar air, 70 mm. 

 for venous blood, 35 mm. for the alveoli; there is therefore a 

 movement of carbon dioxid from the blood to the alveoli. This 

 movement, by lowering the carbon dioxid tension of the blood, 



