RESPIRATION. THE GASEOUS INTERCHANGES 417 



breathing rate and for the percentages of oxygen and carbon dioxid, 

 are round numbers and somewhat higher than the actual averages. 

 The Blood Gases. If fresh blood be rapidly exposed to as com- 

 plete a vacuum as can be obtained, it gives off certain gases, known 

 as the gases of the blood. These are the same in kind, but differ in 

 proportion, in venous and arterial blood; there being more carbon 

 dioxid and less oxygen obtainable from the venous blood going to 

 the lungs by the pulmonary artery, than from the arterial blood 

 coming back to the heart by the pulmonary veins. The gases given 

 off by venous and arterial blood, measured under the normal pres- 

 sure and at the normal temperature, amount to from 58 to 60 

 volumes for every 100 volumes of blood, and in the two cases are 

 about as follows: 



Venous Blood Arterial Blood 



Oxygen 12 20 



Carbon dioxi:! 45 38 



Nitrogen 1.7 1.7 



It is important to bear in mind that while arterial blood contains 

 some carbon dioxid that can be removed by the air-pump, venous 

 blood also contains some oxygen removable in the same way; so 

 that the difference between the two is only one of degree. When 

 an animal is killed by suffocation, however, the last trace of oxygen 

 which can be yielded up in a vacuum disappears from the blood 

 before the heart ceases to beat. All the blood of such an animal 

 is what might be called suffocation blood, and has a far darker 

 color than ordinary venous blood. 



The Cause of the Bright Color of Arterial Blood. The color of 

 the blood depends on its red corpuscles, since pure blood-plasma 

 or blood-serum is colorless, or at most a very faint straw yellow. 

 Hence the color change which the blood experiences in circulating 

 through the lungs must be due to some change in its red corpuscles. 

 We have already seen (Chap. XVII) that the functional sub- 

 stance of the red corpuscles is hemoglobin, which has the prop- 

 erty of combining with oxygen. Hemoglobin itself is of a dark 

 purplish color, when combined with oxygen the resulting com- 

 pound is a bright scarlet. Hemoglobin combined with oxygen is 

 known as oxy hemoglobin, and it is on its predominance that the 

 color of arterial blood depends. Hemoglobin uncombined with 



