CHANGES IN AIR AND BLOOD IN RESPIRATION. 677 



figures were obtained reckoned in volumes per cent. : O, 2.26; C(>2, 

 34.3; N, 1.8. In this case each 100 c.c. of arterial blood contained 

 22.6 c.c. of and 34.3 c.c. of CO 2 measured at O C. and 760 mms. 

 Hg. An analysis of human blood (Setschenow) gave closely similar 

 figures; 0, 21.6 per cent.; C0 2J 40.3 per cent.; and N, 1.6 per cent. 

 When the arterial and the venous bloods are compared it is found 

 that the venous blood has more carbon dioxid and less oxygen. 

 Average figures showing the difference in composition are as follows: 



O. C0 3 . N. 



Arterial blood 20 38 1.7 



Venous blood 12 45 1.7 



Difference 870 



The actual amounts of oxygen and carbon dioxid in the venous 

 blood vary with the nutritive activity of the tissues, and differ 

 therefore in the various organs according to the state of activity of 

 each organ in relation to the volume of its blood supply. This 

 point is well illustrated by some analyses made by Hill and Na- 

 barro * of the gases in the venous blood from the brain and the 

 muscles, respectively. Their average results when both tissues 

 were at rest were as follows: 



OXYGEN. CARBON DIOXID. 



Venous blood from limbs (femoral) .... 6.34 per cent. 45.75 per cent 

 " brain (torcular) . . . 13.49 " " 41.65 " " 



It will be seen that under similar conditions there is much less 

 oxygen used and carbon dioxid formed in the brain than in the 

 limbs (muscles). In the former organ the physiological oxidations 

 must either be small compared with those of the muscles, or the 

 brain tissues receive a relatively ample supply of blood, so that the 

 tissue metabolism has less effect upon the blood composition. The 

 venous blood as it comes to the lungs is a mixture of bloods from 

 different organs, and its composition in gases will be constant only 

 when the conditions of the body are kept uniform. Much work 

 has been done in physiology to determine the condition in which 

 these various gases are held in the blood. The results obtained 

 show that they are held in small part in solution, but mainly in 

 chemical combination. To understand the part played by each 

 factor and the conditions that control the exchange of gases in 

 the lungs and tissues it is necessary to recall some facts regarding 

 the physical and chemical properties of gases. 



The Pressure of Gases and the Terms Expressing these 

 Pressures. The air around us exists under a pressure of one 

 atmosphere and this pressure is expressed usually in terms of the 

 height of a column of mercury that it will support, namely, a 



* Hill and Nabarro, "Journal of Physiology," 18, 218, 1895. 



