CHANGES IN AIR AND BLOOD IN RESPIRATION. 667 



"figures were obtained reckoned in volumes per cent.: 0, 22.6; CO2, 

 31.3; 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 COj measured at 0° C. and 760 mms. 

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

 figures; 0, 21.6 per cent.; COj, 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. CO2. N. 



Arterial blood 20 38 1.7 



Venous blood 12 45 1.7 



Difference 8 7 



The actual amovmts of oxygen and carbon dioxid in the venoua 

 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. According to 

 some recent workf the average loss of oxygen in the capillaries, or 

 the degree of oxygen-unsaturation as it has been called, is about 

 5.5 per cent., that is, 5.5 c.c. of oxygen are given to the tissues by 

 each 100 c.c. of blood. This amount of oxygen represents the 

 average needs of the tissues under normal resting conditions. It 

 will be noted that there is a large factor of safety, the blood flow- 

 ing away from the capillaries still contains at least 13 to 14 per 

 cent, of oxygen. In cases of anemic persons this margin of safety 

 may be much reduced. In such persons the oxygen capacity of 

 the arterial blood may be as low as 6 to 7 per cent, owing to the 

 reduction in the amount of hemoglobin. If in passing through 



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

 t Lundsgaard, "Journal of Exp. Med.," 30, 147, 1919. 



