CHAPTER XXI 

 RESPIRATION (Continued) 



Once or twice each minute all the blood in the body 

 passes through the capillaries of the lungs. As it enters 

 from the right side of the heart it is reckoned venous; 

 it goes on to the left auricle and ventricle arterial blood. 

 It has been stated before that arterial blood contains 

 about all the oxygen which can be attached to the 

 hemoglobin of the corpuscles, while average venous blood 

 is not by any means devoid of oxygen. It is likely to 

 retain upward of half the amount of the gas originally 

 present. The facts concerning the carbon dioxid are 

 perhaps rather unexpected. 



Carbon dioxid is the most abundant gas even in arterial 

 blood. So much of it is constantly present there that 

 the increase in venous blood seems rather slight. The 

 quantity obtainable from 100 volumes of arterial blood 

 may be in the vicinity of 38 volumes. From 100 volumes 

 of venous blood about 45 volumes of carbon dioxid may 

 probably be removed. (Blood exposed to a vacuum will 

 froth and rapidly give off all the mixed gases which it 

 lias been holding.) The oxygen in 100 volumes of arterial 

 blood may be 20 volumes; in venous blood about 12. 

 These figures are not to be accepted as rigid and unva- 

 rying. Other quantities are quite as likely to be reported, 

 but the order of magnitude will not be entirely different. 



The time during which any single corpuscle or any 

 portion of plasma is in a position for respiratory ex- 

 change is only a second or two. It is shorter when the 

 circulation is accelerated to support muscular activity. 

 Yet it appears to suffice for the purpose. There is some 

 uncertainty at the present time whether the delicate 

 cells through which the transfer of oxygen and carbon 

 dioxid must take place act like an inert membrane or 

 apply energy of their own evolution to promote the 



293 



