RESPIRATION BEYOND THE LUNGS 393 



The power of blood to absorb C0 2 at various tensions of this gas, as 

 determined in the same way as for 2 (see page 380), has shown that 

 saturation of the hemoglobin with 2 distinctly diminishes the C0 2 - 

 carrying power of the blood. This is shown hi the accompanying curves 



The various tensions of C0 2 are given along the abscissae and the 

 volume per cents of C0 2 taken up by the blood on the ordinates. The 

 upper curve is drawn from results obtained when the blood was shaken 

 with C0 2 in the presence of hydrogen, and the lower, when in the 

 presence of air. (The dotted curve may be disregarded.) The line AB 

 drawn between the two curves represents the absorption of CO, by the 

 blood within the body. At a tension of 40 mm. C0 2 that present in 

 alveolar air (see page 356) A stands in arterial blood at about 52 vols. 

 per cent; and at a pressure of 62 mm. possibly present in the tissues 

 B stands in venous blood at about 67 vols. per cent. The CO 2 -containing 

 power would.be 7 per cent lower (i. e., 60 vols. per cent) in blood saturated 

 with 2 at the latter pressure. The oxygenation of blood in the lungs, 

 therefore, helps to drive out the C0 2 ; and conversely, its deoxygenation 

 in the tissues enhances its power of absorbing this gas. These discoveries 

 are of fundamental importance. 



Having shown how the blood transports its charge of 2 from the 

 lungs to the tissues, we may now proceed to study the call for 2 by 

 the tissues, and in this connection we have to consider (1) the amount 

 of 2 which they require under varying conditions of rest and activity, 

 and (2) the mechanisms by which their varying demands are met. 



THE OXYGEN REQUIREMENT OF THE TISSUES 



In order to ascertain the average 2 requirement of the different tis- 

 sues of the body, it is necessary to adopt as a standard of measurement 

 the amount of 2 in c.c. absorbed per gram of tissue per minute. To ob- 

 tain it we must know: (1) the weight of the particular organ or tissue 

 under investigation; (2) the bloodflow through the vessels of the organ 

 in c.c. per minute; and (3) the different percentages of 2 in the arterial 

 and venous blood of the tissue. It would be beyond the scope of this 

 book to revieAV in any detail the many experimental investigations which 

 have been undertaken in this connection. A few of the most recent 

 and important results are given in the accompanying table from Halli- 

 burton 's Physiology : 



In the order of their oxygen requirements, or the coefficient of oxida- 

 tion, as it is called, the tissues may be divided into four groups ; glandular, 

 muscular, connective, and nervous. The nervous tissues should possibly 

 stand above the connective, but very little is known regarding their, 

 oxygen consumption, although it appears that this is quite low (Hill and 



