598 PHYSIOLOGY OF RESPIRATION. 



a burette fastened over the opening of the tube in C. To drive off all of the 

 carbon dioxid a little dilute phosphoric acid must be added to the blood in 

 B by means of the syringe, S. The gases thus collected into the burette are 

 first measured and are then analyzed for the three important constituents 

 by some of the accepted gasometric methods. The principle involved is to 

 absorb first from the mixture all of the CO 2 by introducing a solution of sodium 

 or potassium hydrate. The reading of the volume left after this absorption 

 is completed compared with the first reading gives the volume of CO 2 . Next, 

 a freshly made alkaline solution of pyrogallic acid is introduced into the tube. 

 This solution absorbs all of the oxygen, whose volume is thus easily determined. 

 The gas that is left unabsorbed after the action of these two solutions is nitro- 

 gen. The volumes of gases are reduced, as is the jcustom, to unit pressure 

 and temperature, that is, to zero degree centigrade and 760 mms. barometric 

 pressure. A correction must also be made for the tension or pressure exerted 

 by the aqueous vapor in the gases. These corrections are made by means 

 of the folio whig formula: 



yi _ V(B-T) 



760 X (1 + 0.003665 1) 



in which V 1 represents the corrected volume, V the volume actually observed, 

 B the barometric height at the tune and place of the observation, T 

 the aqueous tension at the temperature of the reading, and t the temperature 

 in degrees centigrade. 



By means of such methods the gases in the blood have been de- 

 termined. The quantities vary somewhat, of course, with the con- 

 ditions of the animal and with the species of animal. In a quick 

 analysis of dogs' arterial blood made by Pfliiger the following 

 figures were obtained reckoned in volumes per cent.: O, 22.6; C0 2 , 

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

 22.6 c.c. of O 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; O, 21.6 per cent.; CO 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. co 2 . N. 



Arterial blood 20 38 1.7 



Venous blood 12 45 1.7 



Difference 8 7 



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 " " 



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



