ELIMINATION OF CARBON DIOXIDE 271 



tissues, such as in muscle, a much larger per cent of oxygen will have been 

 disassociated and the oxygen tension correspondingly lowered so that the 

 venous blood returning through such an active organ may not have more 

 than half the average amount of oxygen found in venous blood. 



Considering the pressure relations of oxygen from the time of its intro- 

 duction into the body with the fresh air to its fixation in the tissues we have 

 the following schema: 



Oxygen Pressure in the Atmosphere 159 mm. 



I 



Alveolar Air 122 mm. 



i 

 Venous Blood 22 .04 mm. 



Tension of Oxygen in the Arterial Blood 29 .64 mm. 



i 



" " " Tissues o.oo mm. 



Elimination of Carbon Dioxide by the Blood and the Respiratory 

 Apparatus. The principles of absorption of gas by liquids discussed 

 in the preceding pages apply equally well for carbon dioxide with the exception 

 that carbon dioxide is about three times as soluble in blood as is oxygen. 

 The carbon dioxide results from the oxidative processes going on in the tis- 

 sues, and this gas is present in large quantities in the tissues and their im- 

 mediately surrounding lymph. An analysis of the carbon-dioxide content 

 of venous blood reveals the presence of about 45 c.c. of the gas in 100 c.c. of 

 blood. This gas, like oxygen, is held in such large quantity by virtue of the 

 fact that it forms loose chemical combinations in the blood. Of the total 

 quantity not more than 5 per cent is held in simple solution. From 10 to 

 15 per cent of the total volume is found in firm combination in such forms 

 as carbonates, bicarbonates, etc. The remaining 80 and more volumes 

 per cent is held in loose chemical combination, a combination which is broken 

 up under the same conditions of variation in carbon-dioxide tension as were 

 found to exist for oxygen in combination with hemoglobin. In the case of 

 carbon dioxide an analysis of plasma reveals the fact that the gas is in com- 

 bination with some compound of the plasma, probably a proteid. In fact, 

 there is some evidence to show that carbon dioxide combines with the globulin 

 group. Carbon dioxide also forms loose chemical compounds with the con- 

 stituents of the red corpuscles, probably with the proteid portion of the hemo- 

 globin molecule. The pressure relations of this gas as regards its diffusion 

 in the process of elimination are shown in the following table: 



Carbon -dioxide Tension in the Tissues 58 mm. of mercury 



i 



" " " " Venous Blood 41 " " 



I 

 " " " Alveolar Air ,23 to 38mm. of " 



i 



" " Expired Air 5.8 mm. " " 



