382 RESPIRATION. 



blood taken from the different arteries vary but little, those 

 of venous blood from different parts of the venous system vary to 

 a considerable degree ; and even the blood from the same vein 

 'will have a different composition at different times, as, for instance, 

 that coming from a gland when active or at rest. In general, 

 venous blood may be said to contain O from 8 to 12 per cent., and 

 CO 2 about 46 per cent. Zuntz has made many analyses, and 

 concludes, as a result, that venous blood, as compared with arterial, 

 contains 7.15 volumes per cent, less of O, and 8.2 volumes per 

 cent, of CO 2 . 



Although arterial blood contains but 21.6 per cent, of O, still 

 it can be made to take up as much as 23 per cent., which would 

 about saturate it. But even the 2J .6 per cent, is more than is 

 needed by the tissues in their metabolic processes. Unless, there- 

 fore, the blood contains less oxygen than normal, there is no advan- 

 tage to be derived from inhaling oxygen gas. If, however, the 

 venous condition is marked, then oxygen inhaled under pressure 

 may do good. While the arterial blood is nearly saturated with 

 oxygen, experiments have shown that it can take up nearly four 

 times as much CO 2 as it ordinarily contains. 



Causes of the Interchange between O and CO 2 in the 

 I/ungS. The trachea and bronchi can contain about 140 c.c. of 

 air, so that at each inspiration, when 300 c.c. or more of tidal air 

 are taken in, the difference between these two figures, 160 c.c. or 

 more, must represent the amount which passes at each inspiration 

 into the alveoli of the lungs. When expiration occurs an equal 

 volume is exhaled ; thus by the repeated alternation of inspiration 

 and expiration the air in the lungs is being constantly changed. 



But the most potent factor in bringing about this interchange is 

 the diffusion of the gases, which depends upon their partial pressure 

 i. e., the part of the total pressure of the air which is exerted by 

 each of its different components. This is also spoken of as tension 

 by some writers ; although others use the term partial pressure with 

 reference to gases in a mechanical mixture, as in atmospheric air ; 

 and that of tension with reference to gases in solution, meaning 

 thereby "the pressure required to keep the gas in solution." If 

 we regard 760 mm. of mercury as representing the pressure of 

 the atmosphere, and 20.96 as the percentage of the total volume 



20.96 X 760 

 represented by oxygen, then -,~~ ^ will equal the pressure 



exerted by the oxygen, or its partial pressure or tension, which 



0.04 X 750 

 is 1 59.29 mm. The partial pressure of the CO 2 = 



0.30 mm. If now we ascertain the partial pressure of these gases 

 in the alveoli, we shall have the principal conditions affecting their 

 diffusion. The partial pressure of O in the alveoli is estimated 

 at about 114 mm. and of GO, at 36 mm. The O then in the air 



