THE CHEMISTRY OF RESPIRATION 



493 



blood leaving the heart, in which the tension of the oxygen is 5.3 per 

 cent. = 37.6 mm. Hg and that of carbon dioxid 6 per cent. = 46 mm. 

 Hg. If these values are now contrasted with those previously given 

 for the alveolar air, it is evident that the difference in the tension of 

 the oxygen amounts to 109 — 37, or 72 mm. Hg, and that of the carbon 

 dioxid to 46 — 40, or 6 mm. Hg. Consequently, the difference in the 

 tension of the oxygen on the two sides of the limiting membrane is 

 much greater than that of the carbon dioxid ; in either case, however, 

 it must be clear that the atoms of oxygen flow into the blood and the 

 molecules of carbon dioxid into the alveoli. 



Under normal conditions the lining cells of the alveoli and cap- 

 illaries offer no hindrance to the passage of these gases, the difference in 

 their partial pressures being sufficient to cause them to move in these 

 directions. At times, however, the orderly flow of the gases may be 

 greatly impaired by infiltrations of the lining cells or by serous mate- 

 rial exuded into the alveolar spaces in consequence of inflammatory 

 processes (pneumonia). As may readily be gathered, this difficulty 

 can be overcome in a measure by increasing the driving force behind 

 the atoms of oxygen. With this point in view, pure oxygen is some- 

 times substituted for the atmospheric air, the intention being to in- 

 crease the partial pressure of this gas so that at least a part of it will 

 be driven into the system. Obviously, pure oxygen possesses a 

 partial pressure five times greater than that of the oxygen in atmos- 

 pheric air. 



Under normal conditions, however, the diffusion of the gases in 

 the lungs is amply protected, owing to the enormous expanse of the 

 respiratory surface. Upon the basis of 700,000,000 alveoli, possessing 

 an average diameter of 0.2 mm., Zuntz^ has estimated that the 3000 c.c. 

 of stationary air are in relation with 900,000 sq. cm. or 90 sq. m. of 

 surface. 2 Thus, it will be seen that each square centimeter of alveolar 

 surface is required to supply only 0.0003 c.c. of carbon dioxid in a min- 

 ute, the total diffusion of this gas in this period of time being calculated 

 at 300 c.c, namely, at 500 c.c. of tidal air X 0.04 per cent. X 15 

 respirations. The fact that the diffusion pressure is more than 

 sufficient to furnish the required amount of oxygen, may be gathered 



1 Hermann's Handb. der Physiol., iv, 90. 



^ Aeby, Bronchialbaum der Sauget. und des Menschen, Leipzig, 1880, 90. 



