606 PHYSIOLOGY OF RESPIRATION. 



the proteid portion (globin) of the molecule, whereas the oxygen, as 

 is well known, unites with the pigment portion, the hematin. The 

 condition of the C0 2 in the blood is therefore not so simple as that 

 of the oxygen ; but so far as the mechanism affecting the exchange 

 of CO 2 in the tissues and in the lungs is concerned it must be 

 regarded as dependent upon a reaction between the gas and the 

 alkali of the blood, influenced, as above stated, by the presence of 

 the proteid. There is apparently no specially developed organic 

 substance which acts as a carrier of C0 2 in the same way that 

 hemoglobin behaves toward oxygen. 



The Physical Theory of Respiration. The physical theory 

 of respiration assumes that the gaseous exchange in the lungs and 

 in the tissues takes place in accordance with the physical laws of 

 diffusion of gases. If a permeable membrane separates two vol- 

 umes of any gas, or two solutions of any gas at different pressures, 

 the molecules of the gas will pass through the membrane in both 

 directions until the pressure is equal on both sides. As the excess 

 of movement is from the point of higher pressure to the point of 

 lower pressure, attention is paid only to this side of the process, 

 and we say that the gas diffuses from a point of high tension to 

 one of lower tension. After equilibrium is established and the 

 pressure is the same on both sides we must imagine that the 

 diffusion is equal in both directions, and the condition is the same 

 as though there were no further diffusion. In order for this 

 theory to hold for the exchange in the body it must be shown that 

 the physical conditions are such as it demands. Numerous experi- 

 ments have been made, therefore, to determine the actual pressure 

 of the oxygen and carbon dioxid in the venous blood as com- 

 pared with the pressures of the same gases in the alveolar air, and 

 the pressures in the arterial blood as compared with those in the 

 tissues. Although the actual figures obtained have varied some- 

 what with the method used, the species or condition of the animal, 

 yet, on the whole, the results tend to support the physical theory. 



The Gaseous Exchange in the Lungs. It is impossible to 

 determine the exact composition of the alveolar air. The expired 

 air can, of course, be collected and analyzed, but obviously this is a 

 mixture of the air in the bronchi and the alveoli, and consequently 

 has more oxygen and less carbon dioxid than the air in the alveoli. 

 The probable composition of the alveolar air has been calculated by 

 Zuntz and Loewy for normal quiet breathing in the following way : 

 The capacity of the bronchial tree is 140 c.c, and this air may be 

 considered as similar in composition to atmospheric air. A normal 

 expiration contains 500 c.c; hence the alveolar air constitutes only 

 360 c.c. or y! of the entire amount. If the expired air contains 4.38 

 per cent, of C0 2 , then the alveolar air must contain 4.38 -*- -g-f ov 6 



