CHANGES IN AIR AND BLOOD IN RESPIRATION. 627 



equilibrium is disturbed and some of the alkali is taken from the 

 proteins and combines with the excess of CO 2 to form sodium bicar- 

 bonate. When the blood is brought under conditions of low car- 

 bon dioxid pressure, as in circulating through the lungs, the sodium 

 bicarbonate dissociates in part, giving off some CO 2 . This process 

 takes place in the blood more readily than in a simple aqueous 

 solution, because of the affinity of the protein for the base. 

 According to this view, therefore, the CO 2 is carried in the blood 

 as sodium bicarbonate, and the presence of the protein and its 

 power of combining with the sodium are used to explain the ease 

 with which the bicarbonate gives off CO 2 . It should be added 

 that this hypothesis does not meet with universal acceptance. Bohr,* 

 for example, contends that it is necessary to assume that the CO 2 

 forms a dissociable compound with the proteins of the blood, simi- 

 lar to that which he has shown to be true for hemoglobin. 



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 



* Bohr, in Nagel's "Handbuch d. Physiol. des Menschen," 1905, vol. i., 

 part i, p. 111. 



