CHANGES IN AIR AND BLOOD IN RESPIRATION. 677 



globin to that of reduced hemoglobin, from a stronger to a weaker 

 acid, and this change favors the opportunity for the carbonic acid 

 to combine with the alkah of the hemoglobin. On this view the 

 hemoglobin by virtue of the alkali combined with it acts as the 

 chief carbon dioxid carrier from the tissues to the lungs. In 

 the somewhat complicated relations of the COo to the alkali 

 of the blood, the proteins of the plasma and the corpuscles, and 

 the phosphates and carbonates of the plasma and corpuscles all 

 play a role which is not entirely understood, but the above view 

 as to the way in which the hemoglobin participates in transport- 

 ing CO2 from tissues to lungs seems to have good experimental 

 support and furnishes a comprehensible explanation of the inter- 

 change which we know is taking place constantly. 



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 tliis 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 ani- 

 mal, yet, on the whole, the results tend to support the physical 

 theory. 



The Gaseous Exchange m the Lungs. — It is difficult to deter- 

 mine 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 : 



