RESPIRATION. 411 



tions to each other, are shown in Figure 198, expressed in mm. Hg. and 

 percentages of an atmosphere. 



The Mechanism of the Gaseous Exchange. In these pressure differ- 

 ences sufficient cause is found for the exchange of the gases. The oxygen 

 pressure in the alveoli being in excess of that in the blood, the gas passes 

 through the thin alveolo-capillary wall into the plasma. As the oxygen 

 pressure in the plasma rises and approximates that in the alveoli, a portion of 

 the oxygen combines with the hemoglobin until the latter is almost saturated. 

 The corpuscle is then carried through the arterial system surrounded by 

 oxygen under a definite pressure which is sufficient to keep -the absorbed 

 oxygen in union with the hemoglobin. On passing into the systemic capil- 

 laries, the blood enters a region in which the oxygen tension in the surround- 

 ing tissues is nil. At once the oxygen dissolved in the plasma passes through 

 the capillary wall into the surrounding tissue-spaces. The pressure removed 

 from the corpuscle, a dissociation of the oxygen and of the hemoglobin takes 

 place, after which the dissociated oxygen also passes through the capillary 

 wall into the surrounding lymph and so to the tissue-cells where it is stored 

 and utilized. On passing into the venous system the dissociation of the 

 oxygen and the hemoglobin is checked by the rise of oxygen pressure in the 

 plasma. On reaching the lungs the oxygen again passes into the blood 

 until the former condition is regained. 



The sojourn of the blood in the capillaries being short, the oxy hemoglo- 

 bin can part with but a portion of its oxygen, sufficient, however, to satisfy 

 the needs of the tissues. 



The carbon dioxid pressure in the tissues being in excess of that in the 

 blood, it passes through the capillary wall into the blood, where it exists in the 

 free and combined states. On passing into the pulmonic capillaries the 

 blood enters a region in which the carbon dioxid in the alveoli is less than 

 in the blood. At once a diffusion and dissociation of the carbon dioxid 

 takes place through the alveolo-capillary wall until equilibrium is established. 

 This, however, is of very short duration, for the carbon dioxid so eliminated 

 is rapidly removed from the lungs by the respiratory movements. 



While diffusion, in response to physical and chemic conditions, thus 

 plays a large part in, and is sufficient to account for, the exchanges of gases, 

 it is possible that the alveolar or respiratory epithelium may also play an 

 essential role. It is believed by some investigators that it is active in both 

 the absorption of oxygen and the excretion of carbon dioxid. This view has 

 been suggested as a means of interpreting the results of the experiments of 

 more recent investigators, made with a view of determining the tension of the 

 blood gases. It was found by Bohr that the tension of the oxygen in arterial 

 blood was often as high as 101 to 144 mm. Hg., and in many instances higher 

 than the tension of the oxygen in the trachea, while the carbon dioxid tension 

 in the trachea was higher than in the blood. Haldane and Smith by a dif- 

 ferent method found an oxygen tension in the arterial blood of 200 mm. Hg. 

 If these results should prove to be correct, though they are at present subject 

 to considerable criticism and not generally accepted, some other force than 

 diffusion would have to be found to explain the facts. It would then remain 

 to be determined in how far the alveolar epithelium could be regarded as 

 an active agent in both absorption and excretion in opposition to pressure. 



