EFFECTIVE ABSORPTIVE SURFACE 261 



red blood corpuscles are distorted in their passage through them. 

 This naturally produces a decrease in the rate of blood flow. Tin- 

 rate is further decreased by the increase in the total sectional area 

 of this capillary system which is at least seven times greater than 

 that of the aorta (Chap. XXIV.). The sudden increase in the 

 area over which the blood has to spread itself in a layer less than 

 one corpuscle thick causes a marked decrease in the velocity of 

 the stream. These two conditions, (a) narrow bore and (b) in- 

 creased area of distribution, of course facilitate the processes of 

 unloading and reloading the erythrocytes. The structure is, in 

 principle, just the same as that of the kidney. 



The next problem before us is that of the transference of carbon 

 dioxide from the blood to the air and of the oxygen from the 

 alveolar air to the blood. About the first process there seems to 

 be no difficulty. Everyone is agreed that, as the tension of 

 carbon dioxide in the blood of the pulmonary artery just as it 

 enters the capillary system is greater than its tension in expired 

 air, a simple process of diffusion through a wet membrane is all 

 that is required. The tension of CO 2 in alveolar air and in the 

 blood is 40 and 46 mm. Hg. respectively. There is, therefore, a 

 difference of 6 mm. Hg. in the CO 2 pressure tending to cause a 

 flow of CO 2 from blood to air. Is this gradient of pressure 

 sufficient to account for the 250 c.c. of gas normally expired per 

 minute ? 



The passage of gas through a membrane depends (a) on the 

 nature of the membrane, (b) on the structure of the membrane, 

 (c) on the physical state of the membrane, (d) on the nature of the 

 gas, and (e) on the gradient of pressure. 



(a) The layer of flattened cells separating blood from alveolar 

 air differs little in chemical nature from any other similar structure. 

 One may note, however, its richness in lipoids. (b) It is constructed 

 of large irregular flattened cells forming an extremely delicate 

 layer as thin as the film of a soap bubble. The average thickness 

 of this membranous layer is 0-004 mm. (c) Not only does the 

 protoplasm forming the membrane contain about 90 per cent, 

 of water, but it is kept moist on both sides, (d) Carbon dioxide 

 is very soluble in water. Water at body temperature and atmos- 

 pheric pressure will absorb over half its volume of carbon dioxide. 

 (e) Experiments by Krogh and others seem to have proved beyond 

 question that the differences in tension existing on the two sides 

 of the lung tissue are quite sufficient to account for the passage of 

 the necessary volume of gas. 



