EFFECTIVE ABSORPTIVE SURFACE 347 



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 COg 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 CO2 pressure tending to cause a flow 

 of CO2 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 two layers of flattened cells separating blood from 

 alveolar air differ little in chemical nature from any other similar 

 structure. Ofie may note, however, their richness m lipoids, 

 {b) They are constructed of large irregular flattened cells forming 

 an extremely delicate layer as thin as the film of a soap bubble. 

 The average thickness of the membranous layer is 0-004 mm. 

 (c) Not only does the protoplasm forming the membrane contain 

 about 90 per cent, of water dispersed through it, but its surface is 

 kept moist on both sides, (d) Carbon-dioxide is very soluble in 

 water, and more soluble in lipoid. Water at body temperature 

 and atmospheric 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. 



It is worth while to look a little more closely at this problem. 

 In Chap. XXIII. is given a table (XLV.) of absorption coefficients 

 of the respiratory gases. These values of a indicate the volumes 

 of gas at N.T.P. which will dissolve in 1 c.c. of water. Later in 

 the same chapter, figures which hardly differ from a were given 

 for the solubility of these gases in plasma, etc. The velocity of 

 diffusion depends not merely on the pressure gradient and on the 



