346 LOADING UP 



outer harbour or '• dead space," which has a capacity of about 

 140 c.c. That is, in ordinary quiet breathing each respiration 

 brings about 360 c.c. of air into the inner harbours (air sacs, 

 alveolar sacs, or infundibula). These funnel-like chambers to 

 which the air passages lead are the most expansile structures in 

 the lung, and they are largest where the expansion of the lung is 

 greatest. All round their walls open myriads of small thin-walled 

 air-cells or alveoli — the true wharves of the port. There and 

 there alone takes place the interchange of exported CO., and 

 imported Og. 



Let us look first at the area of wharfage. The interior of the 

 air sacs and their alveoli is lined by a thin transparent layer of 

 endothelium. If the lining could be stripped from all the sacs 

 of both lungs and inflated, it would form a spherical balloon about 

 17 feet in diameter. If it were spread as a continuous flat sheet 

 it would cover a square floor of 30 feet by 30 feet. In other words, 

 the area of wharfage is, at least, over fifty times the surface area 

 of the body. The average diameter of an air sac is 0'2 mm., 

 with a volume of 0*004 cub. mm., and an area of 0*125 sq. mm. 

 Suppose these air cells to be spherical and closely packed together, 

 then the maximum number contained in a cubic millimetre of lung 

 substance would be 250 cells of total surface 31*2 sq. mm. Now 

 the average value for the total volume of lung substance is 1617 c.c. 

 This provides for the possible presence of 404,500,000 air sacs, 

 with a surface of 50*56 square metres. Of course this is a maximum 

 value for the number. From the volume of lung substance has 

 to be deducted the volume of the supporting cells of the lung, of 

 the blood vessels, and of the air passages. On the other hand, a 

 minimal value is given for area, since no account is taken of the 

 increase of surface caused by the projection of the blood capillaries 

 into the lumina of the alveoli. Various estimates have been made 

 of the surface area of the alveoli, ranging from that of von Huschke 

 of 2,000 sq. metres to that of Aeby given above. Hufner's value 

 is generally taken as a mean, viz. 140 sq. metres. Of this area, 

 about three-fourths consists of thin-walled capillary blood vessels. 

 That is, the effective absorptive surface is about 100 sq. metres. 



Over a surface of about 100 sq. metres, interchange between 

 alveolar air and blood is possible. Just behind this surface- 

 epithelium lie capillary blood vessels of such small bore that the 

 red blood corpuscles are distorted in their passage through them. 

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

 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. XXV.). The sudden increase in the 



