260 LOADING UP 



This volume of 4500 c.c. includes not only the volume of the 

 lungs but of the approaches to the lungs the nasal cavity, 

 the trachea, the bronchi and the bronchioles. These constitute the 

 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 2 and 

 imported O 2 . 



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 cell 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 1-617 c.c. 

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

 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 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 

 lumen of the alveoli. Various estimates have been made of the 

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

 2000 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 



