350 LOADING UP 



not resemble those of the lung. The former are deep granular 

 cells typical of secretory tissue, while the latter, like the capsule 

 of Bowman in the kidney, are thin and flat. Moreover, birds, 

 which have, of all animals, the most rapid and efficient respiratory 

 exchange and so should have a lung epithelium exhibiting marked 

 secretory qualities, have no epithelial covering at all, so that the 

 capillaries appear to be almost completely free and surrounded 

 by alveolar air. 



(2) Most modern workers maintain that just as COg diffuses 

 outwards, so does oxygen diffuse from air to blood. The whole 

 controversy turns on the existence of a pressure gradient for 

 oxygen. The earlier investigators got results which indicated 

 that the oxygen tension of the blood frequently exceeded that of 

 the alveolar air. Later workers like Douglas and Haldane dis- 

 agree with the earlier flndings, and by the employment of finer 

 technique have proved definitely that normally the tension of 

 oxygen is always less in the blood than in the alveolar air. They 

 still maintain, however, that under certain more or less abnormal 

 conditions — say, acclimatisation to high altitudes — there is an 

 active absorption and transference of oxygen to the blood on the 

 part of the pulmonary epithelium. 



A man at rest requires about 300 c.c. of oxygen per kilo of 

 body weight per hour. The average man weighs about 66 kilos, 

 i.e. 330 c.c. of oxygen must pass into his blood every minute. 

 During violent exercise the necessary intake of oxygen may be 

 as great as 3,000 c.c. per minute. In order to produce this 

 transference from air to blood a certain pressure difference is 

 necessary. 



Krogh has shown by an ingenious tonometric method that 

 the oxygen tension of the blood is always lower than the alveolar 

 oxygen tension, and the difference is generally 1 to 2 — even 3 to 4 

 — per cent, of an atmosphere. One must now consider whether 

 1 per cent., i.e. 7-6 mm. Hg, is a sufficient pressure gradient for 

 respiratory purposes. 



Employing the same formula as for COo, one finds with a differ- 

 ence of pressure of 7-6 mm. Hg that 



00239 X 3-8 X 013!> ^^ ^^^^ 



V = = 00006 c.c. 



760 X 5-66 X 0-004 



per minute per sq. cm. This gives a value of 



100 X 10,000 X OOOOG = 600 c.c. 



passing through the total effective absorptive surface of the lung. 

 Thus we see that the physical conditions allow for an ample supply 



