264 LOADING UP 



(1) The lungs may be considered as secretory glands. Fish 

 have a swim bladder which is, like the lungs, an outgrowth from 

 the alimentary canal. Oxygen is secreted by it so as to equalise 

 the specific gravities of fish and water. The fish may secrete 

 oxygen against the pressure produced in the bladder by immersion 

 to a great depth, e.g. against the pressure of hundreds of atmo- 

 spheres. Against this view may be opposed the histological fact 

 that cells composing the walls of the swim-bladder structurally do 

 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 CO 2 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 

 disagree with the earlier findings, 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 3000 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. 



