SECRETORY HYPOTHESIS 265 



Employing the same formula as for CO 2 one finds with a differ- 

 ence of pressure of 7 -6 mm. Hg that 



0-0239 X3-8XO-139 

 v= =0-0006c.cm. 



760x5-66x0-004 



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



100 x 10,000 xO -0006 =600 c.cm. 



passing through the total effective absorptive surface of the lung. 

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

 of oxygen for ordinary purposes. As a matter of fact a difference 

 in pressure of less than four millimetres would be quite sufficient 

 to ensure the supply of the 330 c.c. per minute required by the 

 average man resting but awake. 



How is the rate of transference increased to meet the needs of 

 the man doing hard muscular work who uses up 3000 c.c. of oxygen 

 per minute ? One very obvious point of difference between a 

 resting and a working man lies in the volume of air passing into 

 the lungs per unit of time. The following table shows that the 

 ventilation of the lungs is markedly increased by the performance 

 of work. 



TABLE XLIII. 

 VENTILATION OF LUNGS. 



Litres per min. Kespir. per min. 



Resting 6-7 13-14 



Walking 24 14 



Running - 60 15 



Swimming in cold water - 90 



Running up and down stairs 



(Greatest possible effort of a 



noted swimmer) - 190 over 60 



That is, by the constant addition of fresh air to the lungs the 

 tension of oxygen in the alveoli is kept from falling. A tenfold 

 increase in ventilation provides an ample margin for even the 

 most strenuous work. 



Those who hold to the secretory hypothesis maintain that 

 while diffusion is capable of providing a sufficient oxygen supply 

 for a normal existence even with hard muscular work, yet when 

 the pressure of oxygen in the lung is brought much below normal, 

 active secretion by the lung epithelium must be brought into 

 play. Aviators, for instance, rise to great heights, and so come 

 under a low barometric pressure. 



