270 



A TEXTBOOK OF PHYSIOLOGY 



so that the exchange between the air in the bubble and the gases of 

 the blood is a rapid one. The bubble (2) can be drawn into the fine 

 calibrated tube (3) by means of the screw piston (4), and measured 

 therein. The wide part of this tube (1) is first filled with Ringer's 

 solution, and the bubble introduced into it by means of a pipette. 

 It is then drawn into (3). measured, and again returned to (1). The 

 blood is now allowed to flow through (1) for a few minutes by way 

 of (5) and (6). The bubble is once more measured in (3), and then (5) 

 is filled with potash solution, the bubble returned to (5), then to (3), 

 and again measured. Finally, (1) is filled with sodium pyrogallate 

 solution, and the manoeuvre repeated. Thus the percentage of C0 2 

 and 2 is obtained, for the potash absorbs the C0 2 and the pyro- 

 gallate 2 . From the percentage measured at atmospheric pressure 

 the partial pressures are calculated. 



14 



FIG. 141. SCHEMATIC REPRESENTATION OF KROGH'S MICROTONOMETEK. 

 Description in text. 



The invasion coefficient is the amount of gas which enters 1 square 

 centimetre of the surface in one minute at atmospheric pressure. This 

 has been measured in the case of a bubble of air by means of the micro- 

 aerotonometer. The film covering this bubble is comparable in tenuity 

 to that of the pulmonary endothelium. 



A very rough calculation of the invasion coefficient suggests that it 

 demands a difference of pressure on the two sides of the lung surface 

 of 1 mm. Hg for every 100 c.c. of oxygen absorbed by the lung per 

 minute. 



