THE RESPIRATORY SYSTEM. 



26l 



different methods to determine the tension of oxygen in arterial blood, 

 and obtained discordant results. 



An improved form of aerotonometer, which can be used to measure 

 the tension of oxygen in circulating blood, has been devised by Krogh, 

 and the lower part of it is shown in fig. 101. It consists of a cannula A, 

 which is attached to the central end of an artery ; the blood flows from 

 the cannula into a bulb B, from the top of which passes off a narrow 

 graduated tube. A small air bubble C is placed in the bulb, and the 

 blood flows through the bulb and is returned to the distal end of the 

 artery by the tube D, clotting being prevented by the injection of 

 hirudin. An interchange of gases takes place between the blood and 

 the bubble, and, owing to the small size and rela- 

 tively large surface of the latter, the gases in 

 the blood and the bubble soon come into equili- 

 brium, this being reached when the size of the 

 bubble remains constant; it is then withdrawn 

 into the graduated tube, and its composition is 

 analysed. 



Since the oxygen of the blood is in equili- 

 brium with that in the bubble, the tension of 

 oxygen in the blood can be ascertained by deter- 

 mining the percentage of oxygen in the bubble 

 and the total pressure to which it is exposed. If 

 the arterial pressure is 100 mm. Hg, and the 

 bubble contains 10 per cent, of oxygen, the partial 

 pressure of oxygen is 10 per cent, of 860 mm. 

 Hg (atmospheric + arterial pressure), namely 86 



mm. Hg. The tension of oxygen in the blood is thus equal to 86 mm. 

 Hg, or about 1 1 '3 per cent, of oxygen at atmospheric pressure. 



By means of this apparatus, it has been found that in animals the 

 tension of oxygen in arterial blood is distinctly less than that in alveolar 

 air, the difference being usually 1 to 3 per cent, of an atmosphere (fig. 102). 

 It has been estimated that in man the oxygen tension of venous blood 

 is about 40 mm. Hg, whereas the partial pressure of oxygen in alveolar 

 air is usually about 105 mm. Hg. Taking into account (1) the rate at 

 which oxygen can diffuse through a membrane similar to that separating 

 the blood from the alveolar air, and (2) the enormous surface area of 

 the alveoli, it has been calculated that this difference in the tension of 

 oxygen in the lungs and in the venous blood respectively is more than 

 sufficient to provide for the passage of oxygen from the alveolar air into 

 the blood by simple diffusion. 



Although this view is generally accepted, some authorities are still 



FIG. 101. Krogh's tono- 

 meter. 



