a6o RESPIRATION 



The balloon can be inflated so as to block the bronchus into which it 

 is passed, and cut off the corresponding portion of the lung from com- 

 munication with the outer air. A sample of the air below the block can 

 be drawn off through the inner tube, which opens free in the bronchus. 



This method has been applied both to animals and to man. In 

 observations on man the catheter was passed into the right bronchus 

 so as to occlude at will any one of the lobes of the right lung. On the 

 assumption that the gaseous exchange in the lungs depends essentially 

 on the physical process of diffusion, the occluded alveoli will correspond 

 to the gas space of an aerotonometer. When the occlusion has lasted 

 long enough for the gases in the alveoli and the blood gases to come 

 completely into equilibrium say half an hour all that is necessary is 

 to draw off the air, and from its composition to deduce the tensions in 

 the blood. Since the respiratory function of the occluded lobe is in 

 abeyance, the blood circulating in it is all unaltered-venous blood, as it 

 comes from the right ventricle, so that the gas tensions found can be 

 considered those of the mixed venous blood. 



For estimating the oxygen tension in the arterial blood a method 

 was introduced by Haldane and Smith, which differs fundamentally 

 from those described above, in that it does not depend upon the use of 

 aerotonometers. They applied it not only to animals but also to man. 

 The subject of the experiment breathes air containing a definitely 

 known very small percentage of carbon monoxide until the haemoglobin 

 has united with as much of that gas as it will take up for the give* 

 concentration of it in the air. Then the percentage amount to which 

 the haemoglobin has become saturated with carbon monoxide is deter- 

 mined in a sample of blood taken, say, from the finger. Now, the final 

 saturation with carbon monoxide of a haemoglobin solution brought 

 into contact with a gaseous mixture containing carbon monoxide and 

 oxygen, depends on the relative tensions of the two gases in the liquid. 

 But the tension of carbon monoxide in the blood leaving the lungs will 

 (after absorption has ceased) be the same as that in the inspired air. 

 Knowing this tension and the degree of saturation of the haemoglobin 

 with carbon monoxide, the oxygen tension in the blood leaving the 

 lungs i.e., in the arterial blood is known. 



Before proceeding to the consideration of the results obtained by 

 these diverse methods, it may be well to point out that when a gas 

 is stated to be under such and such a tension in the blood, no direct 

 information is given as to the quantity of gas present. For instance, 

 the oxygen tension in blood exposed to atmospheric air will be the 

 same for the erythrocytes as for the serum namely, about 160 mm. 

 of mercury; but 100 c.c. of serum will scarcely contain i c.c. of 

 oxygen, while TOO c.c. of corpuscles will have absorbed about 60 c.c. 

 of the gas. 



When we now turn to the actual blood-gas tensions obtained by 

 different observers and by different methods, these, as displayed in 

 such a table as appears on p. 261, seem to present, at first sight, 

 nothing but a welter of widely diverging and contradictory figures. 



This table, at the time it was drawn up a few years ago, represented 

 the outcome of a vast amount of work by physiologists of the greatest 

 ability, and specially skilled in such studies. To-day, the absolute 

 numbers possess little value, especially in view of the fact that most 



