258 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- 

 munica,tion 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 of man the 

 following method was introduced by Haldane and Smith : 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 given concentration of it 

 in the air. Then the percentage amount to which the haemoglobin has 

 become saturated with carbon monoxide is determined 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 J c.c. of 

 oxygen, while 100 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. 259, seem to present, at first sight, 

 nothing but a welter of widely diverging and contradictory figures. 



As regards the venous blood, we have already learnt that very 

 considerable variations in the content of oxygen and of carbon 

 dioxide are associated with the varying functional activity of the 

 tissues from which the blood comes. This factor, of course, is also 

 not without influence upon the gas tensions of the venous blood. 

 The carbon dioxide tension of arterial blood is affected by variations 



