CH. XXVI.] MEASQHEMENT OF THE QUANTITY OF THE GASES 365 



into the open tube E, and the water will therefore rise in E ; but in 

 practice it is convenient to keep the gas always at the same volume ; 

 this may be done by raising the pressure in the open limb (E) of the 

 pressure gauge by squeezing some of the water, with which the 

 gauge is filled, out of a rubber reservoir (G) which forms the base of 

 the gauge, thus the level of the water in D is maintained at the zero 

 mark, while that in E rises from H to I. The actual measurement 

 then is the increase of pressure (i.e., the height of the column of 

 water H I) which is necessary to keep the gas at the same volume 

 after the oxygen or carbonic acid has been shaken off as it previously 

 occupied. From this the quantity coming off can be calculated. 



The chemical method is not quite so accurate as the vacuum 

 pump, but it is much more convenient for the study of many 

 problems, as it requires less blood, and, owing to its simplicity, 

 a great number of observations can be made upon a single animal, or 



it can be used for the systematic investigation of the blood in man. 

 / 



Relation between Quantity and Tension of Gases in Blood. 



In the preceding paragraphs the methods of measuring the tension 

 and the quantity of gas in a given sample of blood have been described. 

 It is now necessary to consider the relationship between them. 



On page 363 we have seen that for gases in solution in water, 



T 



Q = K x -p- where Q is the quantity of gas dissolved, T the tension, 



K the coefficient of solubility, and P the atmospheric pressure. 

 Since K and P are constant, it follows that Q varies directly in 

 proportion to T ; that is to say, if the tension is doubled, the quan- 

 tity of gas dissolved is also doubled ; if the tension is trebled, the 

 quantity of gas is trebled, and so on. These results might be 

 plotted out on a curve in which the quantities are placed on the 

 ordinate and the tension on the abscissa. Such a curve would give 

 the quantity of gas dissolved at any given tension, and in the case 

 of water the curve would turn out to be a straight line. 



But in the case of both the oxygen and the carbonic acid in 

 blood, the curve is not a straight line. 



Oxygen in Blood. From every 100 c.c. of arterial blood, about 

 20 c.c. of oxygen can be removed by the air-pump. Nearly all of this 

 oxygen is chemically combined with haemoglobin ; the amount in 

 actual solution in the blood is 0-7 c.c. for every 100 c.c. of blood. 

 The quantity of oxygen which 100 c.c. of blood takes up is called the 

 "oxygen capacity" which should not be confounded with the 

 "specific oxygen capacity" defined below. In normal human 

 blood, the figure is 18*5 c.c., and this forms the basis of standardisa- 

 tion of haemoglobinoineters (p. 445). 



