RES PI R A TION 2 5 1 



Pfliiger's gas-pump is given in Fig. 107. The gases obtained are ulti- 

 mately dried and collected in a eudiometer, which is a graduated glass 

 tube with its mouth dipping into mercury. The carbon dioxide is esti- 

 mated by introducing a little potassium hydroxide to absorb it . The 

 diminution in" the volume of the gas contained in the eudiometer 

 gives the volume of the carbon dioxide. The oxygen may be 

 estimated by putting into the eudiometer more than enough hydrogen 

 to unite with all the oxygen so as to form water, and then, after 

 reading off the volume, exploding the mixture by means of an electric 

 spark passed through two platinum wires fused into the glass. One- 

 third of the diminution of volume represents the quantity of oxygen 

 present. It can also be estimated by absorption with a solution of 

 pyrogallic acid and potassium hydroxide.* The remainder of the 

 original mixture of blood-gases, after deduction of the carbon dioxide 

 and oxygen, is put down as nitrogen (with, no doubt, a small propor- 

 tion of argon). For the sake of easy comparison, the observed 

 volume of gas is always stated in terms of its equivalent at a standard 

 pressure and temperature (760 mm., or sometimes on the Continent 

 i metre of mercury, and o C.). 



It is also possible in various ways to estimate the amount of oxygen 

 in blood without the use of the pump. Thus, since a definite volume 

 of oxygen (1*338 c.c. at o C. and 760 mm. pressure) combines with 

 a gramme of haemoglobin, we can calculate the total volume of 

 oxgyen present if we know how much of the blood-pigment is in the 

 form of oxyhaemoglobin ; and this can be determined by means of 

 the spectrophotometer. Or potassium ferricyanide may be added 

 to the blood. This expels the oxygen from its combination with the 

 haemoglobin, which then unites with an exactly equal amount of 

 oxygen obtained from the ferricyanide to form methaemoglobin 

 (Haldane) (p. 67). 



In dog's blood, which has been up to this time chiefly investi- 

 gated, there are considerable variations in the quantity of 

 oxygen and carbon dioxide which can be extracted. This 

 is particularly true of the venous blood, as might naturally be 

 expected, since even to the eye it varies greatly according to the 

 vein it is obtained from, the rapidity of the circulation, and the 

 activity of the tissues which it has just left. On the average, 



Volumes of 



O 2 CO-2 N 2 



100 volumes of arterial blood yield 20 40 1-2 



mixed venous blood (from 



right heart) yield 10-12 45-50 1-2 



(reduced to o C. and 760 mm. of mercury). 



Average venous blood contains 7 or 8 per cent, by volume 

 less oxygen, and 7 or 8 per cent, more carbon dioxide, than 

 arterial blood. Thus, in the lungs the blood gains about twice 

 as many volumes of oxygen per cent, as the air loses, and the air f 

 gains about half as many volumes of carbon dioxide per cent, 

 as the blood loses. It is easy to see that this must be so, for 



* Or an alkaline solution of sodium hydrosulphite, which is more cleanly. 



