THE GASES OF THE BLOOD 



249 



exerted directly between the liquid and the piston, is obviously equal 

 to the pressure or tension of the gas in the liquid. 



From the above principles it follows that a gas held in solution may 

 be extracted by exposure to an atmosphere in which the partial pressure 

 of the gas is made as small as possible. Thus, oxygen can be obtained 

 from liquids in which it is simply dissolved by putting them in an 

 atmosphere of hydrogen or nitrogen, in which the partial pressure of 

 oxygen is zero, or in the vacuum of an air-pump, in which it is extremely 

 small. Heat also aids the expulsion of dissolved gases. Some gases 

 held in weak chemical union, like the loosely-combined oxygen of 

 oxyhaemoglobin, can be obtained by dissociation of their compounds 



Fig. 1 1 8.- Scheme of Gas-Pump. A, the blood 

 bulb; B, the froth chamber; C, the drying tube; 

 D, fixed mercury bulb ; E, movable mercury 

 bulb connected by a flexible tube with D; F, 

 eudiometer; G, a narrow delivery tube; i, 2, 3, 4, 

 taps, 4 being a three-way tap. A is filled with 

 blood by connecting the tap i by means of a 

 tube with a bloodvessel. Taps i and 2 are then 

 closed. The rest of the apparatus from B to D is 

 now exhausted by raising E, with tap 4 turned 

 so as to- place D only in communication with G, 

 till the mercury fills D. Tap 4 is now turned so as 

 to connect C with D, and cut off G from D, and E 

 is lowered. The mercury passes out of D, and air 

 passes into it from B and C. Tap 4 is again turned 

 so as to cut off C from D and connect G and D. E 

 is raised and the mercury passes into D and forces 

 the air out through G, the end of which has not 

 hitherto been placed under F. This alternate 

 raising and lowering of E is continued till a man- 

 ometer connected between C and 4 indicates that 

 the pressure has been sufficiently reduced. The 



tap 2 is now opened ; the gases of the blood bubble up into the froth chamber, pass 

 through the drying-tube C, which is filled with pumice-stone and sulphuric acid, and 

 enter D. The end of G is placed under the eudiometer F, and by raising E, with 

 tap 4 turned so as to cut off C, the gases are forced out through G and collected 

 in F. The movements required for exhaustion can be repeated several times till 

 no more gas comes off. The escape of gas from the blood is facilitated by immersing 

 the bulb A in water at 40 to 50 C. 



when the partial pressure is reduced. More stable combinations may 

 require to be broken up by chemical agents carbonates, for instance, 

 by acids. 



Extraction of the Blood-Gases. This is best accomplished by ex- 

 posing blood to a nearly perfect vacuum. The gas-pumps which have 

 been most largely used in blood analysis are constructed on the principle 

 of the Torricellian vacuum. A diagram of a simple form of Pfliiger's 

 gas-pump is given in Fig. 118. The gases obtained are ultimately dried 

 and collected in a eudiometer, which is a graduated glass tube with its 

 mouth dipping into mercury. The carbon dioxide is estimated 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 



