A MANUAL OF PHYSIOLOGY 



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- 



A, the blood bulb ; B, the froth 

 chamber ; C, the drying tube ; 

 D, fixed mercury tube ; 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 con- 

 tinued till a manometer connected 

 between C and 4 indicates that the 

 pressure has been sufficiently re- 

 duced. 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 im- 

 mersing the bulb A in water at 40 to 50 C. 



combined oxygen of oxyhaemoglobin, can be obtained by dissociation 

 of their compounds 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 

 exposing 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 



FIG. iQ7.'-ScHME OF GAS-PUMP. 



