CHAP. IL] RESPIRATION. 557 



of the processes by which the air in D was originally extracted will now 

 remove the gases which have been given off from the known volume of 

 blood, the only difference being that now the tube /filled with mercury 

 is inverted in the trough e over the upper end of the tube b. In this 

 way the gases originally in D are not allowed to escape into the air, as 

 'was the case when the apparatus was being originally made vacuous, 

 but are collected in f for subsequent analysis. During the extraction 

 of the gases from the blood the bulb D is immersed in a vessel of warm 

 water, to facilitate the exit of the gases and, by causing the formation 

 of large quantities of aqueous vapour, to sweep the gases rapidly over 

 into A. The sulphuric acid chamber C dries the vacuum before the 

 admission of the blood into D, and hence makes it more perfect and 

 causes the most complete and rapid evolution of gases from the blood. 



The average composition of the gas thus obtained from each of 

 the two kinds of blood (the arterial blood being taken from a large 

 artery, and the venous blood from the right side of the heart) is, 

 stated in round numbers, as follows : 



From 100 vols. may be obtained 



Of oxygen, of carbonic acid, of nitrogen. 



Of Arterial Blood, 20 vols. 40 vols. 1 to 2 vols. 



Of Venous Blood, 8 to 12 vols. 46 vols. 1 to 2 vols. 



all measured at 760 mm. and C. 



That is to say, venous blood, as compared with arterial blood, 

 contains 8 to 12 p.c. less oxygen and 6 p.c. more carbonic acid. It 

 must be remembered, however, that while arterial blood from 

 whatever artery taken has always nearly the same proportion 

 of gases, or at all events the same amount of oxygen, the amount 

 of oxygen in venous blood, even when taken from the same vein, 

 may vary a good deal, still more so when it is taken from different 

 veins. The reason of this we shall see hereafter. 



It will be convenient to consider the relations of each of these 

 gases separately. 



The relations of Oxygen in the Blood. 



343. When a liquid such as water is exposed to an 

 atmosphere containing a gas such as oxygen, some of the oxygen 

 will be dissolved in the water, that is to say, will be absorbed from 

 the atmosphere. The quantity which is so absorbed will depend 

 on the pressure of the oxygen in the atmosphere above ; the 

 greater the pressure of the oxygen, the larger the amount which 

 will be absorbed. If the pressure of the whole atmosphere remain 

 the same, at 760 mm. of mercury for instance (the ordinary at- 

 mospheric pressure), the pressure of the oxygen may be increased 

 or diminished by increasing or diminishing the proportion of 

 oxygen in the atmosphere. So that with an atmosphere remain- 



