CHAP, ii.] RESPIRATION. 447 



tated by the admission of a little water into Z>, the last traces of air 

 being driven over into A by the rush of vapour from the water. A 

 known volume of blood having been collected over mercury in a small 

 tube is now allowed to enter D through the tap h and yields up its 

 gases to the vacuum. A repetition 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 dif- 

 ference 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 the case when the apparatus was being originally made vacuous, but 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 /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. 



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

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

 gen 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 



