190 THE BODY AT WORK 



observations. When, for example, it was found that the tension 

 in lymph was less than the tension in blood, the specimen of 

 lymph examined was probably not in the same condition as the 

 lymph in the tissue-spaces where the exchange occurs. The 

 experimenter in such a case was in error in supposing that the 

 specimen of lymph which he examined contained as much 

 carbonic acid as did the lymph in the tissue-spaces from which 

 the blood which he compared with it received its supply of this 



We have already given the figures for the composition of the 

 air in the air-chambers of the lungs. The figures commonly 

 accepted as correct for the percentages of the several gases in 

 the blood are, at C. and 760 millimetres of mercury 

 pressure : 



In 100 vol. of arterial blood 20 39 1-2 



In 100 vol. of venous blood . . 8-12 46 1-2 



This table shows the gain in oxygen and the loss in car- 

 bonic acid which results from the passage of blood through 

 the capillaries of the lungs. The aerated blood returned to the 

 heart by the pulmonary veins contains 8 to 12 volumes per cent. 

 more oxygen, and about 7 volumes per cent, less carbonic acid, 

 than the blood which the pulmonary artery carries to the lungs. 



As to the physics of this exchange, the air in the recesses of 

 the lungs contains about 16-36 per cent, of oxygen, and an 

 amount of carbonic acid variously estimated at from 2-57 per 

 cent, to 3-84 per cent. Of the 760 millimetres of mercury which 

 the atmosphere holds up in a barometric tube, the oxygen 



760 x 16-36 



in the alveoli of the lungs supports - - = 124-33 milli- 



100 



metres ; the carbonic acid, at the lower figure quoted (2-57 per 

 cent.), 19-5 millimetres. 



The tension of gases in arterial blood is ascertained by open- 

 ing an artery into a closed vessel which contains nitrogen mixed 

 with oxygen and carbonic acid at about the tensions which it is 

 computed that they have in the blood. If the amounts of these 

 gases are exactly right, no exchange occurs between the blood 

 and the mixture of gases. The mean of many observations 



