RESPIRATION 



259 



minute when resting, and as often as ten times a minute during hard 

 muscular work. According to the simplest hypothesis, and the one 

 generally accepted, the venous blood, which enters the pulmonary 

 capillaries, has a lower oxygen and higher carbon dioxide concentration 

 or pressure than that in the pulmonary air. An exchange takes place 

 between the blood and the pulmonary tissue lymph, and then be- 

 tween this fluid and the pulmonary air, this exchange being in 

 accordance with the physical laws which govern the solution and 

 diffusion of gases. 



HNITROSOBACTERIA] 

 FlG. 131. To ILLUSTRATE THE CIRCULATION OF NlTROGEN IN NATURE. 



. The Blood Gases. The presence of gases in the blood was first 

 demonstrated when Robert Boyle (1636) placed blood under his 

 vacuum pump and made it boil. The gas content of the blood is 

 obtained (1) by exposing the blood to a vacuum and pumping off 

 the gases; (2) by chemical means, whereby the different gases are 

 displaced chemically from the blood. 



Extraction of Blood Gases by Means of Pump. The general prin- 

 ciple of the pump, of which many forms have been devised for 

 the extraction of blood gases, is that the blood is exposed to a 

 barometric vacuum. A simple form of pump is shown in Fig. 132. 

 The pump consists of a mercury reservoir (A), which is connected 

 with a second reservoir (B) by means of pressure tubing. The upper 

 end of B is closed by a three-way tap. By means of this tap B can 

 be put in connection with either the tube E leading to the blood- 

 receiver F, or with the tube C leading to the eudiometer H. The 



