2 7 6 



RESPIRATION 



blood pressure at the entry to the heart. The effect of forced 

 breathing was to cause a great diminution in venous blood pres- 

 sure. Thus the supply of blood to the heart must have become 

 inadequate to fill the right ventricle. Owing, however, to the 

 diminished outflow of blood from the arterial system there was 

 no fall in arterial blood pressure. It seems to be only when the 

 anoxaemia of forced breathing becomes so intense as to affect the 

 heart muscle seriously that the arterial blood pressure falls. 



Figure 70. 



Measurement of venous blood pressure by placing subject in a head-down 

 position. 



Putting all these facts together, it appears that in general the 

 circulation is so regulated as to keep the pressures of both oxygen 

 and CO 2 approximately steady in the venous blood from any 

 particular organ. The regulation is evidently of a double kind, 

 involving both oxygen and CO 2 . If the oxygen pressure goes 

 down and the CO 2 pressure also goes down, as in a pure anox- 

 aemia, there is comparatively little effect on the circulation rate, 

 because increase due to the lowered oxygen pressure is at once 

 counteracted by the effect of diminution due to the lowered CO 2 

 pressure. Similarly, in an atmosphere containing simple excess of 

 CO 2 increased circulation due to the excess of CO 2 pressure tends 

 to be counteracted by decrease due to increased oxygen pressure. 

 During muscular work, on the other hand, there is both a rise of 

 CO 2 pressure and fall of oxygen pressure, and consequently a 



