BLOOD PRESSURE 137 



of carbon monoxide in the blood before and following its administration 

 we can calculate with how much blood the known inspired quantity of 

 carbon monoxide must Jiave combined. The results vary somewhat in 

 different animals; in the dog, the blood constitutes about 7.7 per cent 

 of the body weight, and in man, about 5 per cent. 



The immediate effect of hemorrhage on the blood pressure depends on 

 the rate of bleeding. If a large artery, such as the femoral, is cut across, 



Fig. 29. Effect of stimulation of the left splanchnic nerve on the arterial blood pressure. 

 Note the primary and secondary rises. 



the pressure will show an immediate but moderate fall, due largely to the 

 fact that we have suddenly decreased the peripheral resistance. If on 

 the other hand only a small artery or a vein is opened, the bleeding will 

 at first produce no effect on the blood pressure, and it is only after some 

 considerable amount of blood has been removed that it begins to fall (Fig. 

 30). To be more exact, we may state that the removal of 5 c.c. of blood per 

 kilogram of body weight does not influence the blood pressure. The re- 

 moval of a second portion of 5 c.c. per kilogram causes the blood pres- 

 sure to begin to fall, the fall of pressure for each subsequent 5 c.c. of 



