138 THE CIRCULATION OF 1HE BLOOD 



3. The Amount of Blood in the Body 



This can be altered by hemorrhage or transfusion, and the results 

 of such procedures are of interest not only on account of their physi- 

 ological bearing, but also because of their great practical importance. 



To appreciate the significance of the results, it is important to bear in 

 mind that the total volume of the ~blood constitutes from 5 to 7 per cent 

 of the weight of the animal (see page 85). 



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, 

 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. 

 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 

 blood per kilogram removed averaging about 6 mm. Hg, until after 20 

 to 25 c.c. of blood per kilogram have been removed, when a more rapid 

 fall in pressure sets in (Downs'*). When the pressure reaches the level 

 of from 20 to 30 mm. Hg, the danger limit is reached, for there now 

 supervenes a train of symptoms known as * ' shock, ' ' and the chances for the 

 animal's recovery become uncertain. That the removal of the first por- 

 tion of blood, if this removal is slow enough, does not influence the blood 

 pressure, indicates that some adjustment has occurred in the vascular 

 system to hold up the pressure in spite of the loss of blood. This adjust- 

 ment is believed to consist in vasoconstriction. 



Recovery from hemorrhage is remarkably rapid, the original volume of 

 blood being restored within a few hours. The chances of recovery de- 

 pend upon the amount of blood lost. A loss equal to 2 or 3 per cent of 

 the body weight can almost always be recovered from in laboratory ani- 

 mals, and in the case of man there is reason to believe that recovery 

 may occur after as much as 3 per cent of the body weight has been lost. 

 The recovery of blood pressure is brought about partly by a transfer 

 of fluid from the tissues to the blood. This abstraction causes a drying 

 out of the tissues, which soon excites an extreme degree of thirst. The 

 dilution of blood by fluid derived from the tissues occurs very rapidly, 

 as can be shown by comparison of the hemoglobin content, or the number 

 of blood corpuscles, in samples of blood removed immediately before 



