138 



THE CIRCULATION OF THE BLOOD 



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

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

 fall in pressure sets in (Downs' 4 ). "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. 



Starr* | n Endof 

 ifowRerrwBKemov/al 



Time in Sees 8t 

 Abscissa 



Fig. 30. The effect of rapid and slow hemorrhage on the arterial blood pressure. Between 

 the second and third pieces of tracing an interval of two minutes elapsed. 



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 



