SEC. 8. CHANGES IN THE QUANTITY OF BLOOD. 



§ 164. In an artificial scheme, changes in the total quantity of 

 fluid in circulation will have an immediate and direct effect on the 

 arterial pressure, increase of the quantity heightening and decrease 

 diminishing it. This effect will be produced partly by the pump 

 being more or less filled at each stroke, and partly by the peri- 

 pheral resistance being increased or diminished by the greater 

 or less fulness of the small peripheral channels. The pressure 

 along the whole system and hence the venous pressure will under 

 all circumstances be raised with the increase of fluid, but an 

 increase of the arterial pressure beyond that of the venous pressure 

 will be observed only so long as the elasticity of the arterial tubes 

 can be brought into play. 



In the natural circulation, the direct results of change of quan- 

 tity are modified by compensatory arrangements. Thus experi- 

 ment shews the following when an animal with normal blood 

 pressure is bled from one carotid. The pressure in the other 

 carotid sinks so long as the bleeding is going on ; this is chiefly 

 because the free opening in the vessel, from which the bleeding is 

 going on, cuts off a great deal of the peripheral resistance, and so 

 leads to a general lowering of the blood pressure. It remains 

 depressed for a brief period after the bleeding has ceased, but 

 in a short time regains or nearly regains the normal height. 

 This recovery of blood pressure, after haemorrhage, is witnessed so 

 long as the loss of blood does not amount to more than about 3 per 

 cent, of the body-weight. Beyond that, a large and frequently a 

 sudden dangerous permanent depression is observed. 



The restoration of the pressure after the cessation of the 

 bleeding is too rapid to permit us to suppose that the quantity of 

 fluid in the blood vessels is replaced by the withdrawal of lymph 

 from the extra-vascular elements of the tissues. In all probability 

 the result is gained by an increased action of the vaso-constrictor 

 nerves increasing the peripheral resistance, the vaso-motor centre 

 being thrown into increased action by the diminution of its 

 blood supply ; when the blood by ligature of the arteries in the 



