SEC. 8. CHANGES IN THE QUANTITY OF BLOOD. 



186. 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 venous 

 pressure will under all circumstances be raised with the increase of 

 fluid, but the arterial pressure will be raised in proportion only so 

 long as the elastic walls of the arterial tubes are able to exert their 

 elasticity. 



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

 tity are modified by compensatory arrangements. Thus experi- 

 ment shews that 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 1 , and remains depressed for a 

 brief period after the bleeding has ceased. In a short time how- 

 ever it 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 repaired 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-motor 

 nerves, increasing the peripheral resistance, the vaso-motor centres 

 being thrown into increased action by the diminution of their 



1 Chiefly in consequence of the free opening in the vessel, from which the bleeding 

 is going on, cutting off a great deal of the peripheral resistance, and so leading to a 

 general lowering of the blood-pressure. 



