CHANGES IN THE QUANTITY OF BLOOD. 237 



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 quantity are 

 modified by compensatory arrangements. Thus experiment shows that 

 when an animal with normal blood-pressure is bled from one carotid, the 

 pressure in the other carotid sinks so long as ihe bleeding is going on, 1 and 

 remains depressed for a brief period after the bleeding has ceased. In a 

 short time, however, it regains or nearly regains, the normal height. This 

 recovery of blood-pressure, after hemorrhage, 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 bloodvessels 

 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 vasomotor nerves, increasing the peripheral resistance, the vasomotor 

 centres being thrown into increased action by the diminution of their blood- 

 supply. When the loss of blood has gone beyond a certain limit, this vaso- 

 motor action is insufficient to compensate the diminished quantity (possibly 

 the vasomotor centres in part become exhausted), and a considerable de- 

 pression takes place ; but at this epoch the loss of blood frequently causes 

 anaemic convulsions. 



Similarly when an additional quantity of blood is injected into the vessels, 

 no marked increase of blood-pressure is observed so long as the vasomotor 

 centre in the medulla oblongata is intact. If, however, the cervical spinal 

 cord be divided previous to the injection, the pressure, which on account of 

 the removal of the medullary vasomotor centre is very low, is permanently 

 raised by the injection of blood. At each injection the pressure rises, falls 

 somewhat afterward, but eventually remains at a higher level than before. 

 This rise is stated to continue until the amount of blood in the vessels above 

 the normal quantity reaches from 2 to 3 per cent, of the body-weight, be- 

 yond which point it is said no further rise of pressure occurs. 



These facts seem to show, in the first place, that when the volume of the 

 blood is increased, compensation is effected by a lessening of the peripheral 

 resistance by means of a vaso-dilator action of the vasomotor centres, so 

 that the normal blood-pressure remains constant. They further show that a 

 much greater quantity of blood can be lodged in the bloodvessels than is 

 normally present in them. That the additional quantity injected does re- 

 main in the vessels is proved by the absence of extravasations and of any 

 considerable increase of the extra-vascular lymphatic fluids. It has already 

 been insisted that, in health, the veins and capillaries must be regarded as 

 being far from filled, for were they to receive all the blood which they can, 

 even at a low pressure, hold, the whole quantity of blood in the body would 

 be lodged in them alone. In these cases of large addition of blood the 

 extra quantity appears to be lodged in the small veins and capillaries (espe- 

 cially of the internal organs), which are abnormally distended to contain 

 the surplus. 



We learn from these facts the two practical lessons, first, that blood- 

 pressure cannot be lowered directly by bleeding, unless the quantity removed 

 be dangerously large, and secondly, that there is no necessary connection 



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. 



