VARIATIONS IN TOTAL QUANTITY OF BLOOD 1011 



These series of changes result in the relief of the vascular system. The 

 heightened pressure in the abdominal veins and capillaries causes a great 

 leakage of fluid in the form of lymph from the capillaries of the intestines and 

 liver, while the increased pressure and velocity of the blood in the glomeruli of 

 the kidney induce a copious secretion of urine, so that within a couple of hours 

 after the injection of salt solution the volume of the circulating fluid may 

 have returned to normal. 



This recovery is effected with geater difficulty if the plethora has been 

 brought about by the injection of defibrinated blood, since this fluid cannot 

 escape rapidly from the capillaries, nor can it be excreted unchanged by the 

 kidneys. Hence it is easy to kill an animal by wearing out its heart, if too 

 large quantities of defibrinated blood be injected. The ultimate fate of the 

 injected blood is to be used as food by the tissues, and to be eliminated by 

 the ordinary channels. 



It must be remembered that the blood-serum of one animal is often poisonous for 

 the corpuscles of another. Thus a few cubic centimetres of dog's serum injected into 

 the peritoneal cavity of a rabbit will cause death. This poisonous action is also shown 

 by mixing dog's serum with defibrinated rabbit's blood, in which case the red corpuscles 

 of the latter are broken up, setting free haemoglobin (hcemolysis). 



THE EFFECTS OF HAEMORRHAGE. ANEMIA 



Any diminution of the total volume of the blood, as by bleeding, would 

 tend to lower the pressure on both sides of the system. The vaso-motor 

 centre, however, strives to maintain the normal arterial pressure, and so the 

 circulation through the brain, unaltered. This object is attained by a 

 general vascular constriction, which diminishes the total capacity of the 

 system and alters the distribution of pressures throughout the system, so 

 as to keep the blood as much as possible on the arterial side. Thus a slight 

 loss of blood has no influence on the arterial blood-pressure, but causes a fall 

 of pressure in the veins, blanching of the abdominal organs, and diminished 

 flow of urine. The heart beats more frequently, and so aids in emptying the 

 venous into the arterial system. 



The deficiency of circulating fluid caused by bleeding is soon remedied by 

 a transfer of fluid from the tissues to the blood. This transfer is independent 

 of the flow of lymph from the thoracic duct into the blood, and is the direct 

 consequence of the universal fall of capillary pressure which results from 

 the bleeding. The abstraction of fluid from the tissues is responsible for 

 the extreme thirst which is the result of hemorrhage, and which directs the 

 animal to take up by the alimentary canal the fluid which is wanting to the 

 body. The transfer of fluid from tissues to blood is extremely rapid ; even 

 during the course of a bleeding it is found that the later samples of blood are 

 more dilute than those obtained at the beginning. This mechanism suffices 

 only to make up the supply of circulating fluid. After a 'bleeding, however, 

 an animal has lost proteins and blood-corpuscles, and these constituents of 

 the blood are but slowly restored, the former directly from the food, the latter 

 by an increased activity of the blood-forming cells in the red marrow. 



