VARIATIONS IN TOTAL QUANTITY OF BLOOD 1131 



heart, and therefore augments both strength and frequency of the 

 beat. Thus the work of the heart is increased in three ways, viz. by : 



(1) Rise of arterial pressure. 



(2) Greater frequency of beat. 



(3) Increased output at each beat (Fig. 476). 



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 capil- 

 laries 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 greater 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 

 (haemolysis). 



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 a normal arterial 

 pressure, and so the circulation through the brain, unaltered. This 

 object is carried out 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 very 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 



