THE BLOODVESSELS 73 



occurring in the general circulation, and are believed not to 

 exceed 15 to 20 mm. (| inch) of Hg. 



In the pulmonary artery the pressure is only about one-seventh 

 of that in the aorta. This pressure decreases throughout the 

 entire pulmonary channel up to the left auricle. The resistance 

 offered by the pulmonary capillaries is less than the peripheral 

 resistance experienced in the systemic circulation. 



Influence of Hemorrhage on Blood-Pressure. — The amount of 

 blood which may be removed from the body without lowering 

 the blood-pressure is surprising. This is explained by the fact 

 that the vessels adjust themselves to the reduced bulk of fluid in 

 circulation ; this adjustment is effected by means of a nervous 

 apparatus to be dealt with presently, and in this way the blood- 

 pressure is kept up. Experiments show that it is not until two- 

 fifths of the blood in the body have been removed that the 

 blood-pressure begins to fall ; after cessation of haemorrhage the 

 pressure again rises, unless the loss of blood amounts to 3 per 

 cent, of the body weight, in which case the low pressure becomes 

 dangerously permanent. 



Stewart's observations on dogs show that an animal may 

 recover even after losing more than half its blood. 



We have previously referred to the question of blood-letting ; 

 the boldness with which our forefathers used the lancet, especially 

 those who believed in a heavy bleeding to begin with, is justified 

 by the physiological evidence above mentioned — viz., that it is 

 not until about one-quarter of the blood in the body is drawn 

 off that any effect on the blood-pressure is evident. 



It is astonishing how rapidly a deficiency in the circulating 

 fluid is made good, the fact being that the tissues give up their 

 fluid in an endeavour to replace the loss of blood, quite apart 

 from the repair which is being effected through the thoracic 

 duct. It is. the loss of fluid by the tissues which causes the thirst 

 of haemorrhage. 



Hill has shown that the effect on the blood-pressure of in- 

 creasing the volume of fluid in circulation is very slight. The 

 arterial pressure and that in the venae cavae rises with the in- 

 jection, but soon falls to its old level, the injected fluid being 

 disposed of in the dilated venous reservoirs, especially those of 

 the splanchnic area. Animals which have been transfused to 

 the extent of 10 or 12 per cent, of the body weight — viz., more 

 than twice the normal amount of blood — suffer from heart 

 failure, the blood-pressure rising and falling as the overloaded 

 heart does its best to cope with the difficulty. Excessive trans- 

 fusion can best be borne if carried on slowly, in order that the 

 large abdominal veins and those of the liver may be given time 

 to accommodate the increasing bulk of fluid. 



