168 THE VASCULAR SYSTEM 



flow 250 per cent, and pressure 1] per cent., so great is the 

 effect of vaso-dilatation coupled with increased cardiac efficiency 

 (Brodie and Miiller). 



In polycythaemia, where the blood is more than doubled in 

 quantity and the number of corpuscles twelve million instead of 

 five, there are no signs of high pressure, hypertrophy of heart, &c. 

 The heart continues to circulate about the normal amount, and 

 the excess is stored away in the peripheral capillary-venous areas. 

 Increased viscosity is compensated for easily by dilatation of the 

 arterioles. The toxic conditions which arise in cases of poly- 

 cythaemia are probably due in part to the long time much of the 

 blood must take to complete the circulation. Thrombosis also 

 results in the cerebral veins. The viscosity of the blood may be 

 increased nearly 100 per cent, by stasis produced by confining it 

 within a limb for twenty minutes by means of a rubber bandage. 

 The increase is due to the passage of blood fluid into the tissues. 

 In cholera and polycythsemia the viscosity co-efficient has been 

 observed to be as high as 20 in place of 4'8, the normal worth. 

 A current of carbonic acid led through the blood, it is said, in- 

 creases the viscosity 50 per cent. The maximal effect of hard 

 work or Turkish baths and sweating is an increase of 20 to 30 

 per cent. 



HYDB^EMIA 



Extraordinary amounts of physiological saline can be infused 

 without harm up to 60 to 70 per cent, of the body weight ! No 

 bad after effects follow the injection of threefold the fatal amount 

 of blood. The arterial pressure is not notably altered, and the 

 vena cava pressure, while rising by some 200 mm. of blood during 

 the injection, soon returns again to the old level. The velocity of 

 flow in the capillaries is greatly increased, as may be seen in the 

 frog's web, owing to the dilated arterioles and lessened viscosity. 

 The output of the heart is immediately increased, and the heart 

 shows no sign of failure. Thus Tigerstedt found on injecting into 

 the rabbit 25 c.c. Ringer sol. (period of injection marked by 

 vertical lines) : 



Arterial pressure . 90 

 Minute volume per kg. 51 



falling in a few minutes to ^' 



96 99 97 



64 78 100 



87 92 89 

 95 122 122 



