139 



BLOOD PRESSURES IN MAN, NORMAL AND PATHOLOGICAL 331 



ordinarily measured having occurred before the capillary region with 

 its apparently low pressures is reached. 



On the other hand there is to be considered the active and strong 

 contractiUty of capillaries as shown by Krogh (76) and others; Lewis 

 (82) estimated their contractile power as being capable of expelling 

 fluid against a pressure of 50 to 60 mm. Hg, and when contracted, of 

 resisting the entry of fluid up to 90 to 100 mm. Hg. Excessive con- 

 striction or closing of an unusually large proportion of the capillary 

 tubes, with diminution of the sectional area of the available capillary 

 bed, must necessarily affect the resistance offered to the outflow from 

 the arterial tree, as well as influencing the capacity of the vascular 

 system, especially in view of the fact that in small capillaries the red 

 corpuscles actually rub against the walls of the tube; it is not simply 

 a matter of internal friction between the layers of the moving blood 

 as in the arteries. 



Conditions that might reduce the disparity between the sectional 

 area of the arterial and capillary fields (e.g., closure of many capil- 

 laries, arterial dilatation etc.) would naturally tend to enhance in some 

 measure the resistance presented in the capillaries. Again, Lombard 

 estimated the fall of pressure between the small arteries and the veins 

 at 40 to 50 mm. Hg which would postulate a resistance in the capil- 

 laries nearly as great as that in the small arteries. But the recent evi- 

 dence favours low values of capillary pressure, involving a great fall 

 from the pressure in the larger arteries (e.g., 120 mm.) before the 

 capillary field is reached with pressures estimated at one-sixth or one- 

 tenth or even much less — indicating the situation of the main resistance 

 in the circulatory system as being in the small arteries and arterioles. 



On the other hand if higher estimates of capillary pressure are cor- 

 rect, especially such as have been reported in some diseased conditions, 

 with a large decline from capillary to venous pressure it is evident 

 that a considerable part of the peripheral resistance must be located in 

 the region of the capillaries and venules. With regard to the possible 

 influence of constriction of the venules, such might obviously have im- 

 portant effects. 



Venous pressure. Venous pressures, easily measured in the veins 

 of the arm or hand by the method of Hooker (67), have been found 

 by that observer to be usually between 10 and 20 mm. H2O, progres- 

 sively increasing with age from 8 cm. in early youth to 25 cm. in old 

 age. Eyster and Middleton (42) report pressures rarely above 11 cm. 

 normally; Briscoe about the same, 11.4 cm. White (132), using a 



