222 CIRCULATION OF THE BLOOD 



pressure of the blood column is excluded i. e., when the capillary region 

 investigated lies at the same level as the heart, amounts to about 33 mm. of 

 mercury (N. v. Kries, gums of the rabbit). Since the aortic pressure in the 

 rabbit amounts to about 100-120 mm. Hg., the capillary pressure would be 

 one-third to two-sevenths of the aortic pressure. 



Poisseuille has given the following formula for the flow of a fluid through 



7? 77 



a horizontal capillary whose wall is wet by the fluid: Q = * $ i r ^ Wftere 



Q represents the volume of fluid flowing through the capillary in the time t, 

 ! the hydraulic pressure at the beginning of the capillary, p 2 that at the end, 

 I the length, and r the radius. ^ is the constant of internal friction If all 

 the dimensions are given in millimeters and milligrams, 77 in milligrams is 

 the retarding force of the friction taking effect upon one square millimeter 

 of surface, when the difference in velocity between two adjacent layers of the 

 fluid one millimeter apart is 1 mm. per second, the change in velocity being 

 uniform. The greater the value of y (i. e., the more viscous is the fluid), the 

 less becomes the volume of fluid which will flow through the tube in unit time. 



It has been shown by B. Lewy that this law is true also for the blood. 

 At a temperature of 36-40 C. he found the mean value of 77 to be 0.00025 

 (swine, sheep), whereas the corresponding value for distilled water is 0.00007. 

 The internal friction of defibr mated blood is thus on the average 3.5 times 

 as great as distilled water. The internal friction of normal blood is some- 

 what greater. According to the researches of Hiirthle, at 37 C. it amounts 

 to 4.7 for the dog, 4.3 for the cat, 3.3 for the rabbit, that of distilled water 

 being taken as 1. Moreover the internal friction of blood varies considerably 

 - under different circumstances. It decreases after bloodletting; it is smaller 

 in starvation than after feeding; and in the dog it reaches its highest value 

 after feeding meat (Burton-Opitz). Relying upon data concerning the in- 

 ternal friction of defibrinated blood, and under certain assumptions as to the 

 length, breadth, etc., in different parts of the vascular system, B. Lewy has 

 calculated the fall in pressure in the capillaries and has found it equal to 

 20-60 mm., or by using the highest value of r; (0.00068) observed by him, 

 equal to 150 mm. of blood. 1 At the most, therefore, about one-fourteenth part 

 of the entire blood pressure is consumed in the capillaries themselves. From 

 which it follows that it is not the capillaries which constitute the chief resist- 

 ance to the blood stream, but rather the smaller arterioles central to them. 



Campbell also has reached the same view from altogether different con- 

 siderations. Among other things he emphasizes the point that if the resist- 

 ance in the capillaries were very great, so that the pressure at the beginning 

 of a capillary were much higher than at its end, the very thin capillaries 

 would be funnel shaped with the wide opening directed toward the arteries, 

 which, as we know, is not the case. 



With the help of Poisseuille's formula, and on the basis of data already 

 at hand for the internal friction of the blood, for the quantity of blood flowing 

 through the aorta, and for the pressure therein, Hiirthle has calculated the 

 absolute resistance in the aortic path (rabbit). As is evident from the formula, 



Approximately 11 mm. Hg. 



