POISEUILLE'S LAW AND THE CAPILLARY CIRCULATION

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Abstract
The diameter, length and rate of blood flow in the arterioles and capillaries of the frog''s mesentery were determined by recording cinematographically the flow of an opaque suspension introduced by intra-arterial injection into pithed Rana temporaria. Evidence is given for the assumption that in blood capillaries the deviations from the conditions required by Poiseuille''s law tend to increase the viscosity of the blood. Under these conditions Poiseuille''s equation can be used only in the limited sense of indicating the lowest possible fall in pressure that can theoretically be associated with the observed rates of capillary blood flow. By how much the true fall in pressure exceeds this calculated figure must be determined by direct observation. The average calculated pressure fall per mm. of length amounted to 3. 3 cm. water in the arterioles and to 0. 8 cm. water in the capillaries. The total fall of pressure in the entire capillary network apparently cannot be less than 1. 0-2. 0 cm. water. A comparison of these calculated figures with the pressure fall determined directly by micro-injection methods indicates that the viscosity of blood is conspicuously increased by the conditions under which blood traverses the capillary network. Poiseuille''s law cannot be applied to the flow of blood through the capillary network except in the very restricted sense mentioned above.

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