THE VISCOSITY OF THE BLOOD IN NARROW CAPILLARY TUBES
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Nearly one hundred years have passed since the French physician Poiseuille (1) took up for consideration the important problem of the resistance of the bloodstream in the narrow parts of the vascular system. As experimental difficulties arose with blood, his fundamental investigations were contied to experiments with water and different fluids in glass capillaries w He found, as is well known, that the time of efflux of a given volume of fluid is directly as the length of the tube, inversely as the difference of pressure at the two ends and inversely as the fourth power of the diameter. During the following time the law of Poiseuille seems to have been generally accepted with regard to the blood also and the results of the first experimental studies by Ewald (2) and Benno Lewy (3) were, like those of many re-examinations, in accordance with the law in question. The now prevailing opinion is that the blood behaves as a real fluid in the vascular system with regard to its viscosity (see the survey by Neuschlosz, 4). Against this opinion, however, objections have been made from a theoretical point of view. Von Kries (5) and Hiirthle (6) have pointed out that the results of investigations of the viscosity of the blood in comparatively wide capillary tubes probably do not apply to the conditions in the narrower parts of the vascular system, whereby these authors especially seem to have had the true capillaries in view. As far as we know, the only statement in the literature regarding the viscosity of the blood being altered in narrow capillary tubes is found in the paper of Denning and Watson (7) In the narrowest capillary employed in their researches, namely, one of a diameter of 0.3 mm., they observed increased values. This result is, however, certainly erroneous, for reasons that will be given below. It is true that the viscosity of the blood has been earlier investigated in capillary tubes, where a divergence from the law of Poiseuille-corresponding to our own results-ought to appear. Thus Hess (8) has examined the viscosity of the same blood in capillaries of a diameter of 0.239 and 0.126 mm. respectively under the same conditions. A calculation on the basis