Microcirculation: Mechanics of Blood Flow in Capillaries

The blood, which is kept in continuous motion within the cardiovascular system, flows in a closed circuit from the heart to arteries, arterioles, cap­ illaries, venules, veins, and then back to the heart. The so-called microcir­ culation includes the smallest blood vessels in this circuit, in particular the capillaries, which range from 4 J.I. to 80 p. in different species, but for the most part are comparable with, and often somewhat smaller than, the red blood cells. The flow in these capillaries is characterized by a low Reynolds number and large flexible particles. Matter and energy exchanges take place along the vessel wall, so that the system is thermodynamically open. The final branches of the arterial tree entering the network of capillaries possess smooth muscle cells so that they in effect act as sphincters, which control the entrance diam­ eters and thus regulate flow into the capillary network. The larger blood vessels on either side of the network are capable of vasomotor readjustments. The capillary bed as a whole is, thus, a complex physical and chemical system serving both transport and exchange functions. In the present review, we have limited ourselves to the mechanics of the flow through the minute blood vessels. We shall define microcirculation as blood flow in vessels whose diameters are comparable with that of the red blood cells. First we shall review relevant concepts of the microcirculation, and then present a description of various kinds of capillary networks in order to clarify the geometrical features of the system. There will follow a discussion of the constitutive equations and boundary conditions, and solutions to several typical problems. The microcirculation can be considered a keystone of life, since it is the end organ for which the cardiovascular system was designed and it is the only place where the cells of the body have direct access to the blood. The field has been relatively neglected insofar as mechanics is considered until recent years and, in fact, only the barest impact has been made in bringing me­ chanics and physiology together.

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