Functional Capillary Density: Active and Passive Determinants

Capillaries have long been envisioned as relatively stable conduits connecting arteries and veins. However, this view is rapidly changing. As the primary effectors of tissue oxygenation, capillaries are now seen as entities with autonomous properties, and it is becoming increasingly apparent that the number of capillaries that sustain red blood cell flow is an important variable in both physiological and pathophysiological situations. This publication reports our current understanding of the mechanisms which control "functional capillary density" and therefore affect tissue perfusion at the level of single capillaries. Blood flow in capillaries is shown to be a dynamic process determined by biologically active controls in the endothelium, as well as by factors related to tissue mechanics and blood viscosity. Surgeons, anaesthesiologists and intensive care physicians confronted with the problem of tissue perfusion and ischaemia as well as basic scientists interested in the regulation of tissue oxygenation should appreciate the up-to-date information provided by this issue.