[Physiological basis of the microcirculation: vascular adaptation]

The microcirculation is the functional "business end" of the cardiovascular system. In vessels with diameters below about 300 µm processes including the regulation of perfusion, exchange processes and relevant components of the immune system are localised. A large number of individual mechanisms are involved, including micro-rheology, the endothelial surface layer, vascular permeability, endothelial function, regulation of smooth muscle tone, leukocyte endothelial interaction, vascular adaptation and angiogenesis. The present article focusses mainly on the role of vascular adaptation. Much more than in large vessels, the microcirculation is characterised by constant adaptation to haemodynamic and metabolic signals. In reaction to changes in parenchymal demand, changes of the diameter of existing vessels (by changes in tone or by structural remodelling) as well as generation of new vessels (angiogenesis) or the pruning of vessels are elicited. These mechanisms are part of the so-called "angioadaptation" which is of great clinical relevance for the pathophysiological consequences of hypertension and age-related macular degeneration.