Microcirculation is an « invisible » world, representing billions of smallest
vessels and about 500m2 endothelium, the largest endocrine organ in human body. It is
organized in a fractal fashion, in essentially three vessel subtypes: mid-and small sized
arterioles (regulating vascular resistance), capillaries (<10µm) and venules which
collect blood from tissues. Microvessel structure, effector mechanisms and their
regulation are strictly adapted to function and fundamental differences exist between
the microvessel segments, as well as between the microvascular and the macrovascular
beds. This chapter aims at giving to the unaware reader a short but complete overview
of these specificities for a better understanding of forthcoming chapters, which are
devoted to describe our concept, namely that microcirculatory defects, eventually
appearing very early in life, can cause and/or aggravate insulin resistance leading to
diabetes and cardiovascular diseases some decades later.