How quantitative studies on single capillaries help us to understand the circulation –Charles Michel (p.31)

When asked to think about the blood flow through a particular organ or tissue, I conjure up a mental picture of the microcirculation in that organ for it is here in the arterioles that blood flow is regulated and distributed and in the capillaries and venules where blood-tissue exchange occurs. Until recently most of these mental pictures were based on qualitative descriptions of microvascular flow and much of our quantitative understanding of microvascular exchange was derived from indirect measurements (such as the disappearance of tracers from blood).