study this picture showing fluid compartments fo the body

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Microcirculation is the delivery of fresh blood to the smallest blood vessels, present in the vasculature embedded within organ tissues.Arterioles carry the blood to the capillaries. Blood flows out of the capillaries into the venules. Arterioles contract and relax, varying their diameter and vascular tone, as the vascular smooth muscle responds to diverse stimuli.The term capillary exchange refers to all exchanges at microcirculatory level, most of which occurs in the capillaries. Sites where material exchange occurs between the blood and tissues are the capillaries. Capillary walls allow the free flow of almost every substance in plasma except plasma protein.Diffusion is the first and most important mechanism that allows the flow of small molecules across capillaries. The process depends on the difference of gradients between the interstitium  and blood. The Starling equation is an equation that describes the roles of hydrostatic and osmotic  forces (the so-called Starling forces) in the movement of fluid acrosscapillary endothelium.