Microcirculation

Microcirculation is the delivery of fresh blood to the smallest blood vessels, present in the vasculature embedded within organ tissues. This contrasts with macrocirculation, which transport blood to and from the organs. The vessels on the arterial side of the microcirculation are called the arterioles, which are well innervated, are surrounded by smooth muscle cells, and are 10-100 µm in diameter. Arterioles carry the blood to the capillaries, which are not innervated, have no smooth muscle, and are about 5-8 µm in diameter. Blood flows out of the capillaries into the venules, which have little smooth muscle and are 10-200 µm. The blood flows from venules into the veins. In addition to these blood vessels, the microcirculation also includes lymphatic capillaries and collecting ducts. The main functions of the microcirculation include the regulation of 1. blood flow and tissue perfusion 2. blood pressure, 3. tissue fluid (swelling or edema), 4. delivery of oxygen and other nutrients and removal of CO2 and other metabolic waste products, and 5. body temperature. The microcirculation also has an important role in inflammation.

Most vessels of the microcirculation are lined by flattened cells, the endothelium and many are surrounded by contractile cells the smooth muscle or pericytes. The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the plasma between the blood and the tissues. The endothelium also produce molecules that discourage the blood from clotting unless there is a leak. The smooth muscle cells can contract and decrease the size of the arterioles and thereby regulate blood flow and blood pressure.

Flow
Flow is determined by the diameter and the length of the vessels of the microcirculation. The Hagen–Poiseuille equation predicts the flow of blood through the vessels.

Capillary Exchange of Water
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 across capillary endothelium.

Capillary Exchange of Solutes, e.g. glucose
Small solutes move across the endothelium by passing through the spaces formed by the tight junctions formed at the edges of adjacent endothelial cells.