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.