Microcirculation

Microcirculation is the circulation of the blood in the smallest blood vessels, present in the vasculature embedded within organ tissues. This contrasts with macrocirculation, which is the circulation of blood to and from the organs. The microcirculation is composed of terminal arterioles, capillaries, and venules that drain capillary blood. The vessels on the arterial side of the microcirculation are called the arterioles, which are well innervated, are surrounded by smooth musclecells, 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 the 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 are the delivery of oxygen and nutrients and the removal of carbon dioxide (CO₂). It also serves to regulate blood flow and tissue perfusion thereby affecting blood pressure and responses to inflammation which can include edema (swelling).

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

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