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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 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 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.
==Sectors of microcirculation==
Microcirculation has three major sectors: pre-capillary (resistive), capillary (swap) and post-capillary (capacitive). In the pre-capillary sector, arterioles and precapillary sphincters participate. Their function is to regulate blood flow before it enters the capillaries and venules by the contraction and relaxation of the smooth muscle found on their walls. The second sector is the capillary sector, which is represented by the capillaries, where substance and gas exchange between blood and interstitial fluid takes place. Finally, the post-capillary sector is represented by the post-capillary venules, which are formed by a layer of endothelial cells that allow free movement of some substances.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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