Response of the Microcirculation: Tissue Oxygenation
The microcirculation can be defined as the circulation between the arterioles and venules; it enables direct flow to the cells. The microcirculation is different from the macrocirculation, because it is not totally enclosed in tubular structures. The microcirculation consists of arterioles 50–100 μm in diameter. This part is under nervous and humoral control and is the main pressure-regulating system of the microcirculation. The arterioles usually terminate in the capillaries, but they can communicate directly via arteriovenous shunts to the veins. Flow is unidirectional. Capillaries 4–15 μm in diameter, have a wall thickness of one endothelial cell and no innervation. Blood flow in these is bidirectional. The postcapillary venules have a diameter of 8–30 μm followed by collecting veins with a diameter of more than 100 μm, with several layers of smooth muscle and no valves. The capillaries allow exchange of gases (O2, CO2) and metabolic substances. The microcirculation has two ways to return to the macrocirculation, one through the venous system, the other through the lymphatics. The flow in the capillaries can be directional or nonexistent, thereby influencing the exchange of oxygen and metabolic substances to the cells. It is therefore possible to have a normal macrocirculation and a disturbed microcirculation reflected in lactic acidosis and eventually cell death.
KeywordsOxygen Tension Oxygen Delivery Hemorrhagic Shock Tissue Oxygenation Tissue Oxygen Tension
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