Hyperaemia.

Hyperaemia is the process by which the body adjusts blood flow to meet the metabolic needs of its different tissues in health and disease. Meticulous control of the microcirculation--the arterioles, capillaries and venules--is essential to life. Reactive hyperaemia, the local vasodilatation which occurs in response to oxygen debt and accumulation of metabolic waste products due to interruption of blood flow; active hyperaemia, the increased blood flow in an organ during a period of activity; and the hyperaemic response to infection and trauma are vitally important. The microcirculation is controlled partly by sympathetic vasoconstrictor impulses from the brain and partly by vasoactive substances secreted locally by the endothelial cells. The most important of the latter is nitric oxide which facilitates flow by causing relaxation of vascular smooth muscle. Neural and endothelial control of blood flow are impaired by illness. Neurological disease and vascular disease which affect the microcirculation, predispose patients to develop ischaemic organ damage, including pressure sores, during periods of intercurrent illness. Severe sepsis or trauma may cause irreversible microcirculatory dysfunction resulting in multi-organ failure and death.