Ischemia occurs when living tissue is deprived of oxygen. While any decrease in the oxygen supply to tissue can be called ischemia, in reality there is a threshold that must be crossed before true ischemia occurs. This is a result of biological processes that give living tissue a certain amount of reserve capacity. This reserve capacity differs among tissue types. For example, the arm muscles have a higher reserve capacity than the heart.
Tissue experiencing ischemia is said to be ischemic.
In the human body, oxygen supply to tissue is accomplished primarily by supplying the tissue with a continuous flow of oxygenated blood. Anything which interrupts or decreases the flow of blood thus causes ischemia. In this sense, depriving tissue of adequate blood flow is the same as depriving tissue of oxygen. Thus, ischemia in living systems is often used to describe situations of impaired blood flow.
Ischemia causes rapid damage to tissue of all types, though some tissues are more sensitive than others. Ischemia in the brain causes stroke, while ischemia in the heart leads to a heart attack (also known as a myocardial infarction).
High blood pressure makes the tissues of the circulatory system more vulnerable to ischemia (by reducing their reserve capacity) while at the same time increasing the risk that ischemia may occur (by increasing their need for oxygen).