Ischemia and Hypoxia
Ischemia is insufficient blood flow to provide adequate oxygenation. This leads to tissue hypoxia (reduced oxygen) or anoxia (absence of oxygen). Ischemia results in hypoxia; however, hypoxia can occur with normal (or elevated) blood flow if, for example, the oxygen content of the arterial blood is decreased by anemia.
The most common causes of ischemia are acute arterial thrombus formation, chronic narrowing (stenosis) of a supply artery that is often caused by atherosclerotic disease, and arterial vasospasm. When blood flow is reduced to an organ, oxygen extraction increases. When the tissue is unable to extract adequate oxygen, the partial pressure of oxygen within the tissue falls (hypoxia) leading to a reduction in mitochondrial respiration and oxidative metabolism. If the hypoxic state is prolonged, cellular death may occur.
Ischemia can also lead to ischemic pain. For example, myocardial ischemia causes chest pain (angina); leg ischemia causes ischemic pain at rest and/or during physical activity (intermittant claudication).