The role of hypoxia in vascular injury and repair.

Although the terms ischemia and hypoxia are often used interchangeably, they represent distinct processes that result in different modulatory effects at the cellular level. Hypoxia is a reduction in oxygen delivery below tissue demand, whereas ischemia is a lack of perfusion, characterized not only by hypoxia but also by insufficient nutrient supply. Hypoxia can be either acute or chronic, and both are centrally regulated by hypoxia-inducible factor, a transcription factor that governs the expression of key response genes such as vascular endothelial growth factor and erythropoietin. Whereas severe chronic hypoxia can cause cell death, less-severe hypoxia can protect against subsequent damage, a phenomenon known as hypoxic conditioning. Several important processes are characterized by hypoxia, including ischemia-reperfusion, tumor growth and progression, inflammation, myocardial ischemia, and a number of ocular pathologies.