Condition in which tissues are starved of oxygen.
The extreme is anoxia (absence of oxygen). There are four types: hypoxemic, from low blood oxygen content (e.g., in altitude sickness ); anemic, from low blood oxygen-carrying capacity (e.g., in carbon monoxide poisoning); distributive, from low blood flow (e.g., generally in shock or locally in atherosclerosis ); and histotoxic, from poisoning (e.g., with cyanide ) that keeps cells from using oxygen. If not reversed quickly, hypoxia can lead to necrosis (tissue death), as in heart attack .