- Hypoxic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to the body as a whole. The term "hypoxic hypoxia" specifies hypoxia caused by low partial pressure of oxygen in arterial blood. In the other causes of hypoxia that follow, the partial pressure of oxygen in arterial blood is normal. Hypoxic hypoxia may be due to:
- Low partial pressure of atmospheric oxygen such as found at high altitude or by replacement of oxygen in the breathing mix either accidentally as in the modified atmosphere of a sewer or intentionally as in the recreational use of nitrous oxide.
- Low partial pressure of oxygen in the lungs when switching from inhaled anaesthesia to atmospheric air, due to the Fink effect, or diffusion hypoxia.
- A decrease in oxygen saturation of the blood caused by sleep apnea or hypopnea
- Inadequate pulmonary ventilation (e.g., in chronic obstructive pulmonary disease or respiratory arrest).
- Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is zero). Normal anatomical shunt occurs in everyone, because of the Thebesian vessels which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen.
- hypemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced.
- Hypoxia when the blood fails to deliver oxygen to target tissues.
- Carbon monoxide poisoning which inhibits the ability of hemoglobin to release the oxygen bound to it.
- Methaemoglobinaemia in which an abnormal version of hemoglobin accumulates in the blood
- Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen due to disabled oxidative phosphorylation enzymes. The effects of drinking alcoholic beverages is a common example.
- Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygenated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia, ischemic heart disease and Intrauterine hypoxia, which is an unchallenged cause of perinatal death.
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