The availbility of oxygen plays a vital and central role in this group of conditions. Anoxia refers to a total lack of oxygen, while hypoxia refers to a partial lack or deficit. The causes of anoxia and hypoxia (please note not necessarily asphyxia) can be classified as follows:
In this group the oxygen tension in the inhaled air is too low to oxygenate the red blood cells sufficiently. This may be the result of low barometric pressure, as found at high altitude. It may also be the result of lung pathology, where the gas exchange capacity of the lungs is decreased, as seen in emphysema. (Emphysema is a swelling due to presence of air, applied especially to a morbid condition of the lungs.)
In this condition the oxygen-carrying capacity of the blood is decreased. This may be the result of a low haemoglobin (the molecule which binds the oxygen in the blood) level, also known as anaemia. Blood loss leads to acute anaemia. In some cases the haemoglobin content may be normal but the molecules cannot bind with oxygen, as another substance, such as carbon monoxide, has already bonded with the haemoglobin molecule (carbon-monoxide poison-ing).
This condition arises when the circulation cannot circulate the blood sufficiently. The most common cause is heart failure, where the heart is not contracting adequately. This may be the result of a myocardial infarct (the heart muscle degenerates because of, for example, a blood clot in a coronary artery which stops the flow of blood), high blood pressure or other cardiac disease.
In this condition the oxygen delivery to the tissues is adequate, but the tissues and the cells cannot use the oxygen. This may be due to a blockage of the enzyme systems in the cell, so that oxygen cannot be used for cell metabolism. Examples include cyanide poisoning. Carbon-monoxide poisoning causes anaemic anoxic/hypoxia, as well as an element of cytotoxic hypoxia due to the inhibition of the enzyme systems in the cells.