The phenomenon whereby the body experiences a decrease in oxygen below normal levels is called hypoxia; extreme cases of hypoxia lead to anoxia, a complete lack of oxygen. The difference between anoxia and asphyxia is that in asphyxia an accumulation of excess carbon dioxide (hypercapnia) takes place, as the normal exchange of oxygen and carbon dioxide in the lungs is obstructed.
Respiration is regulated in the medulla, while chemoreceptors present in the aortic arch and the carotid sinus respond to levels of oxygen, carbon dioxide, and the pH in blood and the cerebrospinal fluid. The concentration of carbon dioxide pressure in the plasma is proportional to the oxygen pressure. Generally, oxygen deprivation may be the consequence of one or more of several conditions. In all cases, damage results that leads first to hypoxia and eventually to death.
Types of oxygen deprivation. In the first condition, respiration may be slowed or stopped by injury or foreign material blocking the air passage. The most common example of this case is asphyxia that results from the inhalation of water by exhausted swimmers or persons who cannot swim. Large quantities of water fill the lungs and cut off the oxygen supply. Other examples include the entrapment of food or liquid in the respiratory tract, strangulation, and residence in high altitude. In these cases, the carbon dioxide pressure is drastically increased. Artificial respiration may save the victim’s life; it should be performed as soon as possible and after the removal of the inhaled foreign substance via vomiting. Strangulation provides the more serious problem of capillary rupturing and internal bleeding.
A second condition, hypoxic anoxia, is caused by an inadequate concentration of oxygen in the atmosphere, which occurs in poorly ventilated enclosed spaces such as mine tunnels, sewers, or industrial areas. Odorless gases such as methane (which is produced in decomposing sewage) or nitrogen may be dangerous because they generally go undetected. A former way of detecting such gases involved taking along a bird in a cage and monitoring its well-being during the exploration of unknown caves or ancient tombs.
In anemic anoxia, respiration may not be effective because of the reduced capacity of the blood to become oxygenated; as a result, less oxygen is transferred to the tissues. Carbon monoxide behaves differently from methane or nitrogen, since it binds much more strongly to hemoglobin than oxygen does. Thus the hemoglobin, which is the oxygen-carrying component of blood, does not transfer oxygen to the tissues, which are starved of it. The passage of oxygen from the lung alveoli to the adjacent blood capillaries may also be affected, such as with chronic lung disease, infections, or developmental effects.
A fourth category is stagnant anoxia, whereby a reduced flow of blood through the blood tissues takes place. This may be a generalized condition, attributable to heart disease, or localized, which may take place in a pilot during aerial maneuvers, for example. A blackout of an aviator is a result of the heart’s inability to pump enough blood to these regions against the high centrifugal force. In some cases, the carbon dioxide pressure cannot be removed in the usual manner by the lung. Any lung disease will decrease the effective removal of carbon dioxide and therefore result in elevated levels of it in the blood. Thus in emphysema, a disease in which the alveoli increase in size and which leads to a reduction of the surface area available for gas exchange, carbon dioxide will be retained in the blood. In bronchopneumonia, the alveoli contain secretions, white cells, bacteria, and fibrin, which prevent an efficient gas exchange.
In histotoxic anoxia, the failure of cellular respiration is observed. The body’s cells are unable to utilize oxygen as a result of poisoning, as from cyanide. The supply of oxygen is normal, but the cells are unable to metabolize the oxygen that is delivered to them.
Symptoms. All cases of anoxia may lead to oxygen deprivation in the brain, which may be fatal if it lasts more than a few minutes. Nerve cell degeneration may start and continue, despite the fact that the original cause of anoxia is removed and normal breathing is resumed. Many health conditions may interfere with the blood transport of oxygen, which is accomplished via the red blood cells. Such diseases include cases of anemia, trauma, hemorrhage, and circulatory disease.
The body responds to oxygen deprivation with an increase in the rate or depth of breathing. The normal, sea-level oxygen pressure of the air is approximately 160 millimeters (6.2 inches) of mercury. When the oxygen pressure is reduced to 110 millimeters (4.2 inches) of mercury at an altitude of about three thousand meters (ten thousand feet), the pulse rate increases and the volume of blood pumped from the heart also increases. Although prolonged exposure to low oxygen pressure may bring the pulse rate back to normal, the output of the heart remains elevated. Despite the lack of oxygen, both the heart and the brain function because of the dilation of their blood cells and the increased oxygen extraction from the blood. Anoxia leads to vision problems first, while hearing is generally the last sense to go. It is not unusual for a person who is suffering from anoxia to be incapable of moving but able to hear.
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