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(oxygen starvation, oxygen deficiency), the lowering of the oxygen content of the tissues. The pathological condition that arises during hypoxia results from the tissues’ receiving a smaller supply of oxygen than they require (in cases of hypoxemia—lowered oxygen content of the blood). It may also arise when the tissues’ capacity to use oxygen is less than their need for oxygen. Irreversible changes take place in vital organs as a result of hypoxia. The central nervous system, heart muscle, and kidney and liver tissues are the most sensitive to oxygen deficiency.
Various hypoxic states are distinguished according to the classification adopted in 1949 at a conference on hypoxia in Kiev. Hypoxic hypoxia is a form of oxygen deficiency caused by a decrease in the oxygen content of inhaled air (for example, when climbing a mountain), by the difficulty of oxygen penetrating into the blood from the lungs because they are diseased and the air passages are obstructed, and by respiratory disturbances. Hemic hypoxia—which arises when the amount of hemoglobin capable of incorporating oxygen is reduced (decreased oxygen capacity of the blood)—develops after loss of blood, carbon monoxide poisoning, and exposure to radiation. Circulatory hypoxia occurs in cases of circulatory disorders that reduce the amount of blood flowing to the tissues per unit of time. Tissue hypoxia is a form of oxygen deficiency that is associated with changes in the activity of the respiratory enzymes. As a result of these changes the tissues are prevented from using the oxygen present in the blood that bathes them. (Tissue hypoxia develops in cases of disturbances of vitamin metabolism and in certain types of poisoning, such as cyanide.)
Hypoxia may be acute or chronic, depending on the rate at which it intensifies. In acute hypoxia, the functioning of the higher sections of the central nervous system is the first to be affected. In chronic hypoxia, the first to be affected is the functioning of the cardiovascular, respiratory, and circulatory systems. Resistance to hypoxia can be increased by training in a pressure chamber or in the mountains. Under these conditions the body develops a number of adaptive mechanisms, including reflex intensification of respiration and blood circulation, increase in the number of red blood cells as a result of their escaping from the blood depots, and increased hemoglobin content of red blood cells. These adaptations improve the individual’s sense of well-being and increase his work capacity under conditions of oxygen deficiency. It has been found that increased resistance to a given injurious factor simultaneously increases resistance to other adverse influences. For example, as the body becomes more resistant to acute hypoxia, it becomes more resistant to acceleration, ionizing radiation, heat, and great physical exertion.
REFERENCEBarbashova, Z. I. Akklimatizatsiia k gipoksii i ee fiziologicheskie mekhanizmy. Moscow-Leningrad, 1960.
Petrov, I. R. Kislorodnoe golodanie golovnogo mozga. [Leningrad] 1949.
N. A. AGADZHANIAN