Hypoxia can be defined as a typical pathological process characterized by insufficiency of biological oxidation that leads to disorders of energy balance and turnover in the cell. Hypoxia may be caused by exogenous and endogenous factors (fig. 11).
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Fig. 11. Types of hypoxia
EXOGENOUS HYPOXIA
Exogenous hypoxia arises from a decrease in the partial oxygen pressure in the inhaled air and therefore it can result from a reduction in the barometric pressure and a percentage of oxygen in the air as well. The former is called hypobaric hypoxia and the latter is called normobaric. The hallmark of exogenous hypoxia is a decrease in the partial oxygen tension (PatmO2) in the inhaled air.
Exogenous hypobaric hypoxia results from the inhalation of the air containing oxygen at subnormal pressure. For example, on ascent to a high altitude the percentage composition of the air does not change, but the oxygen partial pressure decreases. Exogenous normobaric hypoxia can occur when oxygen is displaced by the inert or anesthetic gases such as nitrous oxide and ethylene, or oxygen is consumed by combustion in a closed space.
Exogenous hypoxia is accompanied by a decrease in the partial alveolar oxygen pressure and the arterial carbon dioxide tension in the arterial blood. Hypocapnia is the result of the hyperventilation that develops as an adaptive response to falling arterial oxygen tension.
RESPIRATORY HYPOXIA
Respiratory hypoxia may be induced by one of three pathophysiological mechanisms that impair gas exchange in the lungs. They include: alveolar hypoventilation, diffusion limitation in the alveolar-capillary membrane and ventilation-perfusion mismatching.
The hallmark of respiratory hypoxia is a reduction of the partial arterial oxygen tension (PaO2) in normal (PatmO2). Carbon dioxide tension in the blood depends on the variant of respiratory hypoxia.