Hypoxia

Overview
  • Hypoxia refers to an isolated deficiency of oxygen delivery to perfused cells or tissues. It is important to emphasize that while hypoxic tissues do not receive sufficient oxygen, they do receive nutrients via blood perfusion that can be used, although inefficiently, for metabolic processes. Consequently, tissues can tolerate isolated hypoxia much better than Ischemia, defined as insufficient delivery of both oxygen and tissues due to a major insufficiency of blood perfusion.
Pathogenesis
  • As mentioned, hypoxic tissues do not receive oxygen but can utilize nutrients, such as glucose, within the blood. Consequently, hypoxic tissues can generate some ATP via anaerobic glycolysis but cannot engage cellular respiration. Anaerobic glycolysis results in the generation of Lactic Acid and when hypoxia is widespread this can lead to release of large amounts of lactic acid. In milder cases of hypoxia, the small amount of ATP produced in hypoxic tissues is enough to prevent irreversible cellular injury; however, in extreme cases, mechanisms of Cell Injury Biochemistry do occur, leading to irreversible cell death.
Etiology
  • Overview
    • Etiologies of hypoxia can be largely divided into two basic categories defined by reductions in either blood oxygen content or blood oxygen delivery; however, the special case of carbon monoxide poisoning arises due to a combination of these factors
  • Reduced Oxygen Content
    • Hypoxia may be simply due to an insufficiency in the total quantity of oxygen within the blood perfusing a tissue. This could arise from Hypoxemia, defined as an insufficient partial pressure of oxygen within arterial blood. However, arterial blood may display a normal oxygen tension but not be able to transport significant quantities of oxygen if there is little hemoglobin within the blood, as may occur in Anemia. It may be helpful at this point to highlight the distinction between "Hypoxia" and "Hypoxemia". Hypoxia arises due to reduced oxygen delivery to tissues, whereas Hypoxemia refers to reduced oxygen tension within arterial blood. Although Hypoxemia may cause of Hypoxia, it is not the only etiology.
  • Reduced Oxygen Delivery
    • Hypoxia can still occur even in contexts of normal blood oxygenation if that blood does not perfuse tissues at a sufficient rate. Reduced rates of blood tissue perfusion can lead to profound declines in tissue oxygenation that fall short of being termed outright Ischemia. Such situations are frequently encountered in states of Shock when rapid failure of the heart leads to reduced circulation of well-oxygenated blood.
  • Carbon Monoxide Poisoning
    • As described in Carbon Monoxide and Oxygen Transport, exposure of Carbon Monoxide to a volume of blood results in two major biochemical sequelae. First, the volume of blood cannot absorb and carry as much oxygen, resulting in a reduction of oxygen content. However, carbon monoxide-exposed blood also displays a higher affinity for oxygen; thus, oxygen transported to tissues is not efficiently released, resulting in a reduction in oxygen delivery. Consequently, individuals suffering from carbon monoxide poisoning can display profound isolated hypoxia in well-perfused tissues.