Peripheral vessels dilate in response to low oxygen, whereas the vessels of the pulmonary vasculature constrict to shunt blood away from the poorly ventilated region, thereby matching ventilation to perfusion. In the second part of the review, the role of hypoxia in these pathological conditions will be described. Not only is O 2 homeostasis essential for survival, but hypoxia plays also an important role in the pathogenesis of frequent and severe pathologies including myocardial and cerebral ischemia and cancer. The first part of this review will provide a description of these systems. In addition, all nucleated cells sense O 2 concentration and respond to reduced O 2 availability acutely (within minutes) through the activation of pre-existing proteins and chronically (within hours) through the regulation of gene transcription. Sensing of increased (hyperoxia) or decreased (hypoxia) O 2 level occurs through specialized chemoreceptor cells that regulate cardiovascular and ventilatory rates. The precise establishment of these systems during development and their regulation in organisms provide the basis for oxygen homeostasis. This occurred through the evolution of a complex physiological infrastructure for O 2 delivery that includes an entry (lungs), transport vehicle (erythrocytes), a highway and secondary road system (vasculature), and a propulsion device (heart). Physiological systems have evolved to ensure the optimal oxygenation of all cells in each organism. The ability to maintain oxygen homeostasis is essential to the survival of all vertebrate species.
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