Hypoxia results from the inability of the lungs to oxygenate the blood and remove carbon dioxide sufficiently. Respiratory failure is said to have occurred if arterial pO2 and pCO2 are not held within physiological limits to satisfy tissue requirements. This results in hypoxia.
Hypoxia can occur by hypoventilation, where the volume of gas reaching the alveoli is reduced; diffusion impairment; shunt, where blood flows abnormally between two blood vessels bypassing the lungs; or ventilation-perfusion inequality.
Drowning, emphysema, collapsed lung, carbon monoxide poisoning and other causes of hypoxia reduce arterial oxygen levels by one or more of these mechanisms.