Hypoxemia (also spelled hypoxaemia) is an abnormally low level of oxygen in the blood.

[2][5] While there is general agreement that an arterial blood gas measurement which shows that the partial pressure of oxygen is lower than normal constitutes hypoxemia,[5][6][7] there is less agreement concerning whether the oxygen content of blood is relevant in determining hypoxemia.

The oxygen content of blood is thus sometimes viewed as a measure of tissue delivery rather than hypoxemia.

The compensation may cause symptoms to be overlooked initially, however, further disease or a stress such as any increase in oxygen demand may finally unmask the existing hypoxemia.

Hypoxemia is caused by five categories of etiologies: hypoventilation, ventilation/perfusion mismatch, right-to-left shunt, diffusion impairment, and low PO2.

Hypoxia occurs when the breathing center doesn't function correctly or when the signal is not appropriate: A variety of conditions that physically limit airflow can lead to hypoxemia.

Oxygen entering the lungs typically diffuses across the alveolar-capillary membrane into blood.

There must be a corresponding increase in VCO2 thus a necessity to clear the carbon dioxide to prevent a metabolic acidosis.

Hypoxemia occurs in these individuals due to increased pulmonary blood flow causing: Key to understanding whether the lung is involved in a particular case of hypoxemia is the difference between the alveolar and the arterial oxygen levels; this A-a difference is often called the A-a gradient and is normally small.

In 2022 a research has shown smartwatches can detect short-time hypoxemia as well as standard medical devices.