Exercise
Exercise-induced arterial hypoxemia occurs during exercise when a trained individual exhibits an arterial oxygen saturation below 93%. It occurs in fit, healthy individuals of varying ages and genders. Adaptations due to training include an increased cardiac output from cardiac hypertrophy, improved venous return, and metabolic vasodilation of muscles, and an increased VO2 max. 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:

Reduced capillary transit time due to an increased blood flow within the pulmonary capillary. Capillary transit time (tc), at rest is around 0.8s, allowing plenty of time for the diffusion of oxygen into the circulation and the diffusion of CO2 out of the circulation. After training, the capillary volume is still the same however cardiac output is increased, resulting in a decreased capillary transit time, reducing to around 0.16s in trained individuals at maximal work rates. This does not give sufficient time for gas diffusion and results in a hypoxemia. Intrapulmonary arteriovenous shunts are dormant capillaries within the lungs that become recruited when venous pressures become too high. They are normally located within deadspace area where gas diffusion does not occur, thus the blood passing through does not become oxygenated resulting in a hypoxemia.
Physiology
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. The arterial oxygen partial pressure is obtained directly from an arterial blood gas determination. The oxygen contained in the alveolar air can be calculated because it will be directly proportional to its fractional composition in air. Since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water. History
The term hypoxemia was originally used to describe low blood oxygen occurring at high altitudes and was defined generally as defective oxygenation of the blood.In modern times there are a lot of tools to detects hypoxemia including smartwatches. In 2022 a research has shown smartwatches can detect short-time hypoxemia as well as standard medical devices. References
Further reading


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