Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body. Definition
Hypoxemia refers to the low level of oxygen in blood, and the more general term hypoxia is an abnormally low oxygen content in any tissue or organ, or the body as a whole. Hypoxemia can cause hypoxia (hypoxemic hypoxia), but hypoxia can also occur via other mechanisms, such as anemia.Hypoxemia is usually defined in terms of reduced partial pressure of oxygen (mm Hg) in arterial blood, but also in terms of reduced content of oxygen (ml oxygen per dl blood) or percentage saturation of hemoglobin (the oxygen-binding protein within red blood cells) with oxygen, which is either found singly or in combination.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, there is less agreement concerning whether the oxygen content of blood is relevant in determining hypoxemia. This definition would include oxygen carried by hemoglobin. The oxygen content of blood is thus sometimes viewed as a measure of tissue delivery rather than hypoxemia.Just as extreme hypoxia can be called anoxia, extreme hypoxemia can be called anoxemia. Signs and symptoms
In an acute context, hypoxemia can cause symptoms such as those in respiratory distress.
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.