Blood Oxygen During Exercise

When you are exercising, the muscles that you are using require more energy. If exercise is sustained, this demand is met primarily by aerobic means. The aerobic system uses oxygen, which is transported to working muscles by your blood. Most of the oxygen transported in blood is bound to hemoglobin, which is an iron-containing protein in red blood cells. Oxygen is also transported as a dissolved gas in blood plasma, but this accounts for only approximately 1 percent of the total amount that is delivered.

OXYGEN

Oxygen is a gas that comprises 20.9 percent of atmospheric air. When we breathe, we inhale air into our lungs so that we can remove oxygen from it. Once it is transported to the muscle, this oxygen is used to combine with electrons after their movement has been harnessed for energy. Removal of oxygen from inspired air occurs because a pressure gradient exists between oxygen in air that has filled your lungs and oxygen in blood flowing through them. The pressure in the air is greater, which means that oxygen moves into your blood.

OXYGEN SATURATION

After receiving oxygen, blood leaves your lungs and returns to your heart, which propels it through your arteries to your tissues. In healthy individuals at rest, the pressure of oxygen in arterial blood is high enough to ensure that hemoglobin is 95- to 98-percent saturated. This high arterial oxygen pressure is important because it provides the driving force that pushes oxygen from hemoglobin into the tissues where it is needed. The pressure is reduced in these tissues because they are consuming oxygen to provide for energy transfer.

EXERCISE

During sustained exercise like walking, cycling and jogging, active tissues consume more oxygen than they do at rest. This means that the oxygen transported by the blood must be increased. Given that hemoglobin is already 95- to 98-percent saturated at rest, loading it with more oxygen to satisfy this increased demand is not an option. Your cardiovascular system solves this problem by increasing the rate at which it circulates blood through its vascular network. In most cases, this adjustment allows the increased oxygen demand to be met without your arterial oxygen saturation falling below the levels that are present when hemoglobin is loaded with oxygen at rest. Given that this is even the case during maximal exercise, this has been cited as evidence that the oxygen diffusion capacity of the lung typically exceeds the oxygen delivery capacity of the cardiovascular system in most healthy individuals.

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EXERCISE-INDUCED ARTERIAL HYPOXEMIA

Exercise-induced arterial hypoxemia is a reduction in arterial oxygen saturation to abnormally low levels. This can occur for a variety of reasons. Highly-trained athletes who can consume very large quantities of oxygen sometimes exhibit an appreciable fall in arterial oxygen saturation when exercising at near-maximal or maximal levels. This might be attributable to their extremely developed capacity for blood circulation and correspondingly short transit time of red blood cells through the lungs. Patients with cardiovascular and pulmonary disease might also exhibit arterial hypoxemia due to impaired pulmonary diffusion during exercise or even with mild exertion. In severe cases, supplemental oxygen is required so that these individuals can perform simple activities of daily living.