Red blood cells carry oxygen through your body via the arteries, supplying your internal organs with oxygen, to keep them functioning normally.
Red blood cells pass through the lungs and absorb oxygen before heading back to the heart.
Red blood cells become saturated with oxygen to meet your body's oxygen needs. The normal level of saturation is 95 to 100 per cent, reports Harvard Medical School. Lower oxygen levels tax your brain and heart, making you tired and irritable, and affecting your ability to concentrate.
A pulse oximeter reading is taken via a clip attached to your fingertip. It reads a light shined through your finger---fully saturated blood cells are a different colour than less-saturated blood cells, says the Ohio State University Medical Center (OSUMC).
An arterial blood gas test takes a small amount of blood from an artery instead of a vein. Your blood sample is then checked in the lab for the presence of oxygen.
Any damage to the lungs by disease---or medical conditions such as emphysema or asthma---can cause your oxygen level to drop below 95 per cent. Your doctor may recommend oxygen therapy, says the OSUMC, via a portable or stationary oxygen source with tubing that fits in your nose.