Red blood cells contain hemoglobin, which transports oxygen from the lungs. Arterial blood from the lungs is bright red because it carries oxygen, while venous blood returning to receive more oxygen is a darker red. The pulse oximeter uses this difference, along with the fact that arterial blood flows with a pulsating pattern, to calculate oxygen saturation. The higher the percentage displayed on the device, the more oxygen the blood is carrying to the body.
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