Blood-Oxygen Saturation
Philips Medical Systems notes that pulse oximeters work to measure the blood-oxygen saturation in a patient. This is a measure of how much oxygen is in the arterial blood. Oxygen is carried in the blood by a protein called hemoglobin, which can be found in red blood cells. Hemoglobin exists in two distinct states: oxyhemoglobin (hemoglobin bound to oxygen) and deoxyhemoglobin (hemoglobin that does not have oxygen). Blood-oxygen saturation is a measure of the percentage of the hemoglobin molecules that are bound to oxygen. In healthy patients, blood-oxygen saturation will be between 94 and 100 percent.
Oximetry
Pulse oximeters determine blood-oxygen saturation by taking advantage of the fact that hemoglobin is red when it is bound to oxygen and blue when it is not. Pulseox.info points out that pulse oximeters contain two small diodes that emit light. One diode emits red light and the other one emits infrared light, which is invisible to the human eye. This light is then shone on a narrow portion of the body, such as the tip of a finger. Some of the light will pass all the way through the tissue. The pulse oximeter also has a light sensor that will then detect the amount of light (both red and infrared) that passes through the tissue.
The Role of the Pulse
Pulse oximeters measure the blood-oxygen saturation by detecting changes in the amount of red and infrared light that pass through the skin over time. Because there are a lot of things in the tissue that can absorb the light, the oximeter measures the change in light transmission when the arteries are full of blood and when they are (mostly) empty. This is why the device is called a pulse oximeter. By comparing the light that gets transmitted through the tissue when the arteries are full and empty, it can measure the effects of just the blood. By comparing the amount of red and infrared light that passes through the blood, pulse oximeters can determine how much of the hemoglobin is bound to oxygen.
