Recently, my doctor attached a small clip to my index finger that allowed a machine to not only measure my pulse rate but my blood gasses too. No needles were involved. How does this work? — CM, New York, New York
The red blood cells in your blood contain large amounts of a complicated and brightly colored molecule known as hemoglobin. This molecule’s ability to bind and later release oxygen molecules is what allows blood to carry oxygen efficiently throughout your body.
Each hemoglobin molecule contains four heme groups, the iron-containing structures that actually form the reversible bond with oxygen molecules and that also give the hemoglobin its color. However, this color depends on the oxidization state of the heme group—red when the heme group is binding oxygen and blue-purple when the heme group is alone. That color difference explains why someone who is holding their breath may “turn blue”—their hemoglobin is lacking in oxygen. The clip you wore was analyzing the color of your blood to determine the extent of oxygenation in its hemoglobin. It measured your pulse rate by looking for periodic fluctuations in the opacity of your finger, brought on by changes in your finger’s blood content with each heartbeat.