What Does Oxygen Saturation Measure?
Have you ever been at the doctor’s office and had them put a little clip on your finger? It’s a pulse oximeter. It measures more than one thing. One of the numbers it gives is your oxygen saturation. But what does that percentage actually mean? Is it telling you how much oxygen is in your blood? Not exactly. It’s telling you how much oxygen your hemoglobin is carrying.
You can think of hemoglobin as having spots where oxygen can attach. Some of those spots have oxygen attached and some don’t. Oxygen saturation tells you what percentage of those spots are filled.
The Percentage Shows How Full Hemoglobin Is
Hemoglobin is a protein inside your red blood cells, and one of its jobs is to carry oxygen. When oxygen moves from your lungs into your blood, it attaches to hemoglobin and gets carried through your body.
That’s why the number is shown as a percentage. It’s not saying that your blood is a certain percentage oxygen. It’s showing how full your hemoglobin is with the oxygen it can carry. That’s also what “saturation” is referring to.
The Finger Clip Estimates The Percentage
Here’s the interesting part. A pulse oximeter can estimate all of that without actually taking any blood.
It shines light through your finger and looks at how much of that light passes through. Hemoglobin with oxygen attached absorbs light differently from hemoglobin without oxygen attached. The pulse oximeter uses that difference to estimate how much of your hemoglobin is carrying oxygen.
That’s the number you may see labeled SpO₂ on the screen. The “Sp” tells you it’s an estimate from a pulse oximeter, while the “O₂” refers to oxygen.
And your finger isn’t the part the device is checking for oxygen. It’s just a convenient place for the pulse oximeter to shine light through the blood flowing there and get the information it needs.
The Other Number On The Screen Is Your Pulse
Remember how the pulse oximeter measures more than one thing? The other number you’ll commonly see is your pulse rate.
Oxygen saturation tells you how much of your hemoglobin is carrying oxygen. Your pulse rate tells you how many times your heart is beating.
The pulse oximeter can pick up both from your finger because blood flow changes slightly with each heartbeat. So that little clip is using the same blood flowing through your finger to give you two different measurements: how much oxygen your hemoglobin is carrying and how fast your heart is beating.