What do we know about human blood? Most of us would describe blood as red, warm, and an essential part of the body’s vital functions. If we lose too much blood, bad things happen. Yet, we can donate blood from one human to another. And blood makes up approximately 7% of our body’s total weight.
But what more can we say about blood? As a fluid that circulates throughout our body, blood is an essential substance that’s composed of both liquid and solid parts.
The liquid part, plasma, makes up over 50% of our blood. Meanwhile the solid part is composed of white blood cells, platelets, and red blood cells. While white blood cells fight infection and platelets form clots to stop bleeding, it’s the final element of blood — red blood cells — that may be the most important solid of them all.
The primary function of the red blood cell is to carry oxygen from the lungs to bodily tissue while removing carbon dioxide. “This gas exchange transports oxygen to cells where it’s used to make ATP (adenosine triphosphate) as energy for the cells,” says Dr. Nina Bausek, Chief Scientist at PN Medical. And when we discuss how much oxygen our red blood cells are carrying, we refer to this as blood oxygen saturation.
Blood oxygen saturation, or blood oxygen levels, can tell us a lot about our body’s current state of health. While it’s true that we can use this metric to measure the efficiency and intensity of workouts, we can also use it as an indicator of underlying health risks and disease.
But, before discussing the particulars of blood oxygen saturation, let’s take a few steps back and start with the basics. Read on to learn everything you need to know about blood oxygen saturation.
What Is Blood Oxygen Saturation?
Our red blood cells are nothing without an important protein referred to as haemoglobin. Though it may sound akin to some goblin-like creature, haemoglobin is actually a vital component of the red blood cell. Without it, our red blood cells would quickly perish. That’s because haemoglobin carries oxygen within the red blood cell, transporting it to the places our body needs it most. But not all haemoglobin is carrying oxygen — it’s also responsible for carrying carbon dioxide away from our tissue.
This fraction of oxygen-saturated haemoglobin relative to the total amount of haemoglobin is what we consider blood oxygen saturation. Our body regulates a very precise balance of oxygen in our blood that’s tailored to our current needs. This amount of oxygen will shift based on factors that include exercise, illness, and even our environment.
“When you breathe, the whole point of breathing is the gas exchange that happens in the lungs,” says Dr. Bausek. “The oxygen saturation in the blood gives you an idea of how well you’re breathing as well as how well your respiration works.”
Now that we possess a foundational understanding of what blood oxygen saturation is, let’s press on to see exactly how this component of our health is measured.
How Is Blood Oxygen Saturation Measured?
Measuring blood oxygen saturation requires a bit of sophisticated equipment. To measure blood oxygen levels, we can utilize one of two methods: an arterial blood gas or pulse oximetry.
Arterial Blood Gas
Arterial blood gas (ABG) comes in the form of a blood test. This test can measure the blood’s oxygen levels, detect the amount of other gases in the blood, and measure the blood’s pH. While ABG is a precise way to measure oxygen levels, it also has drawbacks.
According to Dr. Bausek, “It is a little more accurate to measure the arterial saturation, but it’s also more invasive. You may get the blood gas test done before an operation or if you need very accurate results,” but this procedure isn’t something your doctor would routinely perform.
To perform this test, a doctor will take a blood sample from one of the body’s many arteries. It’s important to note that a sample will never be collected from a vein because only arteries contain oxygenated blood — veins do not. Often the blood sample will be drawn from the radial artery in the wrist, and it will likely be uncomfortable, because there is a nerve beside it. Your blood will then be submitted for testing and in the final reading you may see this measurement referred to as PaO2.
If levels come back normal, no further intervention is necessary. If not, your doctor may suggest other steps to bring those levels back to normal.
The other method we can use to measure blood oxygen levels is by performing a test called pulse oximetry. To perform this noninvasive test, a small device known as a pulse oximeter will attach to either the finger, toe, or earlobe and emit an infrared light. “This will assess the viscosity of the blood to measure how much oxygen is in the blood,” notes Bausek.
A reading from a pulse oximeter will indicate what percentage of the blood is saturated with oxygen. We refer to this measurement as SpO2. While a pulse oximetry test may have a 2% window of error, it’s far easier to perform and therefore utilized more often than the ABG test.
The only problem that may arise with pulse oximetry is if the patient is wearing nail polish. This can cause the pulse oximeter to read higher or lower than it should. As a precaution, a doctor may remove nail polish before performing this test.
Pulse oximetry is so easy to perform, in fact, that pulse oximeters are available at your local drug store should your doctor recommend you monitor your blood oxygen saturation at home. Even biometric trackers now contain pulse oximeters to make the process simpler. It is wise, however, to talk with a licensed healthcare provider before tracking your own blood oxygen levels to ensure measurements and readings are accurate.
What Are Normal Blood Oxygen Saturation Levels?
Now that we’ve determined how blood oxygen saturation is measured, let’s discuss what normal levels are, and what they should be. Use the guidelines below to determine what is considered normal, and what isn’t.
Blood oxygen levels that fall between 75 and 100 mmHg (millimeters of mercury) or between 10 and 13 kP (kilopascal) are considered normal when the ABG test is performed. If a pulse oximeter is used, normal readings will fall between 95% and 100%, but this can vary for many reasons.
“Most people tend to have somewhere around 97% or 98% blood oxygen saturation,” notes Dr. Bausek. “But anything above 90% is considered fine and healthy.”
Keeping these normal ranges in mind, it should be noted that normal isn’t the same for everyone. Individuals who suffer from lung diseases such as chronic obstructive pulmonary disease (COPD) will often have low oxygen saturation levels, and their normal value will likely be lower as a result.
Below Normal Levels
A below-normal blood oxygen level is referred to as hypoxemia. Though not uncommon, hypoxemia is a serious complication because it can act as an indicator of more serious issues.
The lower the blood oxygen levels, the more severe the hypoxemia. An ABG reading below 75 mmHg / 10 kP or pulse oximeter reading below 90% is considered below normal. A doctor may intervene to diagnose the issue if low oxygen levels show up in testing, and many reasons exist that may explain low blood oxygen levels.
“The most obvious reason may be that your breathing is dysfunctional,” notes Bausek. “The gas exchange does not work very well, and you see this often with COPD patients. But after exercise the blood oxygen saturation actually drops as well, and that’s because they’ve used up more oxygen.” For this reason it’s important to determine why below normal blood oxygen saturation is occurring, though it’s inadvisable to measure these levels immediately after exercise.
Above Normal Levels
Above-normal oxygen saturation levels are far less common, and therefore, do not typically warrant concern. Should above-normal levels occur for prolonged periods of time, oxygen toxicity may occur that can spell disaster for the central nervous system.
The symptoms of too much oxygen are minimal, but they include headaches, sleepiness, and confusion. See a doctor immediately if you’re working in high altitudes or breathing pure oxygen and encountering such symptoms.
What Happens If Oxygen Levels Are Too Low?
If you detect low blood oxygen levels, this simply means you aren’t receiving enough oxygen. To combat this, a doctor or respiratory therapist may administer oxygen therapy or supplemental oxygen. This practice simply involves providing oxygen from a tank or device to elevate levels back to normal.
We can attempt to treat low oxygen levels with simpler approaches as well. “What we can do is reset the breathing pattern with deeper breaths and more regular breaths,” says Basuek of fixing low oxygen levels. “You can also do respiratory muscle training to get the respiratory muscles in the diaphragm stronger, and this means they have the ability to draw in more air quicker to optimize the gas exchange.”
What Affects Oxygen Levels?
Blood oxygen levels can be influenced by numerous factors. High altitudes that contain less oxygen, for instance, will cause levels to fall below-normal. But over time, your body will adapt.
“If you start training at altitude, the initial physiological response is that your heart rate goes up and your respiratory rate goes up,” says Bausek. “The body tries to compensate by breathing quicker and pumping out more blood. But after a few weeks the blood produces more hemoglobin to catch more oxygen.”
Inhaled carbon monoxide, on the other hand, will take the place of oxygen in haemoglobin and force oxygen levels to drop. Though inhaling carbon monoxide is uncommon, it may occur if there’s a carbon monoxide leak in your house. For this reason, we install carbon monoxide detectors.
Sleep apnea, a disorder in which breathing stops during sleep, influences blood oxygen levels. Other physiological conditions such as asthma, some heart diseases and shortness of breath can all play a role in the body’s ability to store and utilize oxygen.
What Does Blood Oxygen Saturation Really Mean?
At the end of the day, blood oxygen saturation is one of many components to consider when analyzing your health and well-being. Speak with a healthcare professional if this benchmark is abnormal or you experience any symptoms of irregular oxygen saturation levels, especially during sleep.
You can use an advanced, biometric health tracker to monitor this metric at home. Instead of using this tool to determine whether your blood oxygen saturation is normal, determine your own normal and “make sure your oxygen saturation is relatively constant,” says Bausek.
While most of us won’t need to monitor our blood oxygen saturation regularly, we can use this information to get more insight into our overall health.