Blood oxygen can be measured without drawing blood

        The main function of oximeter is to measure the blood oxygen saturation in human arterial blood. For healthy adults, the ideal blood oxygen saturation should be above 95%; Anything below 95% is considered low, and if it falls below 90%, it may be a health risk. The curious question is how accurate is this technique for assessing oxygen levels in blood without drawing it? Once you understand how the oximeter works, the answer becomes clear. It turns out that the red blood cells in the blood are responsible for carrying oxygen throughout the body, and the real oxygen is the hemoglobin inside the red blood cells.

        Hemoglobin is made up of four peptide chains, each of which can bind to an oxygen molecule.

        As blood flows through the lungs, hemoglobin combines with oxygen to form "oxygenated hemoglobin." These oxygen-rich red blood cells are then transported to tissues and organs throughout the body, releasing oxygen and returning to the heart through veins. At this point, hemoglobin that has lost oxygen is called "deoxygenated hemoglobin." The blood oxygen saturation measured by the oximeter is the proportion of oxygenated hemoglobin to the total hemoglobin. In the case of normal lung function and full breathing, the arterial blood that has just left the lungs should be oxygen-rich, so that almost all hemoglobin is bound to oxygen molecules, and the blood oxygen saturation should theoretically reach 100%.

        However, in practice, a variety of factors, such as mild suffocation and changes in breathing rate, can cause a decrease in blood oxygen saturation. However, for people who do not intentionally hold their breath, their blood oxygen saturation usually does not fall below 95%.

        So how do you figure out the ratio of oxygenated hemoglobin to deoxygenated hemoglobin in your blood without drawing blood? We can recall that when we use a flashlight to shine a finger at close range, the finger will appear red because the finger has different transmittance to different wavelengths of light, and the red light is absorbed less, so more light passes through.

        All substances have specific transmittance and absorbance for different wavelengths of light. In simple terms, transmittance refers to the proportion of light remaining after light passes through a substance, while absorbance is the opposite, indicating the proportion of light absorbed after entering a substance. The results show that the absorbance of oxygenated hemoglobin is lower in red band, but higher in infrared band. In contrast, deoxyhemoglobin has a higher absorbance in the red band and a lower absorbance in the infrared band. The oximeter takes advantage of this difference.

        The most common type of oximeter is the transmission oximeter, which is what we often call a clip-on oximeter. The oximeter has red and infrared light transmitters on one side of the clamp and two receivers on the other. After the receiver receives the transmitted light, the absorbance of the blood is judged according to the intensity change of the light, and the bleeding oxygen saturation is calculated. Of course, that's just the fundamentals. In fact, factors such as changes in blood flow rate due to the beating of the heart and interference with skin and muscle tissue need to be taken into account in the calculation.