Blood in Your Veins Is Not Blue — Here's Why It's Always Red
By Marisia Fikiet, June 3, 2018 10:02am ET
Human blood is red because of the protein hemoglobin, which contains a red-colored compound called heme that's crucial for carrying oxygen through your bloodstream. Heme contains an iron atom that binds to oxygen; it's this molecule that transports oxygen from your lungs to other parts of the body.
Chemicals appear particular colors to our eyes based on the wavelengths of light they reflect. Hemoglobin bound to oxygen absorbs blue-green light, which means that it reflects red-orange light into our eyes, appearing red. That's why blood turns bright cherry red when oxygen binds to its iron. Without oxygen connected, blood is a darker red color.
https://www.livescience.com/62724-blood-in-your-veins-not-blue.html
