Blood is red because of a protein called hemoglobin in red blood cells. Hemoglobin contains a red-colored compound called heme, which has an iron atom that binds to oxygen. When oxygen binds to this iron, it causes the blood to reflect red light, making it appear bright red. Without oxygen, the blood appears darker red. This iron-oxygen interaction in hemoglobin is the key reason that blood is red.
Why Hemoglobin Makes Blood Red
- Hemoglobin is a protein in red blood cells responsible for carrying oxygen.
- The heme group in hemoglobin contains iron, which chemically reacts with oxygen.
- This reaction absorbs some light wavelengths and reflects red, giving blood its distinct color.
Oxygenated vs. Deoxygenated Blood Color
- Blood rich in oxygen, pumped from the lungs to the body, is bright red.
- Blood returning to the lungs with less oxygen is darker red.
- The color difference is due to how much oxygen is bound to hemoglobin.
Common Misconceptions
- Blood is never blue; the bluish appearance of veins is an optical illusion caused by light scattering through skin.
- Some animals have different oxygen-carrying molecules that make their blood other colors (e.g., blue blood in some crustaceans) but humans have red blood due to hemoglobin.
This explains why human blood is consistently red and changes shade depending on oxygen levels.
