The idea that deoxygenated blood is blue is a common misconception. Human blood is always red, but its precise shade changes depending on its oxygen content. This variation in color is directly tied to the function and structure of the protein responsible for transporting oxygen throughout the body. Understanding the different hues of blood requires looking closely at the molecule that makes it red.
The True Color of Oxygenated Blood
Oxygenated blood, found in the arteries and pumped from the lungs and heart, exhibits a bright scarlet or cherry-red color. This vivid hue is due to oxyhemoglobin, which is hemoglobin fully saturated with oxygen. Hemoglobin is the protein within red blood cells that contains four iron-containing heme groups.
When an oxygen molecule binds to the iron atom in each heme group, it causes a slight shift in the protein’s three-dimensional structure. This structural change alters how the molecule absorbs and reflects light. The oxyhemoglobin molecule reflects light in the scarlet-red spectrum, giving the blood its characteristic bright color. Arterial blood is typically over 95% saturated with oxygen, which maximizes this bright red appearance.
The Color of Deoxygenated Blood
The color of blood changes significantly after it delivers oxygen to the body’s tissues and begins its return trip through the veins. Once the oxygen is released from the hemoglobin molecule to the surrounding cells, the protein transforms into deoxyhemoglobin. Deoxyhemoglobin has a different molecular structure, which changes its optical properties compared to the oxygen-bound state.
This structural rearrangement causes the blood to absorb and reflect light differently, resulting in a much darker shade of red. Deoxygenated blood is more accurately described as being a dark red, maroon, or reddish-purple color. The color shift is entirely within the red spectrum, and the blood never actually becomes blue.
The difference in color is a direct result of the electronic state of the iron atom within the heme group. As the oxygen dissociates, this causes a shift in the light wavelengths that are reflected. While venous blood is referred to as deoxygenated, it still contains a considerable amount of oxygen, often maintaining a saturation level between 50% and 80%.
The Mystery of Blue Veins
The persistent idea that deoxygenated blood is blue stems from the visual appearance of veins beneath the skin. Veins often look blue or purplish, but this is an optical illusion, not a reflection of the blood’s true color. The blood inside these vessels is the same dark red color described above.
This phenomenon is caused by the way light interacts with the skin and the blood vessels beneath the surface. When light hits the skin, the longer red wavelengths penetrate deeper and are absorbed by the hemoglobin in the blood. Shorter blue light does not penetrate as deeply and is scattered and reflected back to the observer’s eye.
The depth at which veins are located beneath the skin surface, combined with the scattering effect of blue light, makes the vessels appear blue or greenish. This perception is highly dependent on skin tone and the thickness of the subcutaneous fat layer. Since arteries are generally located much deeper beneath the skin, this optical illusion does not typically affect them.