Blood inside the body has a deep red color that can appear darker or brighter depending on factors like oxygen levels. While veins carrying blood under the skin may look blue or green, this is only due to how light is refracted and absorbed rather than the actual color of the blood itself.
What gives blood its red color?
The red color of blood comes from the hemoglobin protein in red blood cells. Hemoglobin contains iron molecules that bind to oxygen, changing color from dark red when oxygen-poor to a bright cherry red when oxygen-rich. This color change allows hemoglobin to transport oxygen from the lungs to tissues throughout the body. The millions of red blood cells in each drop of blood collectively give blood its rich, red hue.
Oxygenated vs Deoxygenated Blood
The specific shade of red depends on the oxygenation level:
- Oxygenated blood leaving the lungs is a bright, cherry red.
- Deoxygenated blood returning to the lungs is a darker, burgundy red.
This color difference allows you to see the change in blood as it circulates through the body. Oxygenated blood flows from the lungs through the arteries, delivering oxygen to tissues, while deoxygenated blood flows back through the veins to pick up fresh oxygen.
Why Do Veins Look Blue?
Although blood is always red inside the body, veins often appear blue or green when visible through the skin. This is not the actual color of the blood, but rather an optical effect from how light interacts with skin and blood vessels:
- Skin absorbs more red light than blue/green light.
- Deeper veins absorb even more red light due to overlying tissue.
- The blue and green wavelengths reflect back to the eye more than red.
So even though the blood inside is red, the veins look blue or green. Pressing on a vein can make it bulge more, changing this effect and temporarily making it appear more red.
Blood Vessel Color Comparison
|Arteries||Red||Oxygenated blood flows through arteries so red color is visible|
|Veins||Blue/Green||Deeper veins absorb more red light, leaving blue/green|
What About Blood Outside the Body?
When blood leaves the body, it begins to change color due to the loss of oxygen. Here’s how its appearance evolves:
- Bright red – Fresh oxygenated blood straight from an artery.
- Dark red – Blood that has lost some oxygen but is still relatively fresh.
- Burgundy red – Blood that has lost most of its oxygen as it ages and dries.
- Dark brown/black – Very old, dried blood continues darkening due to chemical changes in hemoglobin.
The pace of these color changes depends on exposure to air and moisture. Blood may turn brown and blackish within minutes if thinly smeared and left to dry in air. A thicker pool of blood can retain a dark red color for hours before beginning to blacken.
Blood Color Variations
While red is by far the most common, blood can sometimes appear with abnormal coloring due to medical conditions or chemical exposures:
This blood disorder causes hemoglobin to lose its ability to bind and release oxygen effectively. Blood with methemoglobinemia appears chocolate brown or navy blue instead of red since the hemoglobin is altered.
Exposure to sulfur compounds like sulfur dioxide gas can convert hemoglobin into sulfhemoglobin. This causes blood to take on a greenish hue.
Carbon monoxide (CO) exposure leads to carboxyhemoglobin formation, giving blood a cherry red or bright pink color. This is because carboxyhemoglobin remains red regardless of whether it carries oxygen or not.
Summary of Key Points
- Blood gets its red color from hemoglobin in red blood cells.
- Hemoglobin changes from dark red to bright red as it binds to oxygen molecules.
- Veins often look blue/green because skin absorbs more red light.
- When blood leaves the body, it gradually darkens from red to brown/black as it deoxygenates and dries out.
- Certain conditions like methemoglobinemia can alter blood’s normal red appearance.
Blood inside the healthy human body is always some shade of red due to its millions of red blood cells containing iron-rich hemoglobin. The specific brightness and hue of this red depends on the blood’s level of oxygenation. While veins may appear blue or green when seen under the skin, this is just an optical effect caused by how light filters through tissue. Any blood freshly emerging from the body will continue to look bright or dark red until it begins to deoxygenate and chemically change color after exposure to air, eventually drying to a dark brown or black crust. So long as blood remains in active circulation within intact blood vessels, flowing to and from the lungs, expect to see that quintessential red color we associate with blood.