This Is Why Human Blood Is Red

If veins are blue, why is blood red?

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The fake kind is used in scary movies and Halloween decorations. The real kind is as essential as water for giving and maintaining life. That’s right, we’re talking about blood, and this vital bodily fluid deserves some much needed appreciation.

We could go on and on about blood’s impressive resume—you know, the fact that it disperses oxygen to your lungs and tissues, carries cells and antibodies throughout the body to fight infection, and clots when you have a cut (no big deal). But today, we’re going to focus on one specific aspect of blood: Its color.


Why Is Blood Red?

Blood essentially transports important nutrients throughout the body, and it does so through veins, arteries, and capillaries. If you can see your veins through your skin, you may notice they’re a blue or purple color, depending on your skin tone.

You might think: Does that mean blood is sometimes blue? A cool thought, but nope. That hue has more to do with how your eyes absorb and see color than the color of the blood itself.

Human blood is always a shade of red. Some animals, like octopuses and horseshoe crabs, however, do have blue blood. Human blood is red because of a special protein called hemoglobin in red blood cells that helps transport oxygen. Eating foods high in iron keeps hemoglobin levels up so your cells get the oxygen and nutrients they need.

Even though blood is red, it does change its shade as it travels throughout the body—sometimes it’s bright red, sometimes it’s dark red. The brightness of the red is determined by how much oxygen is in the blood. Blood coming directly from the heart is oxygen-rich and bright red, and as the blood circulates and oxygen is removed by tissue, the blood becomes darker.

While humans do share the same hue of blood, not all human blood is created equal. There are more than eight different types of human blood, and some are more in demand at life-saving blood banks than others. Learn more about different blood types and how rare yours is here.

Now that your blood is pumping from learning this fun fact, check out more amazing blood facts here.