Blood is actually red all the time, but different colors of red. When you see blood outside the body it is usually a dark red. This blood is from the veins. Sometimes you may cut an artery. This blood is bright red. It also spurts out in pulses instead of oozing like the blood from veins. So even outside the body, blood can be different colors. (What's the difference between veins and arteries and why would that affect the color? Why does arterial blood spurt?) The blood vessels you see at the surface of your skin are veins. Everyone's skin is slightly different in color, so the veins can look different in different people, but blood is exactly the same color in everyone. It still doesn't look red. That's because we're seeing the *walls* of the veins too. When you see lemon- lime flavored soft drinks in plastic bottles they usually look green, but when you pour them out, they're often clear or yellow. It's not the blood that's bluish; it's the whole vein, including the walls, just like the soft drinks look green because they're in colored bottles.

Have you ever seen a totally white rabbit or mouse? They're called "albinos" because they can't make pigments (the substances that color our eyes, skin, and hair). Their veins look red, even though their blood is the same color as ours. Why is that?

Blood is red ordinarily because the oxygen-containing compound, hemoglobin, contains iron, iron that becomes iron oxide (aka rust) when it combines with oxygen in the air.

First off, it's important to note that blood is always red. Though we typically think of blood as bright red, blood can actually be several different hues, from a dark brown-red to a bright scarlet red. The colors and its variance are caused by a molecule in red blood cells called Hemoglobin, which contains the metal iron. Hemoglobin uses this iron to bind to oxygen molecules and transport the oxygen to all the cells in our body.

When blood passes through the lungs, the hemoglobin absorbs oxygen from the air we breathe and turns bright red. Our heart then pumps this blood all over the body. When it reaches its destination, the oxygen is released from the hemoglobin, and the hemoglobin turns a darker red-brown. Most of the blood vessels you can see on your body are veins, which carry the oxygen-depleted blood back to the lungs and heart. They appear blue because your skin and tissues block some of the light, but are actually filled with the darker red-brown blood.

When you cut a vein, the blood is exposed to all of the oxygen in the air, and the hemoglobin in the red blood cells binds to that oxygen just like it would in your lungs, turning the blood bright red.