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 areterial 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.
Your blood gets its red color from oxygen.Your blood has this molecule in it called hemoglobin. Hemoglobin is like a little transporter for oxygen. It is used by your body to get oxygen to your muscles. This starts when hemoglobin attaches onto oxygen it gets from your lungs. After that, your heart pumps your blood to your muscles so they can use the oxygen. Then, your blood returns to your lungs so that the hemoglobin can pick up more oxygen.
When oxygen attaches to a hemoglobin, the hemoglobin changes “shape” and it looks “red”. After that oxygen is removed by your muscles, the hemoglbin changes “shape” again, and this time it looks “blue”. So…when you’re looking at your veins and arteries, the ones that look “blue” have blood without oxygen in them. The ones that look “red” have blood with oxygen in them.
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