The tiniest tubes
Capillaries are the smallest blood vessels in your body. They are much thinner than one hair from your head. You need a microscope to see one.
They are so narrow that red blood cells have to line up and go through one at a time, in single file, like kids walking down a narrow hallway.
Capillary comes from a Latin word that means like a hair.
Where the trading happens
Capillaries are where blood does its most important work. Their walls are only one cell thick, so things can pass right through them.
Oxygen and food leave the blood and go into the body's cells. Waste, like carbon dioxide, leaves the cells and goes into the blood to be carried away.
It is like a delivery truck dropping off groceries and picking up the trash, at every house on the street.
A net everywhere
Capillaries make a huge net, so thick that almost every cell in your body is close to one. Only a few places, like the clear front of your eye and the outer layer of your skin, do without them.
All your blood vessels laid end to end would stretch for many thousands of miles, and most of that is capillaries. Scientists still argue about the exact number, because they are so hard to count.
Try it: press on one of your fingernails for a few seconds. It turns pale. Let go and watch the pink rush back. That is blood filling the capillaries under your nail.
Pink cheeks and bruises
When you get hot or run hard, more blood flows into the capillaries near your skin. That lets heat out, and it can make your cheeks look pink or red.
When you feel embarrassed, the same thing can happen to your face. That is a blush.
If you bump yourself hard, a few capillaries under your skin can break and leak a little blood. That makes a bruise. It may turn blue, purple, green and yellow as your body cleans it up.