The Body

Grown-ups

Blood

Blood is a red liquid that flows all through you, carrying oxygen, food and helpers to every part.

Red cells, white cells and tiny platelets ride along in yellowish plasma. Tap any part!

What is blood?

Blood looks like a plain red liquid, but it is really crowded with tiny living cells. Under a microscope, one drop is packed with them.

More than half of blood is plasma, a yellowish liquid that is mostly water. Floating in the plasma are three kinds of cells and cell pieces.

Red blood cells carry oxygen. White blood cells fight germs. Platelets help stop bleeding when you get a cut.

How much blood do you have?

A grown-up has about 5 liters of blood. That is about as much as two and a half big bottles of soda.

A kid has less, because a kid's body is smaller. An 8-year-old has about 2 liters, about one big bottle of soda.

As you grow, your body makes more blood to fill your bigger body. Blood is about 7 to 8 percent of how much you weigh.

Blood's big jobs

Delivery: blood carries oxygen from your lungs and food from your stomach and intestines to every cell in your body.

Cleanup: blood picks up waste from your cells. It drops carbon dioxide off at the lungs for you to breathe out, and other waste at the kidneys, which turn it into pee.

Guarding: white blood cells in your blood hunt down germs to keep you healthy.

Fixing: platelets rush to a cut and help make a plug, so you stop bleeding.

Warming: blood carries heat around your body, like warm water in pipes, so your fingers and toes stay cozy.

Always red

Blood is always red, never blue. Blood full of oxygen is bright red. Blood that has given away its oxygen is a darker red.

The red color comes from hemoglobin (HEE-muh-glow-bin), a special part of red blood cells that holds on to oxygen. It has iron in it.

Not every animal has red blood! An octopus has blue blood, because its blood uses copper instead of iron to carry oxygen. A few kinds of lizards even have green blood.

Always on the move

Blood moves all the time. When you are resting, it takes about a minute for blood to go from your heart, around your body and back again.

When you run, your heart pumps faster, so the trip gets even quicker.

Your body makes new blood cells all the time, inside your bones, in a soft middle part called bone marrow. Old blood cells wear out and get recycled.

🤩 Wow facts

  • When you are resting, your blood goes all the way around your body in about a minute.
  • Blood makes up about 7 to 8 percent of your body weight.
  • Octopuses have blue blood, and a few kinds of lizards have green blood.
  • Your bone marrow makes millions of new red blood cells every second.
  • Blood is thicker than water, because it is crowded with cells.
  • Myth: blood is blue inside your body. Truth: blood is always red, just darker or brighter.

💚 Look after it

  • Eat foods with iron, like beans, spinach, eggs, chicken and iron-fortified cereal. Iron helps your body make red blood cells.
  • Drink water. Plasma, the liquid part of blood, is mostly water.
  • Play actively every day to keep your blood moving.
  • Get plenty of sleep, so your body has time to rest and fix itself.

🩹 When something goes wrong

A small cut. If you get a small cut or scrape, tell a grown-up. They can help you rinse it with clean water, press on it gently with a clean cloth until it stops bleeding, and cover it with a bandage. Your platelets get to work right away.

A nosebleed. Nosebleeds happen to lots of kids, often when the air is dry. Stay calm and sit up. Lean forward a little, and pinch the soft part of your nose for about 10 minutes while you breathe through your mouth. Tell a grown-up, and if it does not stop, they will call a doctor.

🙋 Questions kids ask

Why is blood red?

Red blood cells are full of hemoglobin, which holds oxygen and is red. More oxygen makes it brighter red.

Where does blood come from?

Your body makes blood cells inside your bones, in the soft middle part called bone marrow. It makes new ones all the time.

What happens to the blood I lose from a cut?

Your body makes more. A small cut loses only a tiny bit, and your bone marrow replaces it.

Do kids have less blood than grown-ups?

Yes, because kids are smaller. As you grow, your body makes more blood to fill you up.

🧠 Quick quiz

Question 1 of 3About how much blood does a grown-up have?

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