You know what blood is — it's that red stuff that oozes out if you get a paper cut. The average person has about 1 to 1½ gallons (4-6 liters) of it. But what is blood, really, and where does it come from?

How Does the Body Make Blood?

It's not made in a kitchen, but blood has ingredients, just like a recipe. To make blood, your body needs to mix:

• red blood cells, which carry oxygen throughout the body
• white blood cells, which fight infections
• platelets, which are cells that help you stop bleeding if you get a cut
• plasma, a yellowish liquid that carries nutrients, hormones, and proteins throughout the body

Your body doesn't go to the store to buy those ingredients. It makes them. Bone marrow — that goopy stuff inside your bones — makes the red blood cells, the white blood cells, and the platelets. Plasma is mostly water, which is absorbed from the intestines from what you drink and eat, with the liver supplying important proteins.

Put all these ingredients together and you have blood — an essential part of the circulatory system. Thanks to your heart (which pumps blood) and your blood vessels (which carry it), blood travels throughout your body from your head to your toes.

Let's find out more about each ingredient.

Red Blood Cells

Red blood cells (also called erythrocytes, say: ih-rith-ruh-sytes) look like flattened basketballs. Most of the cells in the blood are red blood cells. They carry around an important chemical called hemoglobin (say: hee-muh-glow-bin) that gives blood its red color.

Blood and breathing go hand in hand. How? The hemoglobin in blood delivers oxygen, which you get from the air you breathe, to all parts of your body. Without oxygen, your body couldn't keep working and stay alive.

White Blood Cells

White blood cells (also called leukocytes, say: loo-kuh-sytes) are bigger than red blood cells. There are usually not a whole lot of white blood cells floating around in your blood when you're healthy. Once you get sick, though, your body makes some more to protect you.

There are a couple types of white blood cells that do different things to keep you well:

Granulocytes

You know how your skin gets a little red and swollen around a cut or scrape? That means the granulocytes are doing their jobs. They have a lot to do with how your body cleans things up and helps wounds heal after an injury. Granulocytes also help prevent infection by surrounding and destroying things that aren't supposed to be in your body and by killing germs.

Lymphocytes

There are two types of lymphocytes: B cells and T cells. B cells help make special proteins called antibodies that recognize stuff that shouldn't be in your body, like bacteria or a virus you get from a sick friend. Antibodies are very specific, and can recognize only a certain type of germ. Once the antibody finds it, it gets rid of the germ so it can't hurt you.

The really cool part is that even after you are better, B cells can become memory cells that remember how to make the special antibody so that if the same germ infects you again, it can kill the germ even faster! T cells also battle germs that invade the body, but instead of making antibodies, they work by making special chemicals that help fight the infection.

Monocytes

Monocytes are white blood cells that fight infection by surrounding and destroying bacteria and viruses.

Platelets

Platelets, also called thrombocytes (say: throm-buh-sytes), are tiny round cells that help to make sure you don't bleed too much once you get a cut or scrape. Cuts and scrapes break blood vessels. If a platelet reaches a blood vessel that's been broken open, it sends out a chemical signal that makes other nearby platelets start to stick together inside the vessel.

After the platelets form this plug, they send out more chemical signals that attract clotting factors. These clotting factors work together to make a web of tiny protein threads. The platelets and this web of protein come together to make a blood clot. The clot keeps your blood inside the vessel while the break in the blood vessel heals up. Without platelets, you'd need more than a bandage to catch the blood when you scrape your knee!

Plasma

Plasma is a yellowish liquid that is mostly water. But it also carries important nutrients, hormones, and proteins throughout the body. Nutrients are chemicals from the food you eat that give your body energy and other things your body's cells need to do their work and keep you healthy.

Hormones carry messages throughout your body, telling it what to do and when. An example of a hormone is growth hormone. It gets your bones and muscles to grow. Many proteins in plasma are really important to your body, like the clotting factors that help you stop bleeding if you get a cut or a scrape.

Plasma also carries away cell waste — chemicals that the cell doesn't want anymore. Nutrients, hormones, proteins, and waste are dissolved in the plasma — kind of like the cocoa mix that dissolves in a cup of hot water. What are the marshmallows? The blood cells — they float in the plasma.

Hey, What's Your Type?

Everybody's blood is red, but it's not all the same. There are eight blood types, described using the letters A, B, and O. Those letters stand for certain proteins found on the red blood cells. Not everyone has the same proteins.

In addition to getting a letter or two, a person's blood is either "positive" or "negative." That doesn't mean one person's blood is good and another person's blood is bad. It's a way of keeping track of whether someone's blood has a certain protein called Rh protein. This protein is called "Rh" because scientists found it while studying Rhesus monkeys. If your blood is positive, you have this protein. If it's negative, you don't. Either way is totally fine.

People have one of these eight different blood types:

1. A negative
2. A positive
3. B negative
4. B positive
5. O negative
6. O positive
7. AB negative
8. AB positive

Blood types are important if a person ever wants to donate blood or needs a blood transfusion. Getting blood of the wrong type can make a person sick. That's why hospitals and blood banks are very careful with donated blood and make sure the person gets the right type.

People might need blood transfusions when they're sick or if they lose blood. Without enough healthy blood, the body won't get the oxygen and energy it needs. Healthy blood also protects you from germs and other invaders.

Now that you know how important blood is, what can you do? Kids generally aren't allowed to donate blood, but when you're older consider giving the gift of life!

Reviewed by: Steven Dowshen, MD
Date reviewed: July 2012