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 1-1/2
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
, which are cells that help you stop bleeding if you get a
, 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
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-
-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:
-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:
-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:
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.
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 are white blood cells that fight infection by
surrounding and destroying bacteria and viruses.
Platelets, also called thrombocytes (say:
-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 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.
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:
- A negative
- A positive
- B negative
- B positive
- O negative
- O positive
- AB negative
- AB positive
Blood types are important if a person ever wants to donate blood
or needs a
. 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.
Someone might need a blood transfusion 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!
Steven Dowshen, MD
Date reviewed: May 2006
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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