You know what blood is — it's that red stuff that oozes out if you get a
paper cut. But what is blood, really, and what does it do?
What Is Blood and What Does It Do?
Blood is needed to keep us alive. It brings oxygen and nutrients to all the parts
of the body so they can keep working. Blood carries carbon dioxide and other waste
materials to the lungs, kidneys, and digestive system to be removed from the body.
Blood also fights infections, and carries hormones
around the body.
Blood is made up of blood cells and plasma. Plasma (say:
PLAZ-muh) is a yellowish fluid that has nutrients, proteins, hormones, and waste products.
The different types of blood cells have different jobs.
What Are the Types of Blood Cells?
Red blood cells: Red blood cells (RBCs, also called erythrocytes;
say: ih-RITH-ruh-sytes) are shaped like slightly indented, flattened disks. RBCs contain
hemoglobin (say: HEE-muh-glow-bin), a protein that carries oxygen. Blood gets its
bright red color when hemoglobin picks up oxygen in the lungs.
As the blood travels through the body, the hemoglobin releases oxygen to the different
Each RBC lives for about 4 months. Each day, the body makes new RBCs to replace
those that die or are lost from the body. RBCs are made in the inside part of bones
called the bone marrow.
White blood cells: White blood cells (WBCs, also called leukocytes,
say: LOO-kuh-sytes) are a key part of the immune
system. The immune system helps the body defend itself against infection. Different
types of WBCs fight germs, such as bacteria
and viruses. Some types
of WBCs make antibodies, which are special proteins that recognize foreign materials
and help the body get rid of them.
There are several types of WBCs, and their life spans vary from hours to years.
New cells are constantly being formed — some in the bone marrow and some in
other parts of the body such as the spleen, thymus, and lymph nodes.
Blood contains far fewer WBCs than red blood cells, although the body can increase
WBC production to fight infection. The white blood cell count (the number of cells
in a given amount of blood) in someone with an infection often is higher than usual
because more WBCs are being made or are entering the bloodstream to battle the infection.
Platelets: Platelets (also called thrombocytes, say: THROM-buh-sytes)
are tiny oval-shaped cells that help in the clotting process. When a blood vessel
breaks, platelets gather in the area and help seal off the leak. Platelets work with
proteins called clotting factors to control bleeding inside our bodies and on our
Platelets survive only about 9 days in the bloodstream and are constantly being
replaced by new platelets made by the bone marrow.
How Does Blood Travel in the Body?
With each heartbeat, the heart
pumps blood throughout our bodies, carrying oxygen to every cell. After delivering
the oxygen, the blood returns to the heart. The heart then sends the blood to the
lungs to pick up more oxygen. This cycle repeats over and over again.
The circulatory system is made up of blood vessels that carry blood away from and
toward the heart.
Two types of blood vessels carry blood throughout our bodies:
Arteries carry oxygenated blood (blood that has gotten oxygen
from the lungs) from the heart to the rest of the body.
Blood then travels through veins back to the heart and lungs,
so it can get more oxygen to send back to the body via the arteries.
As the heart beats, you can feel blood traveling through the body at pulse points
— like the neck and the wrist — where large, blood-filled arteries run
close to the surface of the skin.
What if Someone Has Low Numbers of Blood Cells?
Sometimes medicine can be given to help a person make more blood cells. And sometimes
blood cells and some of the special proteins blood contains can be replaced by giving
a person blood from someone else. This is called a transfusion (say:
People can get transfusions the part of blood they need, such as platelets, RBCs,
or a clotting factor. When someone donates blood, the whole blood can be separated
into its different parts to be used in these ways.
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.
Besides getting a letter or two, a person's blood is either "positive" or "negative."
This is a way of keeping track of whether someone's blood has a protein called Rh
protein. 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: