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Why We Need It

Just about everyone knows that we can’t live without blood. And that the blood in our bodies is pumped by the heart through a network of arteries and veins. But beyond those blood basics, what do you know about that red stuff beneath your skin?

Blood is essential for good health because the body depends on a steady supply of fuel and oxygen to reach its billions of cells. Even the heart couldn’t survive without blood flowing through the vessels that bring nourishment to its muscular walls. Blood also carries carbon dioxide and other waste materials to the lungs, kidneys, and digestive system; from there they are removed from the body.

Without blood, we couldn’t keep warm or cool off, we couldn’t fight infections, and we couldn’t get rid of our own waste products.

So how exactly does blood do these things? How is it made, and what’s in it? How does blood clot? It’s time to learn a little about the mysterious, life-sustaining fluid called blood.

What Is Blood and What Does It Do?

Two types of blood vessels carry blood throughout our bodies: The arteries carry oxygenated blood (blood that has received oxygen from the lungs) from the heart to the rest of the body. The blood then travels through the veins back to the heart and lungs, so it can receive more oxygen to be sent back to the body via the arteries. As the heart beats, you can feel blood traveling through the body at your pulse points — like the neck and the wrist — where large, blood-filled arteries run close to the surface of the skin.

The blood that flows through this network of veins and arteries is called whole blood. Whole blood contains three types of blood cells:

  1. red blood cells
  2. white blood cells
  3. platelets

In babies and young children, blood cells are made in the bone marrow of many bones throughout the body. But as kids get older, blood cells are made mostly in the bone marrow of the vertebrae (the bones that make up the spine), ribs, pelvis, skull, sternum (the breastbone), and parts of the humerus (the upper arm bone) and femur (the thigh bone).

Blood cells travel through the circulatory system suspended in a yellowish fluid called plasma. Plasma is 90% water and contains nutrients, proteins, hormones, and waste products. Whole blood is a mixture of blood cells and plasma.

Red Blood Cells

Red blood cells (RBCs, and also called erythrocytes, pronounced: ih-RITH-ruh-sytes) are shaped like slightly indented, flattened disks. RBCs contain an iron-rich protein called hemoglobin (pronounced: HEE-muh-glow-bun). Blood gets its bright red color when the hemoglobin in RBCs picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the tissues.

The body contains more RBCs than any other type of cell, and each has a life span of about 4 months. Each day, the body produces new RBCs to replace those that die or are lost from the body.

White Blood Cells

White blood cells (WBCs, and also called leukocytes, pronounced: LOO-kuh-sytes) are a key part of the body’s immune system, which is the body’s defense system against infection. WBCs can move in and out of the bloodstream to reach affected tissues. The blood contains far fewer white blood cells than red cells, although the body can increase production of WBCs to fight infection.

There are several types of white blood cells, and their life spans vary from a few days to months. New cells are constantly being formed in the bone marrow.

Several different parts of blood are involved in fighting infection. White blood cells called granulocytes (pronounced: GRAN-yuh-low-sytes) and lymphocytes (pronounced: LIM-fuh-sytes) travel along the walls of blood vessels. They fight germs such as bacteria and viruses and may also attempt to destroy cells that have become infected or have changed into cancer cells.

Certain types of WBCs produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. Someone with an infection will often have a higher white cell count than when he or she is well because more WBCs are being produced or are entering the bloodstream to battle the infection. After the body has been challenged by some infections, lymphocytes “remember” how to make the specific antibodies that will quickly attack the same germ if it enters the body again.


Platelets (also called thrombocytes, pronounced: THROM-buh-sytes) are tiny oval-shaped cells made in the bone marrow. They help in the clotting process. When a blood vessel breaks, platelets gather in the area and help seal off the leak. Platelets survive only about 9 days in the bloodstream and are constantly being replaced by new cells.

Blood also contains important proteins called clotting factors, which are critical to the clotting process. Although platelets alone can plug small blood vessel leaks and temporarily stop or slow bleeding, the action of clotting factors is needed to produce a strong, stable clot.

Platelets and clotting factors work together to form solid lumps to seal leaks, wounds, cuts, and scratches and to prevent bleeding inside and on the surfaces of our bodies. The process of clotting is like a puzzle with interlocking parts. When the last part is in place, the clot happens — but if only one piece is missing, the final pieces can’t come together.

When large blood vessels are severed (or cut), the body may not be able to repair itself through clotting alone. In these cases, dressings and stitches are used to help control bleeding.

In addition to the cells and clotting factors, blood contains other important substances, such as nutrients from the food that has been processed by the digestive system. Blood also carries hormones released by the endocrine glands and carries them to the body parts that need them.

An interesting thing about blood is that blood cells and some of the special proteins blood contains can be replaced or supplemented by giving a person blood from someone else. This process is called a transfusion. In addition to receiving whole-blood transfusions, people can also receive transfusions of a particular component of blood that they need. For example, someone can receive only platelets, red blood cells, or a clotting factor. When people donate blood, the whole blood can be separated into its different parts and used in this way.

Things That Can Go Wrong With Blood

Most of the time, blood functions normally, but sometimes, blood disorders or diseases can cause problems. Diseases of the blood that commonly affect young people can involve any or all of the three types of blood cells (red blood cells, white blood cells, or platelets) or the proteins and chemicals in the plasma that are responsible for clotting.

Some of the diseases and conditions involving the blood include:

Diseases of the Red Blood Cells

The most common condition affecting the red blood cells of teens is anemia, a lower-than-normal number of red cells in the blood. Anemia is accompanied by a decrease in the amount of hemoglobin present in the blood. Anemia symptoms — such as pale skin, weakness, and a fast heart rate — happen because of the blood’s reduced capacity for carrying oxygen. Causes of anemia can be grouped into two categories: anemia caused by insufficient RBC production and anemia caused by RBCs being destroyed too soon.

  • Anemia due to inadequate red blood cell production:
    Several conditions can cause a reduced production of red blood cells, including:

    • Iron deficiency anemia. Iron deficiency anemia is the most common type of anemia and can affect people who have a diet low in iron or who’ve lost a lot of RBCs (and the iron they contain) through bleeding. Premature babies, infants with poor nutrition, menstruating teenage girls, and those with ongoing blood loss due to illnesses such as inflammatory bowel disease are especially likely to have iron deficiency anemia.
    • Anemia due to chronic disease. People with chronic diseases (such as cancer or human immunodeficiency virus infection [HIV]) often develop anemia as a complication of their illness.
    • Anemia due to kidney disease. The kidneys produce erythropoietin, a hormone that stimulates production of red cells in the bone marrow. Kidney disease can interfere with the production of this hormone.
  • Anemia due to unusually rapid red blood cell destruction:
    When red blood cells are destroyed more quickly than normal by disease (this process is called hemolysis, pronounced: hih-MOL-uh-sus), the bone marrow will make up for it by increasing production of new red cells to take their place. But if RBCs are destroyed faster than they can be replaced, a person will develop anemia.
    Several causes of increased red blood cell destruction can affect teens:

    • G6PD deficiency. G6PD is an enzyme that helps to protect RBCs from the destructive effects of certain chemicals found in foods and medications. When the enzyme is deficient, these chemicals can cause red cells to hemolyze, or burst. G6PD deficiency is a common hereditary disease among people of African, Mediterranean, and Southeast Asian descent.
    • Hereditary spherocytosis (pronounced: sfeer-o-sye-TOE-sus). In this inherited condition, RBCs are misshapen (like tiny spheres, instead of disks) and especially fragile because of a genetic problem with a protein in the structure of the red blood cell. This fragility causes the cells to be easily destroyed.
    • Autoimmune hemolytic anemia. Sometimes — because of disease or for no known reason — the body’s immune system mistakenly attacks and destroys red blood cells.
    • Sickle cell disease. Most common in people of African descent, sickle cell disease (also called sickle cell anemia) is a hereditary disease that results in the production of abnormal hemoglobin. The RBCs become sickle shaped, cannot carry oxygen adequately, and are easily destroyed. The sickle-shaped cells also tend to abnormally stick together, causing obstruction of blood vessels. This blockage in the blood vessels can seriously damage organs and cause bouts of severe pain.

Diseases of the White Blood Cells

  • Neutropenia (pronounced: new-truh-PEE-nee-uh) occurs when there aren’t enough of a certain type of white blood cell to protect the body against bacterial infections. People who take certain chemotherapy drugs to treat cancer may develop neutropenia. These people are at higher risk of bacterial infection.
  • Human immunodeficiency virus (HIV) attacks certain types of white blood cells (lymphocytes) that work to fight infection. Infection with the virus can result in AIDS (acquired immunodeficiency syndrome), leaving the body prone to infections and certain other diseases. Teens and adults can get the disease from sexual intercourse with an infected person or from sharing contaminated needles used for injecting drugs or tattoo ink.
  • Leukemias are cancers of the cells that produce white blood cells. These cancers include acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphocytic leukemia (ALL), and chronic lymphocytic leukemia (CLL). The most common types of leukemia affecting kids are ALL and AML. Scientists have made great advances in treating several types of childhood leukemia, most notably certain types of ALL.

Diseases of Platelets

  • Thrombocytopenia (pronounced: throm-buh-syte-uh-PEE-nee-uh), or a lower than normal number of platelets, is usually diagnosed because a person has abnormal bruising or bleeding. Thrombocytopenia can happen when someone takes certain drugs or develops infections or leukemia or when the body uses up too many platelets. Idiopathic thrombocytopenic purpura (ITP) is a condition in which the immune system attacks and destroys platelets.

Diseases of the Clotting System

The body’s clotting system depends on platelets as well as many clotting factors and other blood components. If a hereditary defect affects any of these components, a person can have a bleeding disorder. Common bleeding disorders include:

  • Hemophilia (pronounced: hee-muh-FIL-ee-uh), an inherited condition that almost exclusively affects guys, involves a lack of particular clotting factors in the blood. People with severe hemophilia are at risk for excessive bleeding and bruising after dental work, surgery, and trauma. They may experience episodes of life-threatening internal bleeding, even if they haven’t been injured.
  • von Willebrand disease, the most common hereditary bleeding disorder, also involves a clotting-factor deficiency. It affects both guys and girls.

Other causes of clotting problems include chronic liver disease (clotting factors are produced in the liver) and vitamin K deficiency (the vitamin is necessary for the production of certain clotting factors).

Reviewed by: Yamini Durani, MD
Date reviewed: May 2015