Just about everyone knows that we can't live without blood. Without blood, our organs couldn't get the oxygen and nutrients they need to survive, we couldn't keep warm or cool off, we couldn't fight infections, and we couldn't get rid of our own waste products. Without enough blood, we'd weaken and die.

So how exactly does blood do these things? How is it made, and what's in it? How does blood clot? These questions and more are explained in this article 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, where it receives more oxygen. 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, including:

• red blood cells
• white blood cells
• platelets

These three types of blood cells are mostly manufactured in the bone marrow (the soft tissue inside our bones), especially in the bone marrow of the vertebrae (the bones that make up the spine), ribs, pelvis, skull, and sternum (breastbone). These cells travel through the circulatory system suspended in a yellowish fluid called plasma (pronounced: plaz-muh). 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 (also called erythrocytes, pronounced: ih-rith-ruh-sytes) are shaped like slightly indented, flattened disks. Red blood cells contain an iron-rich protein called hemoglobin (pronounced: hee-muh-glow-bun). Blood gets its bright red color when hemoglobin in red blood cells picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the tissues. The body contains more red blood cells than any other type of cell, and each red blood cell has a life span of about 4 months. Each day, the body produces new red blood cells to replace those that die or are lost from the body.

White blood cells (also called leukocytes, pronounced: loo-kuh-sytes) are a key part of the body's system for defending itself against infection. They 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 white blood cells 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 white blood cells produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. When a person has an infection, his or her white cell count (the number of cells in a given amount of blood) often is higher than when he or she is well because more white blood cells 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 or 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.

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 where they are removed from the body.

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, a person can receive only platelets, red blood cells, or a clotting factor. When a person donates 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 without problems, but sometimes, blood disorders or diseases can cause illness in children and teens. Diseases of the blood that commonly affect children can involve any or all of the three types of blood cells (red blood cells, white blood cells, or platelets). Other types of blood diseases affect 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 children and adolescents is anemia (pronounced: uh-nee-mee-uh), 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. The symptoms of anemia - such as pale skin, weakness, a fast heart rate, and poor growth in infants and children - happen because of the blood's reduced capacity for carrying oxygen. The causes of anemia can be grouped into two main categories: those due to inadequate production of red blood cells and those due to unusually rapid red blood cell destruction. In more severe cases of chronic anemia, as well as when a large amount of blood is lost, a child or teen may need a transfusion of red blood cells or whole blood.

• Anemia resulting from inadequate red blood cell production
  There are several conditions that can cause a reduced production of red blood cells, including:
  • Iron deficiency anemia. Iron deficiency anemia is the most common type of anemia and affects kids and teens of any age who have a diet low in iron or who've lost a lot of red blood cells (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.
  • Lead poisoning. When lead enters the body, most of it goes into red blood cells where it can interfere with the production of hemoglobin. This can result in anemia. Lead poisoning can also affect - and sometimes permanently damage - other body tissues including the brain and nervous system. Although lead poisoning is much less common than it once was in the United States, it still is a problem in many larger cities, especially where young children might ingest paint chips or the dust that comes from lead-containing paints peeling off the walls in older buildings.
  • Anemia due to chronic disease. Children with chronic diseases (such as cancer or human immunodeficiency virus infection) 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 resulting from 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-mah-luh-sus), the bone marrow will make up for it by increasing production of new red cells to take their place. But if red blood cells are destroyed faster than they can be replaced, a person will develop anemia. There are several causes of increased red blood cell destruction that can affect teens:
  • G6PD deficiency. G6PD is an enzyme that helps to protect red blood cells 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) is an inherited condition in which red blood cells 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 anemia, most common in people of African descent, is a hereditary disease that results in the production of abnormal hemoglobin. The red blood cells become sickle shaped, they cannot carry oxygen adequately, and they are easily destroyed. The sickle-shaped blood 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: noo-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.
• Human immunodeficiency virus (HIV) is a virus that 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. Newborns can become infected with the virus from their infected mothers while in the uterus, during birth, or from breastfeeding, although HIV infection of the fetus and newborn is usually preventable with proper medical treatment of the mother during pregnancy and delivery. 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 (pronounced: loo-kee-mee-uhz) 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. In the past 25 years, 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 a person takes certain drugs or develops infections or leukemia or when the body uses up too many platelets. Idiopathic thrombocytopenic purpura (ITP) is a condition, which can occur in children, in which the person's immune system attacks and destroys his or her own platelets.

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