Humans can't live without blood. Without blood, the
body's organs couldn't get the oxygen and nutrients
they need to survive, we couldn't keep warm or cool off, fight
infections, or get rid of our own waste products. Without enough
blood, we'd weaken and die.
Here are the basics about the mysterious, life-sustaining
fluid called blood.
A Life-Sustaining Fluid
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 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, and it contains three types of blood
- red blood cells (RBCs)
- white blood cells (WBCs)
In babies and young children, blood cells are made within the
bone marrow (the soft tissue inside our bones) of lots of bones
throughout the body. But, as kids get older, blood cells are made
mostly in the bone marrow of the vertebrae (the bones of the
spine), ribs, pelvis, skull, sternum (the breastbone), and parts of
the humerus (the upper arm bone) and femur (the thigh bone).
The 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.
What are Red Blood Cells?
Red blood cells (also called erythrocytes) are shaped like
slightly indented, flattened disks. RBCs contain the iron-rich
protein hemoglobin. 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 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 red blood
cells to replace those that die or are lost from the body.
What are White Blood Cells?
White blood cells (also called leukocytes) 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 WBCs than red cells, although the body can
increase production of WBCs to fight infection. There are several
types of WBCs, 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 and lymphocytes
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. The white 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 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.
What are Platelets?
Platelets (also called thrombocytes) 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.
Important proteins called clotting factors 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 even 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.
Nutrients in the Blood
Blood contains other important substances, such as nutrients
from 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 to be removed from
Blood cells and some of the special proteins blood contains can
be replaced or supplemented by giving a person blood from someone
else via a transfusion. In addition to receiving whole-blood
transfusions, people can also receive transfusions of a particular
component of blood, 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 this way.
Diseases of the Red Blood Cells
Most of the time, blood functions without problems, but
sometimes, blood disorders or diseases can cause illness. Diseases
of the blood that commonly affect kids can involve any or all of
the three types of blood cells. Other types of blood diseases
affect the proteins and chemicals in the plasma that are
responsible for clotting.
The most common condition affecting RBCs is anemia, a
lower-than-normal number of red cells in the blood. Anemia is
accompanied by a decrease in the amount of hemoglobin. 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.
Anemia typically is caused by either inadequate RBC
production or unusually rapid RBC destruction. In severe cases of
chronic anemia, or when a large amount of blood is lost, someone
may need a transfusion of RBCs or whole blood.
Anemia resulting from inadequate RBC production.
Conditions that can cause a reduced production of red blood cells
Iron deficiency anemia.
The most common type of anemia, it affects kids and teens of
any age 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
When lead enters the body, most of it goes into RBCs 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 now,
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
Anemia due to chronic disease.
Kids 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
When RBCs are destroyed more quickly than normal by disease (a
process called hemolysis), 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
Several causes of increased red blood cell destruction can affect
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.
is an inherited condition in which 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
Autoimmune hemolytic anemia.
Sometimes - because of disease or for no known reason - the
body's immune system mistakenly attacks and destroys
Sickle cell anemia
, most common in people of African descent, is a hereditary
disease that results in the production of abnormal hemoglobin.
The RBCs 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
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
Human immunodeficiency virus (HIV)
is a virus that attacks certain types of WBCs (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 HIV from sex with an infected
person or from sharing contaminated needles used for injecting
drugs or tattoo ink.
are cancers of the cells that produce WBCs. 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
, 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
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:
, an inherited condition that almost exclusively affects boys,
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 males and
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
Steven Dowshen, MD
Date reviewed: March 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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