When you cut yourself accidentally, do you ever wonder what
makes up this thing we call blood? Blood is the most
commonly tested part of the body, and it is truly the river of
life. Every cell in the
body gets its nutrients from blood.
In this edition of HowStuffWorks,
we will examine the river of life. Understanding blood will
help you as your doctor explains the results of your blood
tests. In addition, you will learn amazing things about this
incredible fluid and the cells in it!
Blood is a mixture of two things:
and plasma. The heart pumps
blood through the arteries, capillaries and veins to provide
oxygen and nutrients to every cell of the body. The blood also
carries away waste products.
The adult human body contains approximately 5 liters
(5.3 quarts) of blood and makes up 7 to 8 percent of body
weight. Approximately 2.75 to 3 liters of blood is plasma and
the rest is the cellular portion.
Plasma is the liquid portion of the blood. Blood
cells like red blood cells float in the plasma. Also dissolved
in plasma are electrolytes, nutrients and vitamins (absorbed
from the intestines or produced by the body), hormones,
clotting factors, and proteins such as albumin and
to fight infection). Plasma distributes the substances it
contains as it circulates throughout the body.
The cellular portion of blood contains red blood cells
(RBCs), white blood cells (WBCs) and platelets. The RBCs carry
oxygen from the lungs; the
WBCs help to fight infection; and platelets are parts of cells
that the body uses for clotting.
Red Blood Cells
Red blood cells (RBCs), also
known as erythrocytes, are by far the most abundant
cells in the blood. RBCs give blood its characteristic red
color. In men, there are an average of 5,200,000 RBCs per
cubic millimeter (microliter), and in women there are an
average of 4,600,000 RBCs per cubic millimeter. RBCs account
for approximately 40 to 45 percent of the blood. This
percentage of blood made up of RBCs is a frequently measured
number and is called the hematocrit. The ratio of cells
in normal blood is 600 RBCs for each white blood cell and 40
There are several things about RBCs that make them unusual:
- An RBC has a strange shape -- a biconcave disc
that is round and flat, sort of like a shallow bowl.
- An RBC has no nucleus. The nucleus is extruded
from the cell as it matures.
- An RBC can change shape to an amazing extent,
without breaking, as it squeezes single file through the
capillaries. (Capillaries are minute blood vessels through
which oxygen, nutrients and waste products are exchanged
throughout the body.)
- An RBC contains hemoglobin, a molecule specially
designed to hold oxygen and carry it to cells that need it.
What RBCs Do
function of red blood cells is to transport oxygen from the lungs to the
the body. RBCs contain a protein called hemoglobin that
actually carries the oxygen.
In the capillaries, the oxygen is released to be used by
the cells of the body. Ninety-seven percent of the oxygen that
is carried by the blood from the lungs is carried by
hemoglobin; the other three percent is dissolved in the
plasma. Hemoglobin allows the blood to transport 30 to 100
times more oxygen than could be dissolved in the plasma alone.
Hemoglobin combines loosely with oxygen in the lungs, where
the oxygen level is high, and then easily releases it in the
capillaries, where the oxygen level is low. Each molecule of
hemoglobin contains four iron atoms, and each iron atom
can bind with one molecule of oxygen (which contains two
oxygen atoms, called O2) for a
total of four oxygen molecules (4 * O2) or eight atoms of oxygen for each
molecule of hemoglobin. The iron in hemoglobin gives blood its
Thirty-three percent of an RBC is hemoglobin. The normal
concentration of hemoglobin in blood is 15.5 grams per
deciliter of blood in men, and 14 grams per deciliter of blood
in women. (A deciliter is 100 milliliters of one-tenth of a
Besides carrying oxygen to the cells of the body, the RBCs
help to remove carbon dioxide (CO2) from the body. CO2 is formed in the cells as a byproduct
of many chemical reactions. It enters the blood in the
capillaries and is brought back to the lungs and released
there and then exhaled as we breathe. RBCs contain an enzyme
called carbonic anhydrase which helps the reaction of
carbon dioxide (CO2) and water
(H2O) to occur 5,000 times
faster. Carbonic acid is formed, which then separates into
hydrogen ions and bicarbonate ions:
CO2 + H2O ===> H2CO3 +
H+ + HCO3-
carbon dioxide + water ==> carbonic acid + hydrogen ion
+ bicarbonate ion
The hydrogen ions then combine with hemoglobin and the
bicarbonate ions go into the plasma. Seventy percent of the
CO2 is removed in this way.
Seven percent of the CO2 is
dissolved in the plasma. The remaining 23 percent of the
CO2 combines directly with
hemoglobin and then is released into the lungs.
How Blood Cells Are Made
All blood cells are
produced in the bone marrow. As children, most of our
bones produce blood. As we age this gradually diminishes to
just the bones of the spine (vertebrae), breastbone (sternum),
ribs, pelvis and small parts of the upper arm and leg. Bone
marrow that actively produces blood cells is called red
marrow, and bone marrow that no longer produces blood cells is
called yellow marrow. The process by which the body produces
blood is called hematopoiesis.
All blood cells (RBCs, WBCs and platelets) come from the
same type of cell, called the pluripotential hematopoietic
stem cell. This group of cells has the potential to form
any of the different types of blood cell and also to reproduce
itself. This cell then forms committed stem
cells that will form specific types of blood cells.
During formation, the RBC eventually loses its nucleus and
leaves the bone marrow as a reticulocyte. At this
point, the reticulocyte contains some remnants of organelles.
Eventually these organelles leave the cell and a mature
erythrocyte is formed. RBCs last an average of 120 days in the
bloodstream. When RBCs age, they are removed by macrophages in
the liver and spleen.
A hormone called erythropoietin and low oxygen
levels regulate the production of RBCs. Any factor that
decreases the oxygen level in the body, such as lung disease
or anemia (low number of RBCs), increases the level of
erythropoietin in the body. Erythropoietin then stimulates
production of RBCs by stimulating the stem cells to produce
more RBCs and increasing how quickly they mature. Ninety
percent of erythropoietin is made in the kidney.
When both kidneys are removed, or when kidney failure is
present, that person becomes anemic due to lack of
erythropoietin. Iron, vitamin
B-12 and folate are essential in the production of RBCs.
White Blood Cells
White blood cells (WBCs),
or leukocytes, are a part of the immune
system and help our bodies fight infection. They circulate
in the blood so that they can be transported to an area where
an infection has developed. In a normal adult body there are
4,000 to 10,000 (average 7,000) WBCs per microliter of blood.
When the number of WBCs in your blood increases, this is a
sign of an infection somewhere in your body.
There are five main types of WBCs:
Neutrophils, eosinophils and basophils
are also called granulocytes because they have granules
in their cells that contain digestive enzymes. Basophils have
purple granules, eosinophils have orange-red granules and
neutrophils have a faint blue-pink color.
Neutrophils are the one of the body’s main
defenses against bacteria. They kill bacteria by actually
ingesting them (this is called phagocytosis). Neutrophils can
phagocytize five to 20 bacteria in their lifetime. Neutrophils
have a multi-lobed, segmented or polymorphonuclear nucleus and
so are also called PMNs, polys or segs. Bands are
immature neutrophils that are seen in the blood. When a
bacterial infection is present, an increase of neutrophils and
bands are seen.
Eosinophils kill parasites and have a role in
Basophils are not well understood, but they function
reactions. They release histamine (which causes blood
vessels to leak and attracts WBCs) and heparin (which prevents
clotting in the infected area so that the WBCs can reach the
Monocytes enter the tissue, where they become larger
and turn into macrophages. There they can phagocytize bacteria
(up to 100 in their lifetime) throughout the body. These cells
also destroy old, damaged and dead cells in the body.
Macrophages are found in the liver, spleen, lungs, lymph
nodes, skin and intestine. The system of macrophages scattered
throughout the body is called the reticuloendothelial system.
Neutrophils and monocytes use several mechanisms to get to
and kill invading organisms. They can squeeze through openings
in blood vessels by a process called diapedesis. They
move around using ameboid motion. They are attracted to
certain chemicals produced by the immune system or by bacteria
and migrate toward areas of higher concentrations of these
chemicals. This is called chemotaxis. They kill
bacteria by a process called phagocytosis, in which
they completely surround the bacteria and digest them with
Lymphocytes are complex cells that direct the body’s immune
system. T lymphocytes (T cells) are responsible for
cell-mediated immunity. B lymphocytes are responsible
for humoral immunity (antibody production). Seventy-five
percent of lymphocytes are T cells. Lymphocytes are different
from the other WBCs because they can recognize and have a
memory of invading bacteria and viruses.
There are many types of T cells that have specific
B cells become plasma cells when
exposed to an invading organism or when activated by helper T
cells. B cells produce large numbers of antibodies (also
called immunoglobulins or gamma globulins). There are five
types of immunogloulins (abbreviated Ig): IgG, IgM,
IgE, IgA and IgD. These are Y-shaped molecules that have a
variable segment that is a binding site for only one specific
antigen. These bind to antigens, which causes them to clump,
be neutralized or break open. They also activate the
- Helper T cells - Helper T cells have proteins on
their cell membranes called CD4. Helper T cells direct the
rest of the immune system by releasing cytokines. Cytokines
stimulate B cells to form plasma cells, which form
antibodies, stimulate the production of cytotoxic T cells
and suppressor T cells and activate macrophages. Helper T
cells are the cells the AIDS virus
attacks -- you can imagine that destroying the cells that
direct the immune system has a devastating effect.
- Cytotoxic T cells - Cytotoxic T cells release
chemicals that break open and kill invading organisms.
- Memory T cells - Memory T cells remain afterwards
to help the immune system respond more quickly if the same
organism is encountered again.
- Suppressor T cells - Suppressor T cells suppress
the immune response so that it does not get out of control
and destroy normal cells once the immune response is no
The complement system is a series of enzymes that help or
complement antibodies and other components of the immune
system to destroy the invading antigen by attracting and
activating neutrophils and macrophages, neutralizing viruses
and causing invading organisms to break open. Memory B cells
also remain for prolonged periods, and if the same antigen is
encountered it causes a more rapid response in producing
This is the average percentage of each type of WBC in the
- Neutrophils - 58 percent
- Bands - 3 percent
- Eosinophils - 2 percent
- Basophils - 1 percent
- Monocytes - 4 percent
- Lymphocytes - 33 percent
The Life and Times of a
Most WBCs (neutrophils, eosinophils, basophils
and monocytes) are formed in the bone marrow. T lymphocytes
start in the bone marrow from pluripotent hematopoietic stem
cells, then travel to and mature in the thymus gland. The
thymus is located in the chest between the heart and
sternum (breastbone). B lymphocytes mature in the bone marrow.
When a granulocyte (neutrophil, eosinophil and
basophil) is released into the blood, it stays there for an
average of four to eight hours and then goes into the tissues
of the body, where it lasts for an average of four to five
days. During a severe infection, these times are often
Monocytes stay in the blood for an average of 10 to
20 hours and then go into the tissues, where they become
tissue macrophages and can live for months to years.
Lymphocytes continually pass back and forth between
lymph tissue, lymph fluid and blood. When they are present in
the blood, they stay for several hours. Lymphocytes can live
for weeks, months or years.
Platelets (thrombocytes) help
blood to clot by forming something called a platelet
plug. The other way that blood clots is through
coagulation factors. Platelets also help to promote other
blood clotting mechanisms. There are approximately 150,000 to
400,000 platelets in each microliter of blood (average is
Platelets are formed in the bone marrow from very large
cells called megakaryocytes, which break up into
fragments -- these cellular fragments are platelets. They do
not have a nucleus and do not reproduce. Instead,
megakaryocytes produce more platelets when necessary.
Platelets generally last for an average of 10 days.
Platelets contain many chemicals that assist clotting.
- Actin and myosin, to help them contract
- Chemicals that help the coagulation process to begin
- Chemicals that attract other platelets
- Chemicals that stimulate blood vessel repair
- Chemicals that stabilize a blood clot
Plasma is a clear, yellowish fluid
(the color of straw). Plasma can sometimes appear milky after
a very fatty meal or
when people have a high level of lipids in their blood. Plasma
is 90-percent water. The other 10 percent dissolved in
plasma is essential for life. These dissolved substances are
circulated throughout the body and diffuse into tissues and
cells where they are needed. They diffuse from areas of high
concentration to areas of lower concentration. The greater the
difference in concentration, the greater the amount of
material that diffuses. Waste materials flow in the opposite
direction, from where they are created in the cells into
the bloodstream, where they are removed either in the kidneys or
Hydrostatic pressure (blood
pressure) pushes fluid out of blood vessels. Balancing
this is something called oncotic pressure (caused by
proteins dissolved in blood), which tends to keep fluid inside
the blood vessels.
Proteins make up a large part of the 10 percent of
material dissolved in plasma and are responsible for oncotic
pressure. Protein molecules are much larger than water
molecules and tend to stay in blood vessels. They have more
difficulty fitting through the pores in capillaries, and
therefore have a higher concentration in blood vessels.
Proteins tend to attract water to keep their relative
concentration in blood vessels more in line with fluid outside
the blood vessels. This is one of the ways the body maintains
a constant volume of blood.
Plasma contains 6.5 to 8.0 grams of protein per deciliter
of blood. The main proteins in plasma are albumin (60
percent), globulins (alpha-1, alpha-2, beta, and gamma
globulins (immunoglobulins)), and clotting proteins
(especially fibrinogen). These proteins function to maintain
oncotic pressure (especially albumin) and transport substances
such as lipids, hormones, medications, vitamins, and other
nutrients. These proteins are also part of the immune system
(immunoglobulins), help blood to clot (clotting factors),
maintain pH balance, and are enzymes involved in chemical
reactions throughout the body.
Electrolytes are another large category of substances
dissolved in plasma. They include:
- Sodium (Na+)
- Potassium (K+)
- Chloride (Cl-)
- Bicarbonate (HCO3-)
- Calcium (Ca+2)
- Magnesium (Mg+2)
These chemicals are absolutely essential in many bodily
functions including fluid balance, nerve conduction, muscle
contraction (including the heart),
blood clotting and pH balance.
Other materials dissolved in plasma are carbohydrates
(glucose), cholesterol, hormones and vitamins. Cholesterol is
normally transported attached to lipoproteins such as
low-density lipoproteins (LDLs) and high-density lipoproteins
(HDLs). For more information on cholesterol, read How
When plasma is allowed to clot, the fluid left behind is
called serum. When blood is collected from a patient it
is allowed to clot in a test tube, where the cells and
clotting factors fall to the bottom and the serum is left on
top. Serum is tested for all the numerous items discussed
above to determine if any abnormalities exist.
There are four major blood
types: A, B, AB, and 0. The blood types are determined by
proteins called antigens (also called
agglutinogens) on the surface of the RBC.
are two antigens, A and B. If you have the A antigen on the
RBC, then you have type A blood. When B antigen is present,
you have type B blood. When both A and B antigens are present,
you have type AB blood. When neither are present, you have
type O blood.
U.S. Blood Type
to the American Association of Blood Banking,
these are the percentages of different blood types in
the U.S. population:
When an antigen is present on the RBC, then the opposite
antibody (also called agglutinin) is present in the
plasma. For instance, type A blood has anti-type-B antibodies.
Type B blood has anti-type-A antibodies. Type AB blood has no
antibodies in the plasma, and type O blood has both
anti-type-A and anti-type-B antibodies in the plasma. These
antibodies are not present at birth but are formed
spontaneously during infancy and last throughout life.
In addition to the ABO blood group system, there is an
Rh blood group system. There are many Rh antigens that
can be present on the surface of the RBC. The D antigen
is the most common Rh antigen. If the D antigen is present,
then that blood is Rh+. If the D antigen is missing, then the
blood is Rh-. In the United States, 85 percent of the
population is Rh+ and 15 percent is Rh-. Unlike in the ABO
system, the corresponding antibody to the Rh antigen does not
develop spontaneously but only when the Rh- person is exposed
to Rh antigen by blood transfusion or during pregnancy. When
an Rh- mother is pregnant with an Rh+ fetus, then the mother
forms antibodies that can travel through the placenta and
cause a disease called hemolytic disease of the newborn
(HDN), or erythroblastosis fetalis.
When blood is
transfused into a patient, the blood type must be determined
so that a transfusion reaction does not occur.
A reaction occurs when the antigens on the RBCs of the
donor blood react with the antibodies present in the
recipient’s plasma. In other words, if donor blood of type A
(contains A antigens) is given to someone with type B blood
(they have anti-type A antibodies in their blood), then a
transfusion reaction will occur.
The opposite does not occur. It is unusual for the
antibodies in the plasma of the donated blood to react to the
antigens on the recipients RBCs because very little plasma is
transfused and it gets diluted to a level too low to cause a
When a transfusion reaction occurs, an antibody attaches to
antigens on several RBCs. This causes them to clump together
and plug up blood vessels. Then they are destroyed by the body
(called hemolysis), releasing hemoglobin from the RBCs
into the blood. Hemoglobin is broken down into bilirubin,
which can cause jaundice. These events occur in
hemolytic disease of the newborn (mentioned previously).
When an emergency blood transfusion is necessary and the
recipient's blood type is unknown, anyone can get type O-
blood transfused since type O- blood has no antigen on its
surface that could react with antibodies in the recipient’s
plasma. Therefore, someone with type O- blood is called a
universal donor. Someone with type AB blood is called a
universal recipient because they have no antibodies
that could react with donated blood.
A unit of blood is 1 pint
(450 milliliters) and is mixed with chemicals (CPD) to prevent
clotting. Each year, approximately 12 million to 14 million
units of blood are donated in the United States. Generally, a
blood donor must at least 17 years old, be healthy, and weigh
over 110 pounds.
Prior to donating blood, the donor is given an information
pamphlet to read. A health history is taken to ensure that the
donor has not been exposed to diseases that can be transmitted
by blood, and to determine if donating blood is safe for that
person's own health. The donor’s temperature, pulse, blood
pressure and weight are obtained. A few drops of blood are
obtained to make sure the donor is not anemic. It usually
takes less than 10 minutes for the blood to be removed once
the needle has been placed. Sterile, single-use equipment is
used so there is no danger of infection to the donor. Donors
should drink extra fluids and avoid exercise
that day. Blood can be donated every eight weeks.
Autologous blood donation is the donation of blood
for one’s own use, usually prior to surgery. Apheresis
is the procedure in which only a specific component of a
donor’s blood is removed (usually platelets, plasma or
leukocytes). In this way, more of that specific component can
be removed than can be derived from one unit of blood.
Each unit of blood can be separated into several components
so that each component can be given to someone with a need for
that specific one. Therefore, a single unit of blood can help
many people. These components include:
- Packed RBCs
- Fresh frozen plasma
- Cryoprecipitate anti-hemolytic factor
- Factor VIII concentrate
- Factor IX concentrate
Red blood cells (packed RBCs) are transfused to
increase oxygen-carrying capacity in patients who are bleeding
or extremely anemic. One unit of blood increases the
hemoglobin by 1 g/dl and the hematocrit by 2 to 3 percent.
Plasma (fresh frozen plasma), once thawed, is
transfused to treat bleeding disorders when many clotting
factors are missing. This occurs in liver failure, when too
much of a blood thinner called Coumadin has been given, or
when severe bleeding and massive transfusions result in low
levels of clotting factors.
Platelets are transfused in people with low platelet
count (thrombocytopenia) or abnormally functioning platelets.
Each unit of platelets raises the platelet count by
approximately 5,000 platelets per microliter of blood.
Albumin makes up 60 percent of the protein in
plasma, is produced in the liver and is used when blood volume
needs to be increased and fluids have not worked, as in cases
of severe bleeding, liver failure and severe burns.
Immunoglobulins are given to persons who have been
exposed to a certain disease such as rabies, tetanus or
hepatitis to help prevent that disease.
Factor VIII concentrate and cryoprecipitate
are used in hemophilia A (classic hemophilia) since this is
caused by a factor VIII deficiency.
Factor IX concentrate is used in hemophilia B
("Christmas disease"), which is caused by a deficiency of
clotting factor IX.
There are many tests that are performed on blood to ensure
its safety. These tests include checking for:
If any of these tests are positive, the
blood is discarded. As of 1996, the risk of getting HIV from a
single blood transfusion was 1 in 676,000 units of blood, the
risk of developing Hepatitis B was 1 in 66,000 units and the
risk of getting Hepatitis C was 1 in 100,000 units. However,
newer testing may decrease the risk of Hepatitis C to between
1 in 500,000 and 1 in 1,000,000.
- Hepatitis B surface antigen
- Hepatitis B core antibody
- Hepatitis C antibody
- HIV-1, HIV-2 antibodies
- HIV-1 p24 antigen
- HTLV-1, HTLV-2 antibodies
For more information on blood and related topics, check out
the links on the next page.
About the Author
Bianco, M.D., is an emergency physician practicing at
Dorchester General Hospital in Cambridge, Maryland. Dr. Bianco
attended medical school at Georgetown University School of
Medicine and received his undergraduate degree from Georgetown
University with majors in nursing and pre-med. He Completed an
internship and residency in emergency medicine at Akron City
Hospital in Akron, Ohio.
Dr. Bianco lives near Baltimore with his wife and two
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