An adult human has about 5 to 6 liters
(1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight.
Infants and children have comparably lower volumes of blood, roughly
proportionate to their smaller size. The volume of blood in an individual
fluctuates. During dehydration, for example while running a marathon, blood
volume decreases. Blood volume increases in circumstances such as pregnancy,
when the mother’s blood needs to carry extra oxygen and nutrients to the baby.
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ROLE OF BLOOD
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Blood carries oxygen from the lungs to
all the other tissues in the body and, in turn, carries waste products,
predominantly carbon dioxide, back to the lungs where they are released into
the air. When oxygen transport fails, a person dies within a few minutes. Food
that has been processed by the digestive system into smaller components such as
proteins, fats, and carbohydrates is also delivered to the tissues by the
blood. These nutrients provide the materials and energy needed by individual
cells for metabolism, or the performance of cellular function. Waste products
produced during metabolism, such as urea and uric acid, are carried by the
blood to the kidneys, where they are transferred from the blood into urine and
eliminated from the body. In addition to oxygen and nutrients, blood also
transports special chemicals, called hormones, that regulate certain body
functions. The movement of these chemicals enables one organ to control the
function of another even though the two organs may be located far apart. In
this way, the blood acts not just as a means of transportation but also as a
communications system.
The blood is more than a pipeline for
nutrients and information; it is also responsible for the activities of the
immune system, helping fend off infection and fight disease. In addition, blood
carries the means for stopping itself from leaking out of the body after an
injury. The blood does this by carrying special cells and proteins, known as
the coagulation system, that start to form clots within a matter of seconds
after injury.
Blood is vital to maintaining a stable
body temperature; in humans, body temperature normally fluctuates within a
degree of 37.0° C (98.6° F). Heat production and heat loss in various parts of
the body are balanced out by heat transfer via the bloodstream. This is
accomplished by varying the diameter of blood vessels in the skin. When a
person becomes overheated, the vessels dilate and an increased volume of blood
flows through the skin. Heat dissipates through the skin, effectively lowering
the body temperature. The increased flow of blood in the skin makes the skin
appear pink or flushed. When a person is cold, the skin may become pale as the
vessels narrow, diverting blood from the skin and reducing heat loss.
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COMPOSITION OF BLOOD
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About 55 percent of the blood is
composed of a liquid known as plasma. The rest of the blood is made of three
major types of cells: red blood cells (also known as erythrocytes), white blood
cells (leukocytes), and platelets (thrombocytes).
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Plasma
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Plasma consists predominantly of water
and salts. The kidneys carefully maintain the salt concentration in plasma
because small changes in its concentration will cause cells in the body to
function improperly. In extreme conditions this can result in seizures, coma,
or even death. The pH of plasma, the common measurement of the plasma’s
acidity, is also carefully controlled by the kidneys within the neutral range
of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins,
minerals, nutrients, and waste products. The concentrations of all of these
molecules must be carefully regulated.
Plasma is usually yellow in color due
to proteins dissolved in it. However, after a person eats a fatty meal, that
person’s plasma temporarily develops a milky color as the blood carries the
ingested fats from the intestines to other organs of the body.
Plasma carries a large number of important
proteins, including albumin, gamma globulin, and clotting factors. Albumin is
the main protein in blood. It helps regulate the water content of tissues and
blood. Gamma globulin is composed of tens of thousands of unique antibody
molecules. Antibodies neutralize or help destroy infectious organisms. Each
antibody is designed to target one specific invading organism. For example,
chicken pox antibody will target chicken pox virus, but will leave an influenza
virus unharmed. Clotting factors, such as fibrinogen, are involved in forming
blood clots that seal leaks after an injury. Plasma that has had the clotting
factors removed is called serum. Both serum and plasma are easy to store and
have many medical uses.
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Red Blood Cells
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Red blood cells make up almost 45
percent of the blood volume. Their primary function is to carry oxygen from the
lungs to every cell in the body. Red blood cells are composed predominantly of
a protein and iron compound, called hemoglobin, that captures oxygen molecules
as the blood moves through the lungs, giving blood its red color. As blood
passes through body tissues, hemoglobin then releases the oxygen to cells
throughout the body. Red blood cells are so packed with hemoglobin that they
lack many components, including a nucleus, found in other cells. Hemoglobin
also takes up and releases nitric oxide, which plays an important role in
regulating blood pressure.
The membrane, or outer layer, of the
red blood cell is flexible, like a soap bubble, and is able to bend in many
directions without breaking. This is important because the red blood cells must
be able to pass through the tiniest blood vessels, the capillaries, to deliver
oxygen wherever it is needed. The capillaries are so narrow that the red blood
cells, normally shaped like a disk with a concave top and bottom, must bend and
twist to maneuver single file through them.
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Blood Type
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There are several types of red blood
cells and each person has red blood cells of just one type. Blood type is
determined by the occurrence or absence of substances, known as recognition
markers or antigens, on the surface of the red blood cell. Type A blood has
just marker A on its red blood cells while type B has only marker B. If neither
A nor B markers are present, the blood is type O. If both the A and B markers
are present, the blood is type AB. Another marker, the Rh antigen (also known
as the Rh factor), is present or absent regardless of the presence of A and B
markers. If the Rh marker is present, the blood is said to be Rh positive, and
if it is absent, the blood is Rh negative. The most common blood type is A
positive—that is, blood that has an A marker and also an Rh marker. More than
20 additional red blood cell types have been discovered.
Blood typing is important for many
medical reasons. If a person loses a lot of blood, that person may need a blood
transfusion to replace some of the lost red blood cells. Since everyone makes
antibodies against substances that are foreign, or not of their own body,
transfused blood must be matched so as not to contain these substances. For
example, a person who is blood type A positive will not make antibodies against
the A or Rh markers, but will make antibodies against the B marker, which is
not on that person’s own red blood cells. If blood containing the B marker
(from types B positive, B negative, AB positive, or AB negative) is transfused
into this person, then the transfused red blood cells will be rapidly destroyed
by the patient’s anti-B antibodies. In this case, the transfusion will do the
patient no good and may even result in serious harm. For a successful blood
transfusion into an A positive blood type individual, blood that is type O
negative, O positive, A negative, or A positive is needed because these blood
types will not be attacked by the patient’s anti-B antibodies.
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White Blood Cells
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White blood cells only make up about 1
percent of blood, but their small number belies their immense importance. They
play a vital role in the body’s immune system—the primary defense mechanism
against invading bacteria, viruses, fungi, and parasites. They often accomplish
this goal through direct attack, which usually involves identifying the
invading organism as foreign, attaching to it, and then destroying it. This
process is referred to as phagocytosis.
White blood cells also produce
antibodies, which are released into the circulating blood to target and attach
to foreign organisms. After attachment, the antibody may neutralize the
organism, or it may elicit help from other immune system cells to destroy the
foreign substance. There are several varieties of white blood cells, including
neutrophils, monocytes, and lymphocytes, all of which interact with one another
and with plasma proteins and other cell types to form the complex and highly
effective immune system.
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Platelets and Clotting
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The smallest cells in the blood are the
platelets, which are designed for a single purpose—to begin the process of
coagulation, or forming a clot, whenever a blood vessel is broken. As soon as
an artery or vein is injured, the platelets in the area of the injury begin to
clump together and stick to the edges of the cut. They also release messengers
into the blood that perform a variety of functions: constricting the blood
vessels to reduce bleeding, attracting more platelets to the area to enlarge
the platelet plug, and initiating the work of plasma-based clotting factors,
such as fibrinogen. Through a complex mechanism involving many steps and many
clotting factors, the plasma protein fibrinogen is transformed into long,
sticky threads of fibrin. Together, the platelets and the fibrin create an
intertwined meshwork that forms a stable clot. This self-sealing aspect of the
blood is crucial to survival.
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PRODUCTION AND ELIMINATION OF BLOOD
CELLS
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Blood is produced in the bone marrow, a
tissue in the central cavity inside almost all of the bones in the body. In
infants, the marrow in most of the bones is actively involved in blood cell
formation. By later adult life, active blood cell formation gradually ceases in
the bones of the arms and legs and concentrates in the skull, spine, ribs, and
pelvis.
Red blood cells, white blood cells, and
platelets grow from a single precursor cell, known as a hematopoietic stem
cell. Remarkably, experiments have suggested that as few as 10 stem cells can,
in four weeks, multiply into 30 trillion red blood cells, 30 billion white
blood cells, and 1.2 trillion platelets—enough to replace every blood cell in
the body.
Red blood cells have the longest
average life span of any of the cellular elements of blood. A red blood cell
lives 100 to 120 days after being released from the marrow into the blood. Over
that period of time, red blood cells gradually age. Spent cells are removed by
the spleen and, to a lesser extent, by the liver. The spleen and the liver also
remove any red blood cells that become damaged, regardless of their age. The
body efficiently recycles many components of the damaged cells, including parts
of the hemoglobin molecule, especially the iron contained within it.
The majority of white blood cells have
a relatively short life span. They may survive only 18 to 36 hours after being
released from the marrow. However, some of the white blood cells are
responsible for maintaining what is called immunologic memory. These memory
cells retain knowledge of what infectious organisms the body has previously
been exposed to. If one of those organisms returns, the memory cells initiate
an extremely rapid response designed to kill the foreign invader. Memory cells
may live for years or even decades before dying.
Memory cells make immunizations
possible. An immunization, also called a vaccination or an inoculation, is a
method of using a vaccine to make the human body immune to certain diseases. A
vaccine consists of an infectious agent that has been weakened or killed in the
laboratory so that it cannot produce disease when injected into a person, but
can spark the immune system to generate memory cells and antibodies specific
for the infectious agent. If the infectious agent should ever invade that
vaccinated person in the future, these memory cells will direct the cells of
the immune system to target the invader before it has the opportunity to cause
harm.
Platelets have a life span of seven to
ten days in the blood. They either participate in clot formation during that
time or, when they have reached the end of their lifetime, are eliminated by
the spleen and, to a lesser extent, by the liver.
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BLOOD DISEASES
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Many diseases are caused by
abnormalities in the blood. These diseases are categorized by which component
of the blood is affected.
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Red Blood Cell Diseases
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One of the most common blood diseases
worldwide is anemia, which is characterized by an abnormally low number of red
blood cells or low levels of hemoglobin. One of the major symptoms of anemia is
fatigue, due to the failure of the blood to carry enough oxygen to all of the
tissues.
The most common type of anemia,
iron-deficiency anemia, occurs because the marrow fails to produce sufficient
red blood cells. When insufficient iron is available to the bone marrow, it
slows down its production of hemoglobin and red blood cells. The most common
causes of iron-deficiency anemia are certain infections that result in
gastrointestinal blood loss and the consequent chronic loss of iron. Adding supplemental
iron to the diet is often sufficient to cure iron-deficiency anemia.
Some anemias are the result of
increased destruction of red blood cells, as in the case of sickle-cell anemia,
a genetic disease most common in persons of African ancestry. The red blood
cells of sickle-cell patients assume an unusual crescent shape, causing them to
become trapped in some blood vessels, blocking the flow of other blood cells to
tissues and depriving them of oxygen.
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White Blood Cell Diseases
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Some white blood cell diseases are
characterized by an insufficient number of white blood cells. This can be
caused by the failure of the bone marrow to produce adequate numbers of normal
white blood cells, or by diseases that lead to the destruction of crucial white
blood cells. These conditions result in severe immune deficiencies
characterized by recurrent infections.
Any disease in which excess white blood
cells are produced, particularly immature white blood cells, is called
leukemia, or blood cancer. Many cases of leukemia are linked to gene
abnormalities, resulting in unchecked growth of immature white blood cells. If
this growth is not halted, it often results in the death of the patient. These
genetic abnormalities are not inherited in the vast majority of cases, but
rather occur after birth. Although some causes of these abnormalities are
known, for example exposure to high doses of radiation or the chemical benzene,
most remain poorly understood.
Treatment for leukemia typically
involves the use of chemotherapy, in which strong drugs are used to target and
kill leukemic cells, permitting normal cells to regenerate. In some cases, bone
marrow transplants are effective. Much progress has been made over the last 30
years in the treatment of this disease. In one type of childhood leukemia, more
than 80 percent of patients can now be cured of their disease.
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Coagulation Diseases
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One disease of the coagulation system
is hemophilia, a genetic bleeding disorder in which one of the plasma clotting
factors, usually factor VIII, is produced in abnormally low quantities,
resulting in uncontrolled bleeding from minor injuries. Although individuals
with hemophilia are able to form a good initial platelet plug when blood
vessels are damaged, they are not easily able to form the meshwork that holds
the clot firmly intact. As a result, bleeding may occur some time after the
initial traumatic event. Treatment for hemophilia relies on giving transfusions
of factor VIII. Factor VIII can be isolated from the blood of normal blood
donors but it also can be manufactured in a laboratory through a process known
as gene cloning.
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BLOOD BANKS
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The Red Cross and a number of other
organizations run programs, known as blood banks, to collect, store, and
distribute blood and blood products for transfusions. When blood is donated,
its blood type is determined so that only appropriately matched blood is given
to patients needing a transfusion. Before using the blood, the blood bank also
tests it for the presence of disease-causing organisms, such as hepatitis
viruses and human immunodeficiency virus (HIV), the cause of acquired
immunodeficiency syndrome (AIDS). This blood screening dramatically reduces, but
does not fully eliminate, the risk to the recipient of acquiring a disease
through a blood transfusion. Blood donation, which is extremely safe, generally
involves giving about 400 to 500 ml (about 1 pt) of blood, which is only about
7 percent of a person’s total blood.
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BLOOD IN NON HUMANS
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One-celled organisms have no need for
blood. They are able to absorb nutrients, expel wastes, and exchange gases with
their environment directly. Simple multicelled marine animals, such as sponges,
jellyfishes, and anemones, also do not have blood. They use the seawater that
bathes their cells to perform the functions of blood. However, all more complex
multicellular animals have some form of a circulatory system using blood. In
some invertebrates, there are no cells analogous to red blood cells. Instead,
hemoglobin, or the related copper compound heocyanin, circulates dissolved in
the plasma.
The blood of complex multicellular
animals tends to be similar to human blood, but there are also some significant
differences, typically at the cellular level. For example, fish, amphibians,
and reptiles possess red blood cells that have a nucleus, unlike the red blood
cells of mammals. The immune system of invertebrates is more primitive than
that of vertebrates, lacking the functionality associated with the white blood
cell and antibody system found in mammals. Some arctic fish species produce
proteins in their blood that act as a type of antifreeze, enabling them to
survive in environments where the blood of other animals would freeze.
Nonetheless, the essential transportation, communication, and protection
functions that make blood essential to the continuation of life occur
throughout much of the animal kingdom.
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