The average person has 5 to 6 quarts of blood in their circulatory system. Over half of blood is plasma. Plasma is a slightly yellowish fluid that carries hundreds of different substances in solution or suspension. These substances include nutrients, minerals, antibodies, hormones, gases, enzymes, blood clotting proteins, and the waste products of metabolism. Also suspended in plasma are red and white blood cells.
Red blood cells are called erythrocytes. Erythrocytes carry
oxygen to and carbon dioxide away from the thirty trillion cells
which compose the human body. Mature erythrocytes have no
nucleus, so they do not divide as do normal body cells. An
erythrocyte will last about 120 days before becoming worn out.
It is estimated that erythrocytes are created and die at a rate
of 5,000,000 per second. Erythrocytes are formed in the red
marrow of bones by a process called erythropoiesis.
Erythrocytes are disk-shaped and slightly concave on both sides.
They require ATP energy to maintain this shape; for when ATP
energy is taken away they become more spherical in shape.
Erythrocytes get there color from an iron pigment called heme.
Heme combines with the protein globin to form hemoglobin.
Hemoglobin picks up oxygen in the lungs, hold it loosely, and
exchange it for carbon dioxide in capillaries which serve cells.
In the lungs hemoglobin releases carbon dioxide which we exhale.
White blood cells are called leukocytes, and are much less
numerous than erythrocytes. White blood cells are actually
colorless, but show up white under microscopic light.
Leukocytes are slightly larger than erythrocytes. There are
many different types of leukocytes, and they are named for their
internal structure, or for the stains which they accept.
Granulocytes have tiny granules which are easily seen when they
are stained. These leukocytes are manufactured in the red
marrow. Lymphocytes are manufactured in the lymph nodes, spleen
and Peyer's Patches of the intestines. A third type, called
monocytes, are produced in connective tissue, lymph nodes and
the spleen.
Neutrophils are leukocytes which absorb a neutral stain (neither
acidic or basic). Eosinophils absorb a rose-colored stain
called eosin. Eosin is a stain that is attracted to acids.
Basophils adsorb a basic stain known as methylene blue.
The primary function of leukocytes is to eat anything which does
not belong in the body. They do this through a process called
phagocytosis. Leukocytes gang up on things which don't belong
and destroy these invaders. In this process some leukocytes are
killed, and remain mixed with the dead invader, and a few live
leukocytes, to form what is commonly called pus.
If there is infection in the body the number of leukocytes rises
dramatically to fight the infection. Therefore, a high white
blood cell count is a sign of infection. Unlike erythrocytes
which must go with the flow of blood, leukocytes can move to
where they are needed. They move much like an ameba, and can
exit the capillaries to fight infection. The membrane of a
leukocyte is chemically attracted to the toxins produced by
bacteria, and it is this attraction that causes them to move
toward the infected area.
Another function of leukocytes is to eat damaged or worn out
erythrocytes. This destruction occurs in the spleen, red bone
marrow, lymphatic tissue, and the liver. After erythrocytes are
eaten, the by-products are recycled to form bile. Since most
damaged and worn out erythrocytes are destroyed in the spleen,
the spleen is known as the "graveyard for red blood cells."
Also suspended in plasma are blood platelets called
thrombocytes. Thrombocytes are about one-fourth the size of
erythrocytes and also have no nucleus. Like erythrocytes,
thrombocytes are also formed in the red marrow. Platelets help
initiate clotting by causing certain plasma contents to
coagulate. The word "coagulate" means to change from a liquid
to a solid. Thrombocytes release an enzyme called thrombin
which causes a plasma protein called fibrogen to form fibrin.
Fibrin is a mesh-work of thread which covers the wound and traps
blood cells so they can not escape through the wound. Vitamin
K is vital to thrombin, and is administered to assist clotting.
K is the first letter of the Danish word for coagulation.
Blood does not normally clot in the circulatory system because
of the presence of a substance called heparin. If blood does
clot in a blood vessel the clot is called a thrombus, and the
condition is called thrombosis . Heparin obtained from the lungs
of slaughtered animals is used medically to break up a thrombus.
If a clot should develop in the circulatory system the clot may
break lose to form an embolus. An embolus is a substance, such
as a blood clot, fat globule, air bubble, or clumps of cells--which is swept along in the blood stream until it lodges in a
vessel and blocks the flow of blood beyond that point. Such a
condition is called an embolism.
While all erythrocytes and most leukocytes are made in the red
marrow from mesenchymal stem cells, lymphocytes and monocytes
are formed in the lymphatic system. The lymphatic system is
sort of a secondary circulation system which functions to return
fluids from tissue spaces to the circulatory system. In the
lymphatic circulatory system there is no pump, such as a heart,
to cause a flow. Instead, the movement and action of muscles
causes this fluid to move. Without lymphatic circulation tissue
becomes water-logged producing a condition called edema. The
lymphatic system also serves to filter blood of bacteria to
arrest infection. Along the course of this second circulatory
system are the lymph nodes which do this filtering. There are
more than a hundred lymph nodes spaced along the lymphatic
circulatory system. Lymph nodes manufacture lymphocytes which
kill the trapped bacteria.
An antibody is a protein in erythrocytes which becomes modified
when it comes in contact with a foreign substance (antigen). An
antigen is any substance which will stimulate the production of
an antibody. This modification occurs in the antibody-antigen
reaction. The end result of this reaction is the production of
a leukocyte that will destroy the antigen. This is the key to
immunity, as these reactions result in the production of a
leukocyte which will immediately kill an invader so that we do
not get sick from it. Often viruses or bacteria are killed or
weakened and are then injected into the blood stream so that
this antibody-antigen reaction occurs. This procedure, called
vaccination, allows us to become immune to a bacterial or viral
disease. However, the antibody-antigen reaction can also cause
allergies and auto-immune diseases.
Immunity includes all of the physiological activities that make
it possible for the body to recognize things that do not belong
in the body and render the harmless. These foreign invaders
include bacteria, viruses, unicellular and multicellular
organisms, and toxins (poisons). If such invaders have
penetrated skin, escaped being killed by lacrimal glands,
sebaceous glands or sweat glands, managed to travel through the
mucus which lines passages into the body, they must then be
killed by lymphocytes.
All lymphocytes start as stem cells made by the red bone marrow.
Those stem cells which travel to the thymus are made into T-lymphocytes. From the thymus they travel to the lymph nodes
which becomes their work place. Those stem cells which
associate themselves with globulin to form gamma globulin are
the B-lymphocytes. As gamma globulin they eat foreign objects
circulating in the blood stream.
Human beings are classified as belonging to one of four general blood groups according to the nature of certain protein antigens in plasma and in red blood cells. Because of our inheritance we have different antigens in our blood. Some common antigens have been named A, B and AB. If one's blood lacks these antigens they are blood type O. Other rare factors are M and N. Another is Rh, an antigen first observed in the rhesus monkey. About 85% of people in the United States have the Rh factor in their blood and are designated as being Rh positive (Rh+). The 15% who lack this factor are Rh negative (Rh-). These antigens must be matched when blood is transfused from one person to another.
Group O- is called the universal donor, because type O- lacks
all of these antigens. Therefore, anyone can be transfused with
O- blood. Group AB+ is called the universal recipient because
it contains all of these antigens. A person with blood type AB
can receive blood of any type.
When blood is to be transfused from a donor to a recipient these
factors in blood must be matched. If transfused blood contains
factors which the recipients blood does not have, there occurs
a reaction called agglutination. In agglutination there is a
clumping of blood cells which inhibits normal blood flow, and
damages the kidneys. For this reason blood must be typed, and
donor and recipient matched for compatibility.
If the father of a child is Rh+ and the mother is Rh-, the
child's blood type will be determined by the random combination
of their parent's genes. The gene for Rh+ blood appears to be
dominant, and there is a good chance that the child's blood
forming organs will begin to manufacture Rh+ blood. Normally,
the blood of mother and developing child do not mix; however,
bits of the child's blood cells will enter the mother's blood
stream as waste products. As the Rh+ antigens from waste
products enters the mother's blood stream, she begins to make
antibodies to destroy them. In time, usually after the second
or third child, the mother's blood will have developed a
significant number of antibodies against the child's invading
Rh+ antigens. When the mother's blood contains a high number of
these antibodies, they invade the body of the fetus and cause
the fetus' blood to agglutinate. The result is anemia, which
often results in blindness, deafness and mental damage. This
condition is called erythroblastosis fetalis, and requires
immediate blood transfusions at birth. Erythroblastosis can be
fatal. Children born with this condition are often blue in
appearance, and are commonly called "blue babies."
About 13% of marriages in the United States pair an Rh+ man with
an Rh- woman. However, there has been developed an Anti-Rh
serum which prevents erythroblastosis. Before the development
of this serum, Rh incompatible couples were advised to have no
more than two children, as the third would run a high risk of
erythroblastosis. With this serum, such parents can have almost
any number of healthy children.
Donating blood is not only good for the recipient, it is good
for the donor as well. The giving of blood stimulates the bone
marrow. If a healthy person donates a pint of blood, they will
regenerate that pint in about four weeks. If they then donate
a second pint, they will regenerate that pint in three weeks.
If they then give a third pint, they regenerate that third pint
in two weeks.
Anemia refers to a lower than normal level of hemoglobin, or to
a lower than normal number of erythrocytes. The most common
kind of anemia is due to iron deficiency. The most common cause
for iron deficiency anemia is a diet deficient of iron.
Hemoglobin levels in mature females are slightly lower than in
mature males, making females more likely to develop iron
deficiency anemia. Iron supplements, in the form of pills, will
usually correct this deficiency. The second most likely cause
of anemia is chronic blood loss. Blood loss may occur because
of bleeding peptic ulcers, hookworms (hookworms feed on
erythrocytes), hemorrhoids, and excessive menstrual flow. The
symptoms of anemia are pallor (skin which becomes more pale in
appearance) and lack of energy. Young people in particular must
be careful not to over-dose on iron supplements, as these
effects can be permanent, and can be far worse than anemia.
Anemia may also develop from lead poisoning, as lead accumulates
in the bone marrow and destroys some blood-forming cells.
Diseases such as malaria also cause anemia as the protozoan
transmitted by the mosquito feeds on red blood cells. Bacterial
toxins can also cause anemia, as many of these toxins destroy
red blood cells.
Pernicious anemia is the result of the failure of the red blood
cells to mature normally. It is caused by a vitamin B12
deficiency. This results in fewer than normal red blood cells
being produced, and the production of very large, mis-shaped red
blood cells. It occurs in people over the age of 30, and
usually has its onset in the 50's and 60's. Pernicious anemia
is usually treated with vitamin B12, and the intrinsic factor, a
substance which allows the absorption of this vitamin. The word
"pernicious" means deadly. This was true of this form of anemia
up until the late 1920's. However, with modern medical
understanding and modern treatments, it is seldom life
threatening today.
Sickle-cell disease also causes anemia. It is a genetic disease
inherited in the recessive manner. In sickle-cell disease, some
of the erythrocytes take a sickle-shape because of the
distorting effects of an abnormal hemoglobin molecule. These
sickle-shaped erythrocytes clog capillaries and veins, causing
an impairment of blood flow to organs such as the lungs and
nervous system. Severe symptoms occur in "crisis periods" which
are more common in spring and fall than other times of the year.
In crisis periods there is fever and pain in the arms, legs,
heart, or abdomen.
Hemophilia is a sex-linked genetic disorder. In its most common
form hemophiliacs have less than one percent of the normal
clotting factor in their blood. The numerous bleeding episodes
of hemophiliacs often results in anemia.
Leukemia is sometimes called "cancer of the blood." It is a
disease of the blood-forming organs in which there is an over-production of leukocytes. Leukemia has many different causes
and, there are many forms of leukemia. Some can be easily
treated; other are fatal.
In the plasma of blood are dissolved inorganic substances such
as: sodium, potassium, chloride, and calcium. The organic
portions of plasma include the plasma proteins known as albumins
and globulin, as well as the proteins fibrogen and lipoproteins.
Plasma also carries enzymes, hormones and vitamins.
Plasma proteins are classed as being either albumins or
globulins. These plasma proteins are common in living things.
The white of an egg, for example, is primarily composed of
albumins. The yolk of an egg is primarily composed of
globulins. Plasma albumin functions to control the osmotic
pressure of blood. Globulin also helps control osmotic
pressure, but it has other functions as well. A part of
globulin is gamma globulin, a substance that contains antibodies
which protect us from disease. Gamma globulin is made by
lymphocytes.
Blood circulates throughout the body in a closed system in which
blood is confined in blood vessels. Blood is forced outward
from the heart through arteries. The arteries become
increasingly smaller as they branch out to body tissue. As they
become smaller, they become arterioles. Arterioles then become
tubes with only a single layer of thickness. These tubes are
called capillaries. Capillaries pass between cells, or layers
of cells, bringing blood into immediate contact with the cells.
This allows oxygen to be exchanged for carbon dioxide, and
nutrients to be exchanged for waste products.
Capillaries then join together to form venules. Venules are
small veins and have a larger diameter and thicker walls than do
capillaries. Venules become increasingly larger to form veins.
Veins return blood to the heart.
While gravity causes the used blood in the veins of the upper
body to flow back down to the heart, the veins below the heart
rely upon the muscular action of breathing and muscular
contractions in the arms and legs to force the used blood back
to the heart. These veins also have valves which keep the blood
flowing in the direction of the heart. These valves allow blood
to flow toward the heart, but close to prevent a backflow.
Varicose veins are swollen, tortuous (crossing), veins which can
be seen as bluish cords. Varicose veins most often appear in
the legs, and in hemorrhoids. Varicose veins are caused by a
failure of the valves in veins to prevent a backflow. The blood
which flows back pools in these veins causing them to become
large and blue in color. If varicose veins become infected,
this infection is called phlebitis. Varicose veins commonly
develop in women during pregnancy. Varicose veins can be
stripped surgically, obliterated by lasers, or treated with
anticoagulants.
Because arteries carry blood that is pumped from the heart, they
have more blood pressure than do veins. Arteries and veins are
composed of these layers of tissue: the tunica intima, which is
composed of endothelial tissue, the tunica media, which is
composed of smooth muscle and elastic tissue, and the tunica
adventitia, composed of white fibrous connective tissue.
However, arteries have an additional layer called the internal
elastic membrane which lies between the tunica intima and the
tunica media. This extra layer help arteries cope with blood
pressure. If, however, there is a weakening of an arterial
wall, blood pressure causes a ballooning in this weakened area.
This condition is known as an aneurysm. Aneurysms are
susceptible to rupture and hemorrhage. The word "hemorrhage
means to bleed.