The Human Circulatory System
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The Transport System
Substances must be moved to where they are to be used or
stored. The movement of materials within a cell or between parts of an
organism is called circulation. The
term transport refers to circulation and all other processes by which
substances pass into or out of cells and move within the organism.
In simple organisms, the
processes of diffusion, active transport, and cytoplasmic streaming are
adequate for circulating materials within cells and between cells. However, in
large or complex organisms, many cells are far from the external environment.
Such organisms need a special circulatory
system to move materials to all parts of the organism. The circulatory
system links the cells of the organism to its environment.
A circulatory
system has three components;
(1)
A
fluid in which transported materials are dissolved; Blood
(2)
A
network of tubes or body spaces through which the fluid flows; Blood vessels
(3)
A
means of driving the fluid through the tubes or spaces. Heart
In
animals, the circulatory fluid is usually called blood. The network of
tubes is the blood vessels. The organ that pumps blood through the system is
called the heart
.
Functions of circulatory system
1-To
transport oxygen to the cells: the circulatory system is delivering oxygen to the cells. Oxygen
combines with food inside of body cells to produce usable energy. Without
energy, body cells would soon die. The cells that use the most oxygen—and the
first to die without oxygen — are brain cells.
2- To remove carbon dioxide: When cells combine oxygen and food to produce energy, they
also produce a waste product called carbon dioxide. Removing carbon dioxide is
another important job of the circulatory system.
3-To
transport food and waste products: transport food to all
body cells. At the same time, wastes produced by the cells are carried away by
the blood. If the blood did not remove such wastes, the body would poison
itself with its own waste products!
4-To defense the body against microorganisms: Sometimes the body comes under attack
from microscopic organisms such as bacteria and viruses. At these times,
another transporting function of the circulatory system comes into play—body
defense. The blood rushes disease-fighting cells and chemicals to the area
under attack.
5-To
transport the chemical messengers: The circulatory system transports other chemicals as
well. These chemicals carry messages sent from one part of the body to another.
For example: a chemical messenger from the pancreas is carried by the blood to
the liver. Its message is "Too much sugar in the blood; remove some of the
sugar and store it."
The Human Circulatory System
Humans, like other
vertebrates, have a closed circulatory system. The system includes the fluid
that contains requiring materials, it is blood. A single heart
which pumps the blood, and a network of blood vessels, which carries the
blood to and from all the cells of the body. There are three kinds of blood
vessels—arteries, veins, and capillaries.
Blood Vessels
Arteries:
The blood vessels that carry blood away from
the heart to the organs and tissues of the body are the arteries. The
walls of arteries are thick and elastic. They contain layers of connective
tissue, muscle tissue, and epithelial tissue. As an artery enters a tissue or
organ, it divides and subdivides many times to form smaller and smaller
arteries. The smallest arteries are called arterioles.
Veins:
The blood vessels that return blood from the body tissues to the heart
are the veins. The smallest veins are called venules. The
venules join together to form veins, which also merge, forming larger and
larger veins. The walls of veins are thin and only slightly elastic. Inside the
veins are flap like valves that
allow the blood to flow in only one direction—toward the heart. When the valves
do not function properly, blood tends to accumulate within the vein. The walls
of the vein become stretched and lose their elasticity. This condition is
called varicose veins.
Capillaries:
Arterioles and venules are connected by
networks of microscopic capillaries. The walls of the capillaries consist of a
single layer of epithelial cells. These vessels are so narrow that red blood
cells pass through them in single file. Dissolved nutrients, wastes, oxygen,
and other substances are exchanged between the blood and the body cells while
blood flows through the capillaries.
The Heart
The heart is a pump whose rhythmic contractions force the blood
through the vessels. This muscular organ is larger than your fist and is
located slightly to the left of the middle of the chest cavity, under the
breastbone and between the lungs. It is composed mostly of cardiac muscle.
Structure of heart
The outside of the heart is surrounded by protective membrane, the pericardium.
The space between its two surface is filled with fluid that is facilitates heart function and protects it from
external hazards
The middle layer of heart is called myocardium, composed of
cardiac muscle. Only this layer contains blood vessels or cardiac vessels. It
is thin in the atria but thicker in the ventricles. The cells of the heart
muscle do not obtain their nutrients from the blood with in the heart directly.
The cardiac vessels in the myocardiac
layer supply nutrient to the heart cells. Another way, nutrients of heart are provided by myocardium. The function of this layer are; the pumping by contraction of cardiac
muscle and to supply nutrients for heart .
The inner layer of heart is called endocardium. It is composed of
a single layer of epithelial cells. This layer connected to myocardium by
connective tissue. This layer contain no blood vessels but in a gelatinous structure.
The function of this layer is to prevent the erosion of the heart
during contraction and relaxation.
Parts of heart
The heart has two sides, right and left. These two halves are separated
by a wall called the septum.
Each half has two chambers. There is a thin-walled chamber called the atrium,
and there is a thick, muscular ventricle. The two atria are
reservoirs for the blood that enters the heart. They contract at the same time.
This contraction forces blood into the two ventricles. Next, the muscular walls
of the ventricles contract forcing the blood through the arteries.
The flow of blood through the heart is controlled by four flap-like valves that allow the blood to flow in
only one direction. Two of these valves, called the atrioventricular,
or A-V, valves, allow blood to flow from the atria into
the ventricles. They prevent the flow of blood from the ventricles into the
atria. In the right side of the heart, the A-V valve is called the tricuspid
valve because it has three flaps. In the left side, it is called the bicuspid,
or mitral valve. The other two valves, called the semilunar
valves, allow blood to move from the ventricles into the
pulmonary artery and the aorta. They prevent backflow from these arteries into
the ventricles.
Actually, the heart is
a double pump. The right side of the heart sends oxygen-poor blood to the
lungs, while the left side sends oxygen-rich blood to the rest of the body.
Circulation through the Heart
The blood first enters the right
atrium of the heart from two different directions. Blood enters through the
superior vena cava and the inferior vena cava.
The superior vena cava carries blood from the head and upper parts of the body.
The inferior vena cava returns blood from the lower parts. From the right
atrium, blood goes through the right a-v valve into the right ventricle. Then
the right ventricle contracts. This forces blood through a set of s-l valves
into the pulmonary arteries. These arteries carry the
blood to the lungs. The blood passes through the lung and into the right and
left pulmonary veins. These vessels return blood to the
heart and open into the left atrium. From there, the blood passes
through the left a-v valve into the left ventricle; finally, blood
passes out the aorta and goes to all parts of the body.
The heart muscle cells are
nourished by special arteries called coronary arteries. The right
and left coronary arteries branch off from aorta . They curve downward around
each side of the heart. Each sends off smaller vessels that penetrate the heart
muscle.
The heartbeat cycle:
The pumping action
of the heart involves two main periods. During one of these periods, the heart
muscle is relaxed. This period of relaxation is called diastole. During
the other period, the heart muscle is contracting. The period of contraction is
called systole.
Have you ever listened to your heart in a
stethoscope?
The heart of an average adult beats about 70 times per minute. This is
when the person is resting. During hard work or exercise, the heart rate may
be as high as 180 beats per minute.
Control of the heartbeat
The cardiac muscle is different from the other muscle tissues of
the body. Cardiac muscle fibers form a network.
The contraction of other types of muscle is controlled by the nervous
system. Cardiac muscle has ability to contract. Even when it is removed from
the body, the heart will keep beating for a while if kept in a special
solution. Each heart-muscle fiber has its own innate rate of contraction. This
is made possible by a structure in the heart called the sinoatrial node,
also called the pacemaker. This pacemaker is a specialized group of
muscle cells in the wall of the right atrium. Contraction of the heart is
initiated by electrical impulses from the pacemaker. A specialized system of
fibers carries the impulses to all parts of the heart, causing the atria to
contract first, and then the ventricles.
The minute electrical current produced
each time the heart contracts can be recorded on a machine that produces an electrocardiogram,
or EKG. Physicians use electrocardiograms to check the health of
the heart.
The rate of the heartbeat is regulated by
certain nerves that enter the pacemaker. Impulses from the vagus nerves
slow down the pacemaker, while impulses from the cardioaccelerator
nerves speed up the pacemaker. The built-in rhythm of the heart is
also affected by changes in body temperature and by certain chemicals
circulating in the blood.
When the natural pacemaker of the heart
does not function properly, the wires from a battery-powered electronic
pacemaker can be attached surgically to the heart to regulate the heartbeat
THE BLOOD:COMPOSITION OF BLOOD
Functions
and Components of Blood
Blood is the liquid tissue of transport in
humans and other vertebrates. Because it is a liquid, blood can
transport dissolved and suspended materials. It carries respiratory gases,
nutrients, cellular wastes, and regulatory substances, such as enzymes and
hormones.
Blood contributes to the regulation of all bodily functions. It
maintains and regulates the chemical state, pH, and water content of
cells and body fluids. Blood is also involved in the regulation of body
temperature.
Blood protects the body. The white blood cells
and certain substances found in the blood protect the body from disease-causing
microorganisms. The ability of the blood to clot protects the circulatory
system from collapse that could be caused by loss of fluid from a wound.
The average human body contains about 5.5 liters of blood. Blood is a
unique tissue in that it is made up of blood cells and a liquid called
plasma. 55 percent of the
total volume of the blood is plasma, while the blood cells, red blood cells,
white blood cells, and platelets, make up about 45 percent.
PLASMA
The major portion of the blood is a yellow-colored liquid called plasma.
It consists mainly of water (over 90 percent) and dissolved proteins (7
percent). It also contains salts, glucose, amino acids, fatty acids, vitamins,
hormones, and cellular wastes.
The three types of protein found in blood plasma are albumin,
globulins, and fibrinogen.
Albumin, which is the most
abundant of the plasma proteins, causes an osmotic gradient that regulates the
diffusion of plasma out of the capillaries into the intercellular spaces.
The globulins serve a number of different functions. Some
globulins are involved in the transport of proteins and other substances from
one part of the body to another. Other globulins, particularly the gamma
globulins, play a major role in the body's defense against infection.
Fibrinogen is important in the clotting of blood.
BLOOD CELLS
There are three types of blood cells. Red blood
cells, white blood cells, and platelets.
Red blood cells:
Red blood
cells, or erythrocytes, are the most numerous of the
cells in the blood (about 5 million per cubic millimeter of blood). Their
major function is to transport oxygen from the lungs to the body tissues
and carbon dioxide from the body tissues to the lungs.
Red blood
cells are disk-shaped cells that are thinner in the center than around the rim.
However, they easily change shape. They are filled with the iron-containing
pigment hemoglobin, which gives blood its characteristic
red color. Hemoglobin functions in the transport of oxygen and carbon dioxide.
During the
development of the human embryo, red blood cells are produced by various
organs, including the liver, spleen, and lymph nodes.
After birth, however, they are normally produced only by the bone marrow.
Mature red blood cells contain no nucleus. They live for about 120
days. Worn-out red cells are removed from the circulation by the liver and
spleen and broken down. The iron from the hemoglobin molecule is reused by the
body.
Anemia is a condition in which a person has too few red
blood cells or insufficient hemoglobin. In anemia, the cells of the body do not
receive an adequate supply of oxygen. Some forms of anemia can be treated by
injections of vitamin B12 or by eating iron-rich foods.
White Blood Cells
The white
blood cells, or leucocytes, protect the body against infection by
bacteria and other microorganisms.
White
blood cells are larger than red blood cells, and unlike red cells, they contain
one or more nuclei.
Leukocytes
are produced by the bone marrow and by lymphatic tissues. The
mature leukocytes enter the bloodstream. They can squeeze between the cells of
capillary walls and move through the body tissues. When there is an infection
at a particular site in the body, the leukocytes collect there.
Structurally, there are several different kinds of white blood cells.
However, in terms of function, leukocytes fall into two groups. One type
acts as phagocytes, engulfing microorganisms and other matter.
The second type is involved in the production of antibodies,
which are protein molecules that attack foreign substances or microorganisms
that enter the body.
Normally,
there are only 6,000 to 8,000 white blood cells per cubic millimeter of blood.
However, when there is an infection in the body, the number may increase to
30,000 per cubic millimeter. Among the phagocytic leukocytes, most can ingest
from 5 to 25 bacteria before they die. The pus that forms at the site of an
infected wound consists mainly of white blood cells that have died after
ingesting bacteria.
Leukemia
is a form of blood cancer in which there is an uncontrolled increase in the
number of white blood cells. Some forms of leukemia can now be controlled or
even cured by drugs.
Platelets
Platelets are small, round or oval fragments of a type of blood
cell formed in the bone marrow. A platelet, which has no nucleus or color,
consists of a bit of cytoplasm surrounded by a cell membrane. There are generally
from 200,000 to 400,000 platelets per cubic millimeter of blood. Platelets
start the blood clotting process to repair injured blood vessels.
BLOOD CLOTTING: The Clotting Process
When a blood vessel is broken, the escape of blood is stopped by the
formation of a solid mass that plugs up the hole, a blood clot.
The solidification of blood is called clotting. Clotting is carried out
primarily by the platelets and the plasma protein fibrinogen. The
overall process of blood clotting may be summarized as follows;
1. Clotting is started by the release of a substance
called thromboplastin from the wall of the injured blood
vessel..
2. As soon as the vessel is injured, platelets begin
to stick to the broken vessel wall and to release thromboplastin as
well.
3. The presence of thromboplastin and of
several other factors at the site of the injury causes a complex series of
enzyme-controlled reactions convert the plasma protein prothrombin
to thrombin. Vitamin K is needed for producing of prothrombin.
4. Thrombin,
which is an enzyme, converts another plasma protein, fibrinogen, into
insoluble strands of fibrin. Thrombin also makes platelets sticky
so that the hole in the vessel wall becomes filled with a mass of platelets and
fibrin strands.
5. Red blood cells become trapped in the mass of
fibrin strands and platelets and fill in the wound. As water evaporates from
the clot, it hardens into a scab.
6. The wound is repaired by the growth of cells called
fibroblasts and by an outer layer of epithelial cells.
Clotting must be prevented if blood is to be used for transfusions.
Calcium ions are necessary for many of the clotting reactions and are present
in plasma. If sodium citrate is added to blood, calcium ions bind to the
citrate and clotting cannot occur. Citrated blood is used for most blood
transfusions
Disorders of the Blood
Anemia :
Body produces too few red blood cells
Leukemia:
Body produces too many white blood cells; many do not work properly
Hemophilia:
Blood does not clot properly
Sickle
Cell Disease: Red blood cells have
abnormal sickle shape, causing them to clog small blood vessels
Any
substance that can cause an immune response is called an antigen. Most antigens
are proteins, but carbohydrates and nucleic acids may also be antigens. Most
microorganisms and most toxins (poisonous substances produced by
bacteria) contain substances that are antigens. Each human body contains a
unique combination of proteins that no other human has. As a result, tissue
from one person transplanted into another will contain "foreign"
proteins that act as antigens. The presence of antigens in the body brings
about an immune response that acts to destroy the antigens or the foreign
tissue carrying them.
Lymphocytes and antibodies
The recognition and destruction of foreign antigens in the body tissues
is carried out by the Lymphocytes. Lymphocytes are produced originally
in the bone marrow of developing embryos. They enter the bloodstream,
pass into the body tissues, and finally collect in the lymphoid tissues. There
are two types of lymphocytes —
B-lymphocytes and
T-lymphocytes.
Before becoming established in the
lymphoid tissue, both B and T lymphocytes undergo "processing" at
special sites in the lymphatic system. Without this processing, they cannot
recognize antigens. It is estimated that there are between 10,000 and 100,000
different kinds of antigen receptors on human lymphocytes. However, each
individual lymphocyte has receptors for only one kind of antigen.
When an antigen enters the body, only those lymphocytes with receptors that
recognize that particular antigen become activated. Depending on the antigen,
B Iymphocytes, T Iymphocytes, or both may be stimulated.
When B Iymphocytes are activated by antigens, they enlarge
and undergo repeated cell divisions, forming two different types of cells—plasma
cells and memory cells. Plasma cells secrete
antibodies, which are proteins that react specifically with antigens and
inactivate them. Antibodies have active sites that fit a particular site on a
par ticular antigen. There are several different classes of antibodies,
and they inactivate antigens in different ways (see Table).
The memory cells produced by the activated
B-lymphocytes remain in the lymphoid tissue. If the same antigen enters the
body again, the memory cells immediately begin to produce antibodies against
it, thereby providing immunity to that disease.
When a T lymphocyte is stimulated by an antigen, it also undergoes rapid
cell division, forming more lymphocytes sensitive to that antigen. Some of
these newly formed T lymphocytes remain in the lymphoid tissue and serve as
memory cells. The rest pass from the lymphoid tissue into the circulatory
system and body tissues. When they come in contact with the antigens to which they
are sensitive, they combine with them and destroy them.The Human Circulatory System
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