Solved by a verified expert:Lecture – The
Circulatory System
Blood
The functions of blood
fall into three categories: transport, defense, and regulation. Blood
moves from the heart to all of the different organs and tissue where exchange
of gases and other materials takes place. Blood picks up oxygen from the
lungs and nutrients from the digestive tract and carries these to the tissues.
It also picks up and transports cellular wastes away from the tissues.
The blood defends the body against invasion by pathogens. Blood also removes
dead and dying cells as well as destroying mutated cells. Certain white
blood cells engulf and destroy pathogens or cancer cells; others produce and
secrete antibodies. These antibodies incapacitate pathogens, making it
easier for other white blood cells to destroy them. When an injury
occurs, blood forms a clot which prevents blood loss. Blood helps
regulate body temperature by picking up heat, mostly from active muscles, and
transporting it throughout the body. If the blood is too warm, the heat
dissipates to the environment from dilated blood vessels in the skin.
Blood also contains buffers which help regulate body pH keeping it relatively
constant.
The adult human body
contains about 5 liters of blood with just over 50% being plasma. Although
blood appears to be homogeneous, it is actually made up of formed elements—blood
cells and platelets—suspended in plasma, the liquid part of the blood.
Blood is the only fluid tissue in the body but is classified as connective
tissue. It is heavier and more viscous—thicker—than water, and slightly
alkaline with a pH of 7.35-7.45.
Plasma is about 92%
water with the remaining 8% being various salts and organic molecules.
There are three major types of plasma proteins: albumins, globulins, and
fibrinogen. Albumin is important in regulation of osmotic pressure.
Globulins generally are involved in the immune system, transport, and
clotting. Fibrinogen is important in clotting with the production of
fibrin, a threadlike protein that forms blood clots.
The formed elements
consist of red blood cells, white blood cells and platelets. The largest
number of formed elements are the red blood cells— erythrocytes. These
cells are quite unique in that they are biconcave and as mature cells, they
lack a nucleus. (See Figure 1) The erythrocytes are formed in the red
bone marrow (typically found in long bones). As the cell matures, it
gains the hemoglobin and loses the nucleus and most of the organelles.
The primary function of the red blood cells is to carry oxygen to all of the
other cells. Besides carrying oxygen to cells of the body, RBCs help to
remove CO 2.
About one-third of the
RBC is hemoglobin.The hemoglobin combines readily with oxygen which it
then carries from the lungs to the remaining cells of the body. When the
hemoglobin drops off the oxygen at the cells, it picks up carbon dioxide which
it carries back to the lungs for expulsion. The life span of RBCs is about 120
days. It is estimated that about 2 million red blood cells are destroyed
every second; therefore it is important that an equal number be made to
maintain balance.
Leukocytes—white blood
cells—retain their nucleus and are present in much smaller numbers than
erythrocytes. Leukocytes help provide a defense against disease organisms, and
certain cells either promote or decrease inflammatory responses. There are five
types of WBCs. All of which must be stained in order to be distinguishable.
Notice how many more of
the RBCs than WBCs there are. Also each type of WBC has a different function. Neutrophils
are the most abundant of the white blood cells and are the first to respond
to an infection. They can surround and destroy bacteria and other foreign
particles. Eosinophilsincrease rapidly during allergic reactions
neutralizing histamines and also act to destroy parasitic worm infections. Basophils
release histamine during allergic reactions as well as heparin which
prevents clotting and promotes blood flow. Lymphocytesare the
smallest leukocytes and play a vital role in immunity (which we will discuss
later). Monocytesare the largest WBCs and are active in phagocytosis
trapping and destroying bacteria and cellular debris. Monocytes migrate out of
the vessels and take up residence in the tissues.
Platelets
(thrombocytes) are actually
just fragments of cells that clump together to plug breaks in blood vessels,
and they start the clotting process. The clotting process is calledhemostasisand consists of three
parts: vascular spasm, platelet plug formation, and coagulation. The
immediate response to a broken blood vessel is a vascular spasm
(which reduces blood loss). The next event is the platelet plug
formation; platelets will stick to the collagen fibers which are exposed. Two
plasma proteins, fibrinogen and prothrombin, participate in the actual clotting
action. Vitamin K is necessary for prothrombin formation, so your diet
even plays a role in blood clotting.
Human Blood Types
We will spend a little
more time on blood types since this is such an important topic and one that
everyone needs to know about. While several different blood types occur in
humans, the most familiar ones involve the ABO blood groups (types A, B, AB,
and O) and the Rh factor. There are three different alleles for this blood
group.
Blood type alleles
Blood Type
I A
A
I B
B
i
O
Since there are three
different alleles, there are a total of six different genotypes at the human
ABO genetic locus.
Allele
from
Parent 1
Allele
from
Parent 2
Genotype
of
offspring
Blood
types of
offspring
I A
I A
I AI A
A
I A
I B
I AI B
AB
I A
i
I Ai
A
I B
I A
I AI B
AB
I B
I B
I BI B
B
I B
i
I Bi
B
i
i
i i
O
What type you have
depends on whether or not there are certain proteins, called antigens,
on the red blood cells or if there are antibodies to these substances.
If you havetype A blood, you can only receive
types A and O blood. Type A individuals have A antigens and anti-B
antibodies.
If you havetype B blood, you can you can only
receive types B and O blood. Type B individuals have B antigens and
anti-A antibodies.
If you havetype AB blood, you can only receive
types A, B, AB, and O blood. This type has both A and B antigens but neither
antibodies.
If you havetype O blood, you can you can only
receive type O blood. Type O has neither A nor B antigens but both
antibodies.
Blood typing also
routinely tests for the presence of the Rh antigen. If the Rh antigen
is present on the RBCs, the blood is typed as Rh positive (Rh +);
if the antigen is absent, the blood is Rh negative (Rh -). Anti-Rh
antibodies are formed only when Rh +RBCs are introduced into an
individual with Rh -blood. Erythroblastosis fetalisresults
from destruction of the fetal erythrocytes by maternal antibodies. This is not
really a problem during the first pregnancy and childbirth, but because of the
buildup of anti-Rh antibodies in subsequent pregnancies, problems may result.
Heart
and Blood Vessels
The heart and blood
vessels are part of the cardiovascular system. The heart pumps blood
through a closed system of blood vessels. Arteries carry blood away from the
heart to capillaries in body tissues. Veins carry blood from capillaries in
body tissues back to the heart.
This system can be
divided into two functional systems: the pulmonaryand systemic
circuits.The pulmonary includes the vessels that transport blood to and
from the lungs while the systemic is for transport of blood to and from all
parts of the body except the lungs.
The heartis a
four-chambered muscular pump that is located between the lungs in the thoracic
cavity just superior to the diaphragm. From the moment it begins beating until
the moment it stops, the human heart works tirelessly. In an average lifetime,
the heart beats more than two and a half billion times without ever pausing to
rest. The apex (pointed end) points down and to the left. The typical heart is
5 inches (12 cm) long, 3.5 inches (8-9 cm) wide and 2.5 inches (6 cm) from
front to back, and is roughly the size of your fist. The average weight of a
female human heart is 9 ounces and a male’s heart is 10.5 ounces.
The heart has three layers. The smooth, inside lining of the heart
is called the endocardium. The middle layer is heart
muscle called themyocardium(by far the thickest).
It is surrounded by a two-layered serous membrane called the pericardium.
Each chamber of the heart has a sort of one-way valve at its exits
that prevents blood from flowing backwards. When the chamber contracts, the
valve at its exit opens. When it is finished contracting, the valve closes so
that blood does not flow backwards. (See Figure 5)
The
tricuspid valveis
at the exit of the right atrium into the right ventricle.
The
pulmonary semilunar valveis
at the exit of the right ventricle to the pulmonary artery.
The
mitral valve(also
called the bicuspid) is at the exit of the left atrium to the left
ventricle.
The
aortic semilunar valveis
at the exit of the left ventricle to the aorta.
When the heart muscle
contracts or beats (called systole), it pumps blood out of the heart.
The heart contracts in two stages. In the first stage, the right and left atria
contract at the same time, pumping blood to the right and left ventricles; then
the ventricles contract together to propel blood out of the heart. Then the
heart muscle relaxes (called diastole) before the next heartbeat. This
allows blood to fill up the heart again.
The sounds of the heartbeat are usually described as lub-dup(pause)
lub-dup. These sounds are made by the closing of the heart valves. The
first sound results from the tricuspidand mitral(called atrioventricular
valves) closing. The second sound results from the closing of the pulmonary
and aortic( semilunar) valves.
The right and left sides of the heart have separate functions. The
right side of the heart collects oxygen-poor blood from the body and pumps it
to the lungs where it picks up oxygen and releases carbon dioxide. The left
side of the heart then collects oxygen-rich blood from the lungs and pumps it
to the body so that the cells throughout your body have the oxygen they need to
function properly.
Circuit of Vessels
The natural pacemaker of
the heart is a special group of cells that have the ability to generate
electrical on their own and is called the sinoatrialnode(SA node). It is located
in the right atrium. The heart also contains specialized fibers that conduct
the electrical impulse from the pacemaker (SA node) to the rest of the heart
(see Figure 6).
The electrical impulse leaves the SA node(1) and travels
to the right and left atria, causing them to contract together. This takes .04
seconds. There is now a natural delay to allow the atria to contract and the
ventricles to fill up with blood. The electrical impulse has now traveled to
the atrioventricular node (AV node)(2). The electrical impulse now goes
to the Bundle of His(3), then it divides into the right and left
bundle branches(4) where it rapidly spreads using Purkinje fibers(5)
to themuscles of the right and
left ventricle, causing them to contract at the same time.
As the heart undergoes depolarization and repolarization,
the electrical currents that are generated spread not only within the heart,
but also throughout the body. This electrical activity generated by the heart
can be measured by an array of electrodes placed on the body surface. The
recorded tracing is called an electrocardiogram(ECG, or EKG). A
“typical” ECG tracing is shown below in Figure 7. The different waves
that comprise the ECG represent the sequence of depolarization and repolarization
of the atria and ventricles.
BLOOD VESSELS
There are three basic
types of blood vessels that form a closed system: arteries, capillaries,
and veins. The walls of arteries and veins have the same basic
structure. However, arterial walls have more smooth muscle and elastic
connective tissues because of the higher blood pressure found in them. Another
difference is that large veins have valves that prevent a backflow of blood.
Capillaries are the most numerous and smallest blood vessels. A capillary’s
diameter is so small that RBCs must pass through them single file. The walls
have only one layer of cells which allows the exchange of materials between the
blood in them and the body cells.
Lymphatic System
During the exchange of materials between the capillary blood and
interstitial fluid, more
fluid enters the tissue than is returned to the blood. The lymphatic systemreturns
the excess fluid to the blood for reuse. By recycling this fluid, the system
helps to maintain homeostasis by conserving water and dissolved substances. The
lymphatic capillaries are small, closed-ended vessels that are made of simple
squamous epithelium but without a basement membrane. This allows the
fluid to easily flow into the capillary. See Figure 9 for the
relationship between the cardiovascular and lymphatic system.
Once the fluid enters a lymphatic capillary it is called lymph.
The lymphatic vessels carry the lymph back through lymphatic glands and
eventually empty into a large vein near the heart. There are several lymphatic
organs or glands including lymph nodes, tonsils, spleen,
and thymus gland. A major function of lymph nodes is the filtration and
cleansing of the lymph. The tonsils, spleen and thymus all function to
intercept and destroy pathogens as well as the maturation of T lymphocytes(
T cells). Other cells become B lymphocytes( B cells).
All cells have surface recognition molecules called antigens.
(Remember that antigens are used in typing blood.) During the specialization
process, lymphocytes “learn” to distinguish “self” antigens. They are then able
to tell the difference between ‘invaders’ and our own tissue (which can be both
good and bad).
The immune response involves one or both of two different
mechanisms. Antibody-mediated immunityinvolves both T and B cells. Cell-mediated
immunityinvolves only the T cells. The first step in an immune response
against a pathogen is recognizing that its antigens are foreign. This involves
an antigen-presenting celland a T cell. When a T cell is activated it
becomes either a cytotoxic (killer) T cellor a helper T cell.
The helper T is what actually gets the immune response started. The helper T
cell activates B cells to divide; some become plasmacells which
actually produce the antibodies and others become memoryB cells. Memory
B cells remain to launch an even stronger immune response if the same pathogen
ever reenters the body. This antibody defense is referred to as humoral
defense. The killer T cells participate in the cell-mediated defenseby
directly attacking the abnormal and foreign cells.
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