# Human Physiology/The cardiovascular system

!Model of a human
heart{width="500"}

## Introduction

The heart is the life-giving, ever-beating muscle in your chest. From
inside the womb until death, the thump goes on. The heart for the
average human will contract about 3 billion times; never resting, never
stopping to take a break except for a fraction of a second between
beats. At 80 years of age, a person\'s heart will continue to beat an
average of 100,000 times a day. Many believe that the heart is the first
organ to become functional. Within weeks of conception the heart starts
its mission of supplying the body with nutrients even though the embryo
is no bigger than a capital letter on this page. The primary function of
the heart is to pump blood through the arteries, capillaries, and veins.
There are an estimated 60,000 miles of vessels throughout an adult body.
Blood transports oxygen, nutrients, disease causing viruses, bacteria,
hormones and has other important functions as well. The heart is the
pump that keeps blood circulating properly. Americans today have many
options to take care of their heart and circulatory system. Expanding
medical technology has made it much easier to do so. This chapter is
dedicated to the heart and its many parts.

## The Heart

The heart is a hollow, muscular organ about the size of a fist. It is
responsible for pumping blood through the blood vessels by repeated,
rhythmic contractions. The heart is composed of cardiac muscle, an
involuntary muscle tissue that is found only within this organ. The term
\"cardiac\" (as in cardiology) means \"related to the heart" and comes
from the Greek word kardia, for \"heart.\" It has a four-chambered,
double pump and is located in the thoracic cavity between the lungs. The
cardiac muscle is self-exciting, meaning it has its own conduction
system. This is in contrast with skeletal muscle, which requires either
conscious or reflex nervous stimuli. The heart\'s rhythmic contractions
occur spontaneously, although the frequency or heart rate can be changed
by nervous or hormonal influence such as exercise or the perception of
danger.

### Endocardium

The endocardium is the innermost lining of the heart which consists of
the endothelial cells forming a smooth membrane in places, and a pocked
and tribeculated surface in others (mainly the ventricles, or lower
pumping chambers).

### Myocardium

The myocardium is the muscular tissue of the heart. The myocardium is
composed of specialized cardiac muscle cells with an ability not
possessed by muscle tissue elsewhere in the body. Cardiac muscle, like
other muscles, can contract, but it can also conduct electricity, like
nerves. The blood to the myocardium is supplied by the coronary
arteries. If these arteries are occluded by atherosclerosis and/or
thrombosis, this can lead to angina pectoris or myocardial infarction
due to ischemia (lack of oxygen). Failure of the heart to contract
properly (for various reasons) is termed heart failure, generally
leading to fluid retention, edema, pulmonary edema, renal insufficiency,
hepatomegaly, a shortened life expectancy and decreased quality of life.

### Epicardium

The outer most layer next to the myocardium is known as the Epicardium.
This is the outer layer after endocardium and myocardium that consists
of a thin layer of connective tissue and fat.

### Pericardium

The pericardium is the thick, membranous sac that surrounds the heart.
It protects and lubricates the heart. There are two layers to the
pericardium: the fibrous pericardium and the serous pericardium. The
serous pericardium is divided into two layers; in between these two
layers there is a space called the pericardial cavity.

### Heart Chambers

The heart has four chambers, two atria and two ventricles. The atria are
smaller with thin walls, while the ventricles are larger and much
stronger.

#### Atrium

There are two atria on either side of the heart. On the right side is
the atrium that contains blood which is poor in oxygen. The left atrium
contains blood which has been oxygenated and is ready to be sent to the
body. The right atrium receives de-oxygenated blood from the superior
vena cava and inferior vena cava. The left atrium receives oxygenated
blood from the left and right pulmonary veins. Atria facilitate
circulation primarily by allowing uninterrupted venous flow to the
heart, preventing the inertia of interrupted venous flow that would
otherwise occur at each ventricular systole.

#### Ventricles

The ventricle is a heart chamber which collects blood from an atrium and
pumps it out of the heart. There are two ventricles: the right ventricle
pumps blood into the pulmonary artery which takes the blood through the
pulmonary circuit, and the left ventricle pumps blood into the aorta for
systemic circulation to the rest of the body. Ventricles have thicker
walls than the atria, and thus can create the higher blood pressure.
Comparing the left and right ventricle, the left ventricle has thicker
walls because it needs to pump blood to the whole body. This leads to
the common misconception that the heart lies on the left side of the
body.

### Septum

The inter ventricular septum (ventricular septum, or during development
septum inferius) is the thick wall separating the lower chambers (the
ventricles) of the heart from one another. The ventricular septum is
directed backward and to the right, and is curved toward the right
ventricle. The greater portion of it is thick and muscular and
constitutes the muscular ventricular septum. Its upper and posterior
part, which separates the aortic vestibule from the lower part of the
right atrium and upper part of the right ventricle, is thin and fibrous,
and is termed the membranous ventricular septum.

### Valves

The two atrioventricular (AV) valves are one-way valves that ensure that
blood flows from the atria to the ventricles, and not the other way. The
two semilunar (SL) valves are present in the arteries leaving the heart;
they prevent blood from flowing back into the ventricles. The sound
heard in a heart beat is the heart valves shutting. The right AV valve
is also called the tricuspid valve because it has three flaps. It is
located between the right atrium and the right ventricle. The tricuspid
valve allows blood to flow from the right atrium into the right
ventricle when the heart is relaxed during diastole. When the heart
begins to contract, the heart enters a phase called systole, and the
atrium pushes blood into the ventricle. Then, the ventricle begins to
contract and blood pressure inside the heart rises. When the ventricular
pressure exceeds the pressure in the atrium, the tricuspid valve snaps
shut. The left AV valve is also called the bicuspid valve because it has
two flaps. It is also known as the mitral valve due to the resemblance
to a bishop\'s mitre (liturgical headdress). This valve prevents blood
in the left ventricle from flowing into the left atrium. As it is on the
left side of the heart, it must withstand a great deal of strain and
pressure; this is why it is made of only two cusps, as a simpler
mechanism entails a reduced risk of malfunction. There are two remaining
valves called the Semilunar Valves. They have flaps that resemble half
moons. The pulmonary semilunar valve lies between the right ventricle
and the pulmonary trunk. The aortic semilunar valve is located between
the left ventricle and the aorta.

### Subvalvular Apparatus

The chordae tendinae are attached to papillary muscles that cause
tension to better hold the valve. Together, the papillary muscles and
the chordae tendinae are known as the subvalvular apparatus. The
function of the subvalvular apparatus is to keep the valves from
prolapsing into the atria when they close. The subvalvular apparatus
have no effect on the opening and closing of the valves. This is caused
entirely by the pressure gradient across the valve.

### Complications with the Heart

The most common congenital abnormality of the heart is the bicuspid
aortic valve. In this condition, instead of three cusps, the aortic
valve has two cusps. This condition is often undiagnosed until the
person develops calcific aortic stenosis. Aortic stenosis occurs in this
condition usually in patients in their 40s or 50s, an average of 10
years earlier than in people with normal aortic valves. Another common
complication of rheumatic fever is thickening and stenosis (partial
blocking) of the mitral valve. For patients who have had rheumatic fever
dentists are advised to prophylactically administer antibiotics prior to
dental work to prevent bacterial endocarditis that occurs when bacteria
from the teeth enter the circulation and attach to damaged heart valves.

The aortic valve is a semilunar valve, but it´s called bicuspid because
of it´s regular three \"cusps\" or \"semilunar\" valves, and is not to
be confused with the left atrioventricular valve, which is more commonly
called the mitral valve, and is one of the two cuspidal valves.

## Passage of Blood Through the Heart

!Diagram of the human
heart.svg "Diagram of the human heart"){width="300"}

While it is convenient to describe the flow of the blood through the
right side of the heart and then through the left side, it is important
to realize that both atria contract at the same time and that both
ventricles contract at the same time. The heart works as two pumps, one
on the right and one on the left that works simultaneously. The right
pump pumps the blood to the lungs or the pulmonary circulation at the
same time that the left pump pumps blood to the rest of the body or the
systemic circulation. Venous blood from systemic circulation
(deoxygenated) enters the right atrium through the superior and inferior
vena cava. The right atrium contracts and forces the blood through the
tricuspid valve (right atrioventricular valve) and into the right
ventricles. The right ventricles contract and force the blood through
the pulmonary semilunar valve into the pulmonary trunk and out the
pulmonary artery. This takes the blood to the lungs where the blood
releases carbon dioxide and receives a new supply of oxygen. The new
blood is carried in the pulmonary veins that take it to the left atrium.
The left atrium then contracts and forces blood through the left
atrioventricular, bicuspid, or mitral, valve into the left ventricle.
The left ventricle contracts forcing blood through the aortic semilunar
valve into the ascending aorta. It then branches to arteries carrying
oxygen rich blood to all parts of the body.

### Blood Flow After the Heart

Aorta-Arteries-Arterioles-Capillaries-Venules-Veins-Vena Cava

### Blood Flow Through Capillaries

From the arterioles, the blood then enters one or more capillaries. The
walls of capillaries are so thin and fragile that blood cells can only
pass in single file. Inside the capillaries, exchange of oxygen and
carbon dioxide takes place. Red blood cells inside the capillary
releases their oxygen which passes through the wall and into the
surrounding tissue. The tissue then releases waste, such as carbon
dioxide, which then passes through the wall and into the red blood
cells.

## The Circulatory System

The circulatory system is extremely important in sustaining life. It's
proper functioning is responsible for the delivery of oxygen and
nutrients to all cells, as well as the removal of carbon dioxide, waste
products, maintenance of optimum pH, and the mobility of the elements,
proteins and cells, of the immune system. In developed countries, the
two leading causes of death, myocardial infarction and stroke are each
direct results of an arterial system that has been slowly and
progressively compromised by years of deterioration.

### Arteries

Arteries are muscular blood vessels that carry blood away from the
heart, oxygenated and deoxygenated blood . The pulmonary arteries will
carry deoxygenated blood to the lungs and the systemic arteries will
carry oxygenated blood to the rest of the body. Arteries have a thick
wall that consists of three layers. The inside layer is called the
endothelium, the middle layer is mostly smooth muscle and the outside
layer is connective tissue. The artery walls are thick so that when
blood enters under pressure the walls can expand.

#### Arterioles

An arteriole is a small artery that extends and leads to capillaries.
Arterioles have thick smooth muscular walls. These smooth muscles are
able to contract (causing vessel constriction) and relax (causing vessel
dilation). This contracting and relaxing affects blood pressure; the
higher number of vessels dilated, the lower blood pressure will be.
Arterioles are just visible to the naked eye.

### Capillaries

framed\|right Capillaries are
the smallest of a body's vessels; they connect arteries and veins, and
most closely interact with tissues. They are very prevalent in the body;
total surface area is about 6,300 square meters. Because of this, no
cell is very far from a capillary, no more than 50 micrometers away. The
walls of capillaries are composed of a single layer of cells, the
endothelium, which is the inner lining of all the vessels. This layer is
so thin that molecules such as oxygen, water and lipids can pass through
them by diffusion and enter the tissues. Waste products such as carbon
dioxide and urea can diffuse back into the blood to be carried away for
removal from the body.

The \"capillary bed\" is the network of capillaries present throughout
the body. These beds are able to be "opened" and "closed" at any given
time, according to need. This process is called autoregulation and
capillary beds usually carry no more than 25% of the amount of blood it
could hold at any time. The more metabolically active the cells, the
more capillaries it will require to supply nutrients.

### Veins

Veins carry blood to the heart. The pulmonary veins will carry
oxygenated blood to the heart awhile the systemic veins will carry
deoxygenated to the heart. Most of the blood volume is found in the
venous system; about 70% at any given time. The veins outer walls have
the same three layers as the arteries, differing only because there is a
lack of smooth muscle in the inner layer and less connective tissue on
the outer layer. Veins have low blood pressure compared to arteries and
need the help of skeletal muscles to bring blood back to the heart. Most
veins have one-way valves called venous valves to prevent backflow
caused by gravity. They also have a thick collagen outer layer, which
helps maintain blood pressure and stop blood pooling. If a person is
standing still for long periods or is bedridden, blood can accumulate in
veins and can cause varicose veins. The hollow internal cavity in which
the blood flows is called the lumen. A muscular layer allows veins to
contract, which puts more blood into circulation. Veins are used
medically as points of access to the blood stream, permitting the
withdrawal of blood specimens (venipuncture) for testing purposes, and
enabling the infusion of fluid, electrolytes, nutrition, and medications
(intravenous delivery).

### Venules

A venule is a small vein that allows deoxygenated blood to return from
the capillary beds to the larger blood veins, except in the pulmonary
circuit where the blood is oxygenated. Venules have three layers; they
have the same makeup as arteries with less smooth muscle, making them
thinner.

## The Cardiovascular Pathways

!Human circulatory system. Arteries are shown in red, veins
blue.

The double circulatory system of blood flow refers to the separate
systems of pulmonary circulation and the systemic circulation in
amphibians, birds and mammals (including humans.) In contrast, fishes
have a single circulation system. For instance, the adult human heart
consists of two separated pumps, the right side with the right atrium
and ventricle (which pumps deoxygenated blood into the pulmonary
circulation), and the left side with the left atrium and ventricle
(which pumps oxygenated blood into the systemic circulation). Blood in
one circuit has to go through the heart to enter the other circuit.
Blood circulates through the body two to three times every minute. In
one day, the blood travels a total of 19,000 km (12,000 miles), or four
times the distance across the U.S. from coast to coast.

### The Pulmonary Circuit

In the pulmonary circuit, blood is pumped to the lungs from the right
ventricle of the heart. It is carried to the lungs via pulmonary
arteries. At lungs, oxygen in the alveolae diffuses to the capillaries
surrounding the alveolae and carbon dioxide inside the blood diffuses to
the alveolae. As a result, blood is oxygenated which is then carried to
the heart\'s left half -to the left atrium via pulmonary veins. Oxygen
rich blood is prepared for the whole organs and tissues of the body.
This is important because mitochondria inside the cells should use
oxygen to produce energy from the organic compounds.

### The Systemic Circuit

The systemic circuit supplies oxygenated blood to the organ system.
Oxygenated blood from the lungs is returned to the left atrium, then the
ventricle contracts and pumps blood into the aorta. Systemic arteries
split from the aorta and direct blood into the capillaries. Cells
consume the oxygen and nutrients and add carbon dioxide, wastes, enzymes
and hormones. The veins drain the deoxygenated blood from the
capillaries and return the blood to the right atrium.

### Aorta

The aorta is the largest of the arteries in the systemic circuit. The
blood is pumped from the left ventricle into the aorta and from there it
branches to all parts of the body. The aorta is an elastic artery, and
as such is able to distend. When the left ventricle contracts to force
blood into the aorta, the aorta expands. This stretching gives the
potential energy that will help maintain blood pressure during diastole,
as during this time the aorta contracts passively.

### Superior Venae Cavae

The superior vena cava (SVC) is a short vein that carries de-oxygenated
blood from the upper half of the body to the heart\'s right atrium. It
is formed by the left and right brachiocephalic veins (also referred to
as the innominate veins) which receive blood from the upper limbs and
the head and neck. The azygous vein (which receives blood from the
ribcage) joins it just before it enters the right atrium.

### Inferior Venae Cavae

The inferior vena cava (or IVC) is a large vein that carries
de-oxygenated blood from the lower half of the body into the heart. It
is formed by the left and right common iliac veins and transports blood
to the right atrium of the heart. It is posterior to the abdominal
cavity, and runs along side of the vertebral column on its right side.

### Coronary Arteries

!Heart showing the Coronary
Arteries

Heart showing the Coronary Arteries The coronary circulation consists of
the blood vessels that supply blood to, and remove blood from, the heart
muscle itself. Although blood fills the chambers of the heart, the
muscle tissue of the heart, or myocardium, is so thick that it requires
coronary blood vessels to deliver blood deep into the myocardium. The
vessels that supply blood high in oxygen to the myocardium are known as
coronary arteries. The vessels that remove the deoxygenated blood from
the heart muscle are known as cardiac veins. The coronary arteries that
run on the surface of the heart are called epicardial coronary arteries.
These arteries, when healthy, are capable of auto regulation to maintain
coronary blood flow at levels appropriate to the needs of the heart
muscle. These relatively narrow vessels are commonly affected by
atherosclerosis and can become blocked, causing angina or a heart
attack. The coronary arteries are classified as \"end circulation\",
since they represent the only source of blood supply to the myocardium:
there is very little redundant blood supply, which is why blockage of
these vessels can be so critical. In general there are two main coronary
arteries, the left and right. • Right coronary artery • Left coronary
artery Both of these arteries originate from the beginning (root) of the
aorta, immediately above the aortic valve. As discussed below, the left
coronary artery originates from the left aortic sinus, while the right
coronary artery originates from the right aortic sinus. Four percent of
people have a third, the posterior coronary artery. In rare cases, a
patient will have one coronary artery that runs around the root of the
aorta.

### Hepatic Veins

In human anatomy, the hepatic veins are the blood vessels that drain
de-oxygenated blood from the liver and blood cleaned by the liver (from
the stomach, pancreas, small intestine and colon) into the inferior vena
cava. They arise from the substance of the liver, more specifically the
central vein of the liver lobule. They can be differentiated into two
groups, the upper group and lower group. The upper group of three
typically arises from the posterior aspect of the liver and drain the
quadrate lobe and left lobe. The lower group rise from the right lobe
and caudate lobe, are variable in number, and are typically smaller than
those in the upper group. None of the hepatic veins have valves.

## Cardiac Cycle

Cardiac cycle is the term used to describe the relaxation and
contraction that occur, as a heart works to pump blood through the body.
Heart rate is a term used to describe the frequency of the cardiac
cycle. It is considered one of the four vital signs. Usually it is
calculated as the number of contractions (heart beats) of the heart in
one minute and expressed as \"beats per minute\" (bpm). When resting,
the adult human heart beats at about 70 bpm (males) and 75 bpm
(females), but this rate varies between people. However, the reference
range is nominally between 60 bpm (if less termed bradycardia) and 100
bpm (if greater, termed tachycardia). Resting heart rates can be
significantly lower in athletes, and significantly higher in the obese.
The body can increase the heart rate in response to a wide variety of
conditions in order to increase the cardiac output (the amount of blood
ejected by the heart per unit time). Exercise, environmental stressors
or psychological stress can cause the heart rate to increase above the
resting rate. The pulse is the most straightforward way of measuring the
heart rate, but it can be deceptive when some strokes do not lead to
much cardiac output. In these cases (as happens in some arrhythmias),
the heart rate may be considerably higher than the pulse. Every single
\'beat\' of the heart involves three major stages: atrial systole,
ventricular systole and complete cardiac diastole. Throughout the
cardiac cycle, the blood pressure increases and decreases. As ventricles
contract the pressure rise, causing the AV valves to slam shut.

### Systole

!The heart in the systole
phase.{width="200"}

The heart in the systole phase. Systole, or contraction, of the heart is
initiated by the electrical cells of the sinoatrial node, which is the
heart\'s natural pacemaker. These cells are activated spontaneously by
depolarization of their membranes beyond a certain threshold for
excitation. At this point, voltage-gated calcium channels on the cell
membrane open and allow calcium ions to pass through, into the main, or
interior, of the muscle cell. Some calcium ions bind to receptors on the
sarcoplasmic reticulum causing an influx of calcium ions into the
sarcoplasm. The calcium ions bind to the troponin, causing a
conformation change, breaking the bond between the protein tropomyosin,
to which the troponin is attached, and the myosin binding sites. This
allows the myosin heads to bind to the myosin binding sites on the actin
protein filament and contraction results as the myosin heads draw the
actin filaments along, are bound by ATP, causing them to release the
actin, and return to their original position, breaking down the ATP into
ADP and a phosphate group. The action potential spreads via the passage
of sodium ions through the gap junctions that connect the sarcoplasm of
adjacent myocardial cells. Norepinephrine (noradrenaline) is released by
the terminal boutons of depolarized sympathetic fibers, at the
sinoatrial and atrioventricular nodes. Norepinephrine diffuses across
the synaptic cleft binds to the β1-adrenoreceptors -- G-protein linked
receptors, consisting of seven transmembrane domains -- shifting their
equilibrium towards the active state. The receptor changes its
conformation and mechanically activates the G-protein which is released.
The G-protein is involved in the production of adenosine 3\',5\'-cyclic
monophosphate (cAMP) from adenosine triphosphate (ATP) and this in turn
activates the protein kinase (β-adrenoreceptor kinase). β-adrenoreceptor
kinase phosphorylates the calcium ion channels in the sarcolemma, so
that calcium ion influx is increased when they are activated by the
appropriate transmembrane voltage. This will of course, cause more of
the calcium receptors in the sarcoplasmic reticulum to be activated,
creating a larger flow of calcium ions into the sarcoplasm. More
troponin will be bound and more myosin binding sites cleared \[of
tropomyosin\] so that more myosin heads can be recruited for the
contraction and a greater force and speed of contraction results.
\[Phosphodiesterase catalyses the decomposition of cAMP to AMP so that
it is no longer able to activate the protein kinase. AMP will of course,
go on to be phosphorylated to ATP and may be recycled.\] Noradrenaline
also affects the atrioventricular node, reducing the delay before
continuing conduction of the action potential via the bundle of HIS.

### Diastole

!The heart in the diastole
phase.{width="200"}

The heart in the diastole phase. Cardiac Diastole is the period of time
when the heart relaxes after contraction in preparation for refilling
with circulating blood. Ventricular diastole is when the ventricles are
relaxing, while atrial diastole is when the atria are relaxing. Together
they are known as complete cardiac diastole. It should be noted that
even this relaxation is an active, energy-spending process. During
ventricular diastole, the pressure in the (left and right) ventricles
drops from the peak that it reaches in systole. When the pressure in the
left ventricle drops to below the pressure in the left atrium, the
mitral valve opens, and the left ventricle fills with blood that was
accumulating in the left atrium. Likewise, when the pressure in the
right ventricle drops below that in the right atrium, the tricuspid
valve opens and the right ventricle fills with blood that was in the
right atrium

### \"Lub-Dub\"

The first heart tone, or S1, \"Lub\" is caused by the closure of the
atrioventricular valves, mitral and tricuspid, at the beginning of
ventricular contraction, or systole. When the pressure in the ventricles
rises above the pressure in the atria, these valves close to prevent
regurgitation of blood from the ventricles into the atria. The second
heart tone, or S2 (A2 and P2), \"Dub\" is caused by the closure of the
aortic valve and pulmonic valve at the end of ventricular systole. As
the left ventricle empties, its pressure falls below the pressure in the
aorta, and the aortic valve closes. Similarly, as the pressure in the
right ventricle falls below the pressure in the pulmonary artery, the
pulmonic valve closes. During inspiration, negative intrathoracic
pressure causes increased blood return into the right side of the heart.
The increased blood volume in the right ventricle causes the pulmonic
valve to stay open longer during ventricular systole. This causes an
increased delay in the P2 component of S2. During expiration, the
positive intrathoracic pressure causes decreased blood return to the
right side of the heart. The reduced volume in the right ventricle
allows the pulmonic valve to close earlier at the end of ventricular
systole, causing P2 to occur earlier, and \"closer\" to A2. It is
physiological to hear the splitting of the second heart tone by younger
people and during inspiration. During expiration normally the interval
between the two components shortens and the tone becomes merged.

## The Heart\'s Electrical Conduction System

The heart is primarily made up of muscle tissue. A network of nerve
fibers coordinates the contraction and relaxation of the cardiac muscle
tissue to obtain an efficient, wave-like pumping action of the heart

How Stuff Works (The
Heart)

#### Control of Heartbeat

The heart contains two cardiac pacemakers that spontaneously cause the
heart to beat. These can be controlled by the autonomic nervous system
and circulating adrenaline. If the cardiac muscles just contracted and
relaxed randomly at a natural rhythm the cycle would become disordered
and the heart would become unable to carry on its function of being a
pump. Sometimes when the heart undergoes great damage to one part of the
cardiac muscle or the person incurs an electric shock, the cardiac cycle
can become uncoordinated and chaotic. Some parts of the heart will
contract whilst others will relax so that instead of contracting and
relaxing as a whole, the heart will flutter abnormally. This is called
fibrillation and can be fatal if not treated within 60 seconds.

!Schematic representation of the sinoatrial node and the
atrioventricular bundle of His. The location of the SA node is shown in
blue. The bundle, represented in red, originates near the orifice of the
coronary sinus, undergoes slight enlargement to form the AV node. The AV
node tapers down into the bundle of HIS, which passes into the
ventricular septum and divides into two bundle branches, the left and
right bundles. The ultimate distribution cannot be completely shown in
this
diagram.{width="300"}

**SA Node**\
The sinoatrial node (abbreviated SA node or SAN, also called the sinus
node) is the impulse generating (pacemaker) tissue located in the right
atrium of the heart. Although all of the heart\'s cells possess the
ability to generate the electrical impulses (or action potentials) that
trigger cardiac contraction, the sinoatrial node is what normally
initiates it, simply because it generates impulses slightly faster than
the other areas with pacemaker potential. Because cardiac myocytes, like
all nerve cells, have refractory periods following contraction during
which additional contractions cannot be triggered, their pacemaker
potential is overridden by the sinoatrial node. The SA node emits a new
impulse before either the AV or purkinje fibers reach threshold. The
sinoatrial node (SA node) is a group of cells positioned on the wall of
the right atrium, near the entrance of the superior vena cava. These
cells are modified cardiac myocytes. They possess some contractile
filaments, though they do not contract. Cells in the SA node will
naturally discharge (create action potentials) at about 70-80
times/minute. Because the sinoatrial node is responsible for the rest of
the heart\'s electrical activity, it is sometimes called the primary
pacemaker. If the SA node doesn\'t function, or the impulse generated in
the SA node is blocked before it travels down the electrical conduction
system, a group of cells further down the heart will become the heart\'s
pacemaker. These cells form the atrioventricular node (AV node), which
is an area between the right atrium and ventricle, within the atrial
septum. The impulses from the AV node will maintain a slower heart rate
(about 40-60 beats per a minute). When there is a pathology in the AV
node or purkinje fibers, an ectopic pacemaker can occur in different
parts of the heart. The ectopic pacemaker typically discharges faster
than the SA node and causes an abnormal sequence of contraction. The SA
node is richly innervated by vagal and sympathetic fibers. This makes
the SA node susceptible to autonomic influences. Stimulation of the
vagus nerve causes decrease in the SA node rate (thereby causing
decrease in the heart rate). Stimulation via sympathetic fibers causes
increase in the SA node rate (thereby increasing the heart rate). The
sympathetic nerves are distributed to all parts of the heart, especially
in ventricular muscles. The parasympathetic nerves mainly control SA and
AV nodes, some atrial muscle and ventricular muscle. Parasympathetic
stimulation from the vagal nerves decreases the rate of the AV node by
causing the release of acetylcholine at vagal endings which in turn
increases the K+ permeability of the cardiac muscle fiber. Vagal
stimulation can block transmission through AV junction or stop SA node
contraction which is called \"ventricular escape.\" When this happens,
the purkinje fibers in the AV bundle develops a rhythm of their own. In
the majority of patients, the SA node receives blood from the right
coronary artery, meaning that a myocardial infarction occluding it will
cause ischemia in the SA node unless there is a sufficiently good
anastomosis from the left coronary artery. If not, death of the affected
cells will stop the SA node from triggering the heartbeat

#### AV Node

The atrioventricular node (abbreviated AV node) is the tissue between
the atria and the ventricles of the heart, which conducts the normal
electrical impulse from the atria to the ventricles. The AV node
receives two inputs from the atria: posteriorly via the crista
terminalis, and anteriorly via the interatrial septum. \[1\] An
important property that is unique to the AV node is decremental
conduction. This is the property of the AV node that prevents rapid
conduction to the ventricle in cases of rapid atrial rhythms, such as
atrial fibrillation or atrial flutter. The atrioventricular node delays
impulses for 0.1 second before spreading to the ventricle walls. The
reason it is so important to delay the cardiac impulse is to ensure that
the atria are empty completely before the ventricles contract (Campbell
*et al.*, 2002). The blood supply of the AV node is from a branch of the
right coronary artery in 85% to 90% of individuals, and from a branch of
the left circumflex artery in 10% to 15% of individuals. In certain
types of supraventricular tachycardia, a person could have two AV Nodes;
this will cause a loop in electrical current and uncontrollably-rapid
heart beat. When this electricity catches up with itself, it will
dissipate and return to normal heart-beat speed.

#### AV Bundle

The bundle of HIS is a collection of heart muscle cells specialized for
electrical conduction that transmits the electrical impulses from the AV
node (located between the atria and the ventricles) to the point of the
apex of the fascicular branches. The fascicular branches then lead to
the Purkinje fibers which innervate the ventricles, causing the cardiac
muscle of the ventricles to contract at a paced interval. These
specialized muscle fibers in the heart were named after the Swiss
cardiologist Wilhelm His, Jr., who discovered them in 1893. Cardiac
muscle is very specialized, as it is the only type of muscle that has an
internal rhythm; i.e., it is myogenic which means that it can naturally
contract and relax without receiving electrical impulses from nerves.
When a cell of cardiac muscle is placed next to another, they will beat
in unison. The fibers of the Bundle of HIS allow electrical conduction
to occur more easily and quickly than typical cardiac muscle. They are
an important part of the electrical conduction system of the heart as
they transmit the impulse from the AV node (the ventricular pacemaker)
to the rest of the heart. The bundle of HIS branches into the three
bundle branches: the right left anterior and left posterior bundle
branches that run along the intraventricular septum. The bundles give
rise to thin filaments known as Purkinje fibers. These fibers distribute
the impulse to the ventricular muscle. Together, the bundle branches and
purkinje network comprise the ventricular conduction system. It takes
about 0.03-0.04s for the impulse to travel from the bundle of HIS to the
ventricular muscle. It is extremely important for these nodes to exist
as they ensure the correct control and co-ordination of the heart and
cardiac cycle and make sure all the contractions remain within the
correct sequence and in sync.

#### Purkinje Fibers

Purkinje fibers (or Purkyne tissue) are located in the inner ventricular
walls of the heart, just beneath the endocardium. These fibers are
specialized myocardial fibers that conduct an electrical stimulus or
impulse that enables the heart to contract in a coordinated fashion.
Purkinje fibers work with the sinoatrial node (SA node) and the
atrioventricular node (AV node) to control the heart rate. During the
ventricular contraction portion of the cardiac cycle, the Purkinje
fibers carry the contraction impulse from the left and right bundle
branches to the myocardium of the ventricles. This causes the muscle
tissue of the ventricles to contract and force blood out of the heart
--- either to the pulmonary circulation (from the right ventricle) or to
the systemic circulation (from the left ventricle). They were discovered
in 1839 by Jan Evangelista Purkinje, who gave them his name.

#### Pacemaker

The contractions of the heart are controlled by electrical impulses,
these fire at a rate which controls the beat of the heart. The cells
that create these rhythmical impulses are called pacemaker cells, and
they directly control the heart rate. Artificial devices also called
pacemakers can be used after damage to the body\'s intrinsic conduction
system to produce these impulses synthetically.

#### Fibrillation

Fibrillation is when the heart flutters abnormally. This can be detected
by an electrocardiogram which measures the waves of excitation passing
through the heart and plotting a graph of potential difference (voltage)
against time. If the heart and cardiac cycle is functioning properly the
electrocardiogram shows a regular, repeating pattern. However if there
is fibrillation there will be no apparent pattern, either in the much
more common \'Atrial Fibrillation\', or the less likely but much more
dangerous \'Ventricular Fibrillation\'. In a hospital during VF the
monitor would make a sound and alert the doctors to treat the
fibrillation by passing a huge current through the chest wall and
shocking the heart out of its fibrillation. This causes the cardiac
muscle to stop completely for 5 seconds and when it begins to beat again
the cardiac cycle would have resumed to normal and the heart will be
beating in a controlled manner again. Fibrillation is an example of
\"circus movement\" of impulses through the heart muscle.

Circus movement occurs when an impulse begins in one part of the heart
muscle and spreads in a circuitous pathway through the heart then
returns to the originally excited muscle and \"re-enters\" it to
stimulate it once more. The signal never stops. A cause of circus
movement is long length pathway in which the muscle is no longer in a
refractory state when the stimulus returns to it. A \"flutter\" is a
circus movement in coordinated, low frequency waves that cause rapid
heart rate. If the Bundle of HIS is blocked, it will result in
dissociation between the activity of the atria and that of the
ventricles, otherwise called a third degree heart block. The other cause
of a third degree block would be a block of the right, left anterior,
and left posterior bundle branches. A third degree block is very serious
medical condition that will most likely require an artificial pacemaker.

## The ECG

E.C.G stands for Electrocardiogram and represents the electrophysiology
of the heart. Cardiac electrophysiology is the science of the
mechanisms, functions, and performance of the electrical activities of
specific regions of the heart. The ECG is the recording of the heart\'s
electrical activity as a graph. The graph can show the heart\'s rate and
rhythm, it can detect enlargement of the heart, decreased blood flow, or
the presence of current or past heart attacks. ECG\'s are inexpensive,
Non-invasive, quick, and painless. Depending on the results, the
patient's medical history, and a physical exam; further tests or a
combination of medications and lifestyle changes may be ordered.

#### How To Read An ECG

  ECG Waveform
  --------------------------------------
  ![](QRS_normal.svg "QRS_normal.svg")

### Cardiac Muscle Contraction

After an action potential excites the plasma membrane of the cardiac
muscle cell the contraction is due to an increase in the cytoplasmic
concentration of Calcium ions. Similar to skeletal muscle, the release
of Ca+ ions from the sarcoplasmic reticulum binds to troponin which
allows actin to bind with myosin. The difference between skeletal muscle
and cardiac muscle is that when the action potential opens voltage gated
calcium ion channels in the T-tubules. The increase in cytosolic calcium
causes calcium ions to bind to receptors on the surface of the
sarcoplasmic reticulum. The binding of calcium ions to these receptors
causes the opening of more calcium ion channels in the SR membrane.
Calcium ions then rush out of the SR and bind to troponin and allow the
myosin and actin to bind together which causes contraction. This
sequence is called calcium-induced calcium release. Contraction ends
when the level of cytosolic calcium returns to normal resting levels.

### Blood Pressure

Blood pressure is the pressure exerted by the blood on the walls of the
blood vessels. Unless indicated otherwise, blood pressure refers to
systemic arterial blood pressure, i.e., the pressure in the large
arteries delivering blood to body parts other than the lungs, such as
the brachial artery (in the arm). The pressure of the blood in other
vessels is lower than the arterial pressure. Blood pressure values are
universally stated in millimeters of mercury (mmHg). The systolic
pressure is defined as the peak pressure in the arteries during the
cardiac cycle; the diastolic pressure is the lowest pressure (at the
resting phase of the cardiac cycle). The mean arterial pressure and
pulse pressure are other important quantities. Typical values for a
resting, healthy adult are approximately 120 mmHg systolic and 80mm Hg
diastolic (written as 120/80 mmHg), with individual variations. These
measures of blood pressure are not static, but undergo natural
variations from one heartbeat to another, and throughout the day (in a
circadian rhythm); they also change in response to stress, nutritional
factors, drugs, or disease.

#### Systolic Pressure

Systolic Pressure is the highest when the blood is being pumped out of
the left ventricle into the aorta during ventricular systole. The
average high during systole is 120 mmHg.

#### Diastolic Pressure

Diastolic blood pressure lowers steadily to an average low of 80 mmHg
during ventricular diastole.

## Cardiovascular Disease

Cardiovascular disease refers to the class of diseases that involve the
heart and/or blood vessels (arteries and veins). While the term
technically refers to any disease that affects the cardiovascular
system, it is usually used to refer to those related to atherosclerosis
(arterial disease). These conditions have similar causes, mechanisms,
and treatments. Over 50 million Americans have cardiovascular problems,
and most other Western countries face high and increasing rates of
cardiovascular disease. It is the number 1 cause of death and disability
in the United States and most European countries. By the time that heart
problems are detected, the underlying cause (atherosclerosis) is usually
quite advanced, having progressed for decades. There is therefore
increased emphasis on preventing atherosclerosis by modifying risk
factors, such as healthy eating, exercise and avoidance of smoking.

#### Hypertension

Hypertension or high blood pressure is a medical condition wherein the
blood pressure is chronically elevated. Hypertension is defined by some
authors as systolic pressure over 130 and diastolic over 85 mmHg. [^1]
Hypertension often has an insidious or un-noticed onset and is sometimes
called the *silent killer* because stretching of the arteries causes
microscopic tears in the arterial wall and accelerates degenerative
changes. Persistent hypertension is one of the risk factors for strokes,
heart attacks, heart failure and arterial aneurysm, and is a leading
cause of chronic renal failure

#### Atherosclerosis

!Severe atherosclerosis of the aorta.
Autopsy
specimen.{width="230"}

Atherosclerosis is a disease affecting the arterial blood vessel. It is
commonly referred to as a \"hardening\" or \"furring\" of the arteries.
It is caused by the formation of multiple plaques within the arteries.
Arteriosclerosis (\"hardening of the artery\") results from a deposition
of tough, rigid collagen inside the vessel wall and around the atheroma.
This increases the stiffness, decreases the elasticity of the artery
wall. Atherosclerosis typically begins in early adolescence, is usually
found in most major arteries, and yet is asymptomatic and not detected
by most diagnostic methods during life. It most commonly becomes
seriously symptomatic when interfering with the coronary circulation
supplying the heart or cerebral circulation supplying the brain, and is
considered the most important underlying cause of strokes, heart
attacks, various heart diseases including congestive heart failure and
most cardiovascular diseases in general.

#### Plaque

Plaque Atheroma or commonly known as plaque is an abnormal inflammatory
accumulation of macrophage white blood cells within the walls of
arteries.

#### Circulatory Shock

Circulatory Shock is a severe condition that results from reduced blood
circulation.

#### Thrombus

A thrombus, or blood clot, is the final product of the blood coagulation
step in hemostasis. It is achieved via the aggregation of platelets that
form a platelet plug, and the activation of the humoral coagulation
system (i.e. clotting factors). A thrombus is physiologic in cases of
injury, but pathologic in case of thrombosis.

Preventing blood clots reduces the risk of stroke, heart attack and
pulmonary embolism. Heparin and warfarin are often used to inhibit the
formation and growth of existing blood clots, thereby allowing the body
to shrink and dissolve the blood clots through normal methods.

#### Embolism

An embolism occurs when an object (the embolus) migrates from one part
of the body (through circulation) and causes a blockage (occlusion) of a
blood vessel in another part of the body. Blood clots form the most
common embolic material by far: other possible embolic materials include
fat globules (a fat embolism), air bubbles (an air embolism), septic
emboli (containing pus and bacteria), or amniotic fluid.

#### Stroke

A stroke, also known as cerebrovascular accident (CVA), is an acute
neurological injury whereby the blood supply to a part of the brain is
interrupted. Strokes can be classified into two major categories:
ischemic and hemorrhagic. \~80% of strokes are due to ischemia.

-   **Ischemic Stroke**: In ischemic stroke, which occurs in
    approximately 85-90% of strokes, a blood vessel becomes occluded and
    the blood supply to part of the brain is totally or partially
    blocked. Ischemic stroke is commonly divided into thrombotic stroke,
    embolic stroke, systemic hypoperfusion (Watershed or Border Zone
    stroke), or venous thrombosis
-   **Hemorrhagic Stroke**: A hemorrhagic stroke, or cerebral
    hemorrhage, is a form of stroke that occurs when a blood vessel in
    the brain ruptures or bleeds. Like ischemic strokes, hemorrhagic
    strokes interrupt the brain\'s blood supply because the bleeding
    vessel can no longer carry the blood to its target tissue. In
    addition, blood irritates brain tissue, disrupting the delicate
    chemical balance, and, if the bleeding continues, it can cause
    increased intracranial pressure which physically impinges on brain
    tissue and restricts blood flow into the brain. In this respect,
    hemorrhagic strokes are more dangerous than their more common
    counterpart, ischemic strokes. There are two types of hemorrhagic
    stroke: intracerebral hemorrhage, and subarachnoid hemorrhage.

The term \"brain attack\" is starting to come into use in the United
States for stroke, just as the term \"heart attack\" is used for
myocardial infarction, where a cutoff of blood causes necrosis to the
tissue of the heart. Many hospitals have \"brain attack\" teams within
their neurology departments specifically for swift treatment of stroke.
If symptoms of stroke are detected at early on-set, special \"clot
busting\" drugs may be administered. These clot busters will dissolve
clots before they can cause tissue death and restore normal circulation.
One of the initial drugs used to dissolve clots was **streptokinase**,
although its use creates a possibility of clot destruction throughout
the entire body, leading to serious hemorrhage. There are newer, third
generation thrombolytics that are safer.

#### Heart Attack

Acute myocardial infarction (AMI or MI), commonly known as a heart
attack, A heart attack occurs when the supply of blood and oxygen to an
area of heart muscle is blocked, usually by a clot in a coronary artery.
Often, this blockage leads to arrhythmias (irregular heartbeat or
rhythm) that cause a severe decrease in the pumping function of the
heart and may bring about sudden death. If the blockage is not treated
within a few hours, the affected heart muscle will die and be replaced
by scar tissue. It is the leading cause of death for both men and women
all over the world

#### Angina Pectoris

Angina Pectoris is chest pain due to ischemia (a lack of blood and hence
oxygen supply) of the heart muscle, generally due to obstruction or
spasm of the coronary arteries (the heart\'s blood vessels).

#### Coronary Bypass

Coronary artery bypass surgery, coronary artery bypass graft surgery and
heart bypass are surgical procedures performed on patients with coronary
artery disease for the relief of angina and possible improved heart
muscle function. Veins or arteries from elsewhere in the patient\'s body
are grafted from the aorta to the coronary arteries, bypassing coronary
artery narrowing caused by atherosclerosis and improves the blood supply
to the myocardium (heart muscle).

#### Congestive Heart Failure

Congestive heart failure (CHF), also called congestive cardiac failure
(CCF) or just heart failure, is a condition that can result from any
structural or functional cardiac disorder that impairs the ability of
the heart to fill with or pump a sufficient amount of blood throughout
the body. It is not to be confused with \"cessation of heartbeat\",
which is known as asystole, or with cardiac arrest, which is the
cessation of normal cardiac function in the face of heart disease.
Because not all patients have volume overload at the time of initial or
subsequent evaluation, the term \"heart failure\" is preferred over the
older term \"congestive heart failure\". Congestive heart failure is
often undiagnosed due to a lack of a universally agreed definition and
difficulties in diagnosis, particularly when the condition is considered
\"mild\". Right sided heart failure commonly causes peripheral edema, or
swelling of the extremities. Left sided heart failure commonly causes
pulmonary edema, or fluid buildup in the lungs.

#### Aneurysm

An aneurysm (or aneurism) is a localized dilation or ballooning of a
blood vessel by more than 50% of the diameter of the vessel and can lead
to instant death at anytime. Aneurysms most commonly occur in arteries
at the base of the brain (the circle of Willis) and in the aorta (the
main artery coming out of the heart) - this is an aortic aneurysm. This
bulge in a blood vessel, much like a bulge on an over-inflated inner
tube, can lead to death at anytime. The larger an aneurysm becomes, the
more likely it is to burst. Aneurysms are also described according to
their shape: Saccular or fusiform. A saccular aneurysm resembles a small
sack; a fusiform aneurysm is shaped like a spindle.

#### Dissolving Blood Clots

To dissolve blood clots you would use a drug that converts plasminogen
(molecule found in blood), to plasmin, (enzyme that dissolves blood
clots).

#### Clearing Clogged Arteries

One way to unblock a coronary artery (or other blood vessel) is
percutaneous transluminal coronary angioplasty (PTCA), which was first
performed in 1977. A wire is passed from the femoral artery in the leg
or the radial artery in the arm up to the diseased coronary artery, to
beyond the area of the coronary artery that is being worked upon. Over
this wire, a balloon catheter is passed into the segment that is to be
opened up. The end of the catheter contains a small folded balloon. When
the balloon is hydraulically inflated, it compresses the atheromatous
plaque and stretches the artery wall to expand. At the same time, if an
expandable wire mesh tube (stent) was on the balloon, then the stent
will be implanted (left behind) to support the new stretched open
position of the artery from the inside.

### Dilated and Inflamed Veins

#### Varicose Veins

Varicose veins are veins on the leg which are large, twisted, and
ropelike, and can cause pain, swelling, or itching. They are an extreme
form of telangiectasia, or spider veins. Varicose veins result due to
insufficiency of the valves in the communicating veins. These are veins
which link the superficial and deep veins of the lower limb. Normally,
blood flows from the superficial to the deep veins, facilitating return
of blood to the heart. However, when the valve becomes defective, blood
is forced into the superficial veins by the action of the muscle pump
(which normally aids return of blood to the heart by compressing the
deep veins). People who have varicose veins are more at risk of getting
a Deep Vein Thrombosis (DVT) and pulmonary embolisms.

#### Phlebitis

Phlebitis is an inflammation of a vein, usually in the legs. This is
usually the most serious if found in a deep vein. However, most people
with the condition, perhaps 80 to 90 percent, are women. The disease may
also have a genetic component, as it is known to run in families.

### Congenital Heart Defects

!Illustration of VSD Heart
defects present at birth are called congenital heart defects. Slightly
less than 1% of all newborn infants have congenital heart disease. Eight
defects are more common than all others and make up 80% of all
congenital heart diseases, whereas the remaining 20% consist of many
independently infrequent conditions or combinations of several defects.

#### Acyanotic Defects

Acyanotic heart defects are those in which there is a normal amount of
oxygen in the bloodstream. The most common congenital heart defect is a
ventral septal defect, which occurs in about 20% of all children with
congenital heart disease. In VSD blood from the left ventricle is
shunted to the right ventricle, resulting in oxygenated blood returning
into pulmonic circulation. One of the potential problems of VSD is
pulmonary hypertension.

#### Cyanotic Defects

Cyanotic heart defects refer to defects that result in decreased amounts
of oxygen in the blood. In cyanotic heart defects deoxygenated blood
from the right ventricle flows into the systemic circulation. Cyanotic
defects include tetrogy of Fallot and transposition of the great
arteries.

## Homeostasis

Homeostasis in the body is only possible if the cardiovascular system is
working properly. This means that the system needs to deliver oxygen and
nutrients to the tissue fluid that surrounds the cells and also take
away the metabolic waste. The heart is composed of arteries that take
blood from the heart, and vessels that return blood to the heart. Blood
is pumped by the heart into two circuits: the pulmonary and systemic
circuits. The pulmonary circuit carries blood through the lungs where
gas exchange occurs and the systemic system transports blood to all
parts of the body where exchange with tissue fluid takes place. The
cardiovascular system works together with all other systems to maintain
homeostasis.

### The Lymphatic System

The lymphatic system is closely related to the cardiovascular system.
There are three main ways that they work together to maintain
homeostasis: the lymphatic system receives the excess tissue fluid and
returns it to the bloodstream, lacteal take fat molecules from the
intestinal villi and transport them to the bloodstream and both systems
work together to defend the body against disease.The lymphatic system
can create white blood cells that fight off disease and infections.

## Interesting Facts

• Heart Disease is the number one killer in American women.\
• 16.7 million deaths are result forms of cardiovascular disease, heart
disease and stroke.\
• Stress, eating high fat foods, obesity, tobacco and alcohol use are
just some risk factors of developing heart disease.\
• Recent research suggests that taking a small dose of aspirin daily may
help prevent a heart attack (because aspirin inhibits platelet
clumping).\
• The length of all your blood vessels lined up is about 60,000 miles
long! To put this in perspective, the Earth\'s circumference is
40,075.02 kilometres and 60,000 miles is around 96,000 km - so your
blood vessels would go twice around the world and still have some to
spare!

## Ways to a Healthy Heart

• Eating healthy, good nutrition.\
• Fitness and Exercise.\
• Having a healthy lifestyle; don\'t drink, smoke, or do drugs.\
• Lowering LDL cholesterol and high blood pressure.\
• Reduce the fat, sodium, and calories in your diet.\

## Aging

The heart muscle becomes less efficient with age, and there is a
decrease in both maximum cardiac output and heart rate, although resting
levels may be more than adequate. The health of the myocardium depends
on its blood supply, and with age there is greater likelihood that
atherosclerosis will narrow the coronary arteries. Atherosclerosis is
the deposition of cholesterol on and in the walls of the arteries, which
decreases blood flow and forms rough surfaces that may cause
intravascular clot formation High blood pressure (hypertension) causes
the left ventricle to work harder. It may enlarge and outgrow its blood
supply, thus becoming weaker. A weak ventricle is not an efficient pump,
and may progress to congestive heart failure. This process may be slow
or rapid. The heart valves may become thickened by fibrosis, leading to
heart murmurs and less efficient pumping. Arrhythmias are also more
common with age, as the cells of the conduction pathway become less
efficient.

## Shock

Physiological Stress

Physiological stress can be any kind of injury from burns, to broken
bones; the body\'s response to stress is categorized in two phases the
ebb phase (early phase) begins immediately after the injury. And the
second phase is about 36 to 48 hours after injury is called the flow
phase. In the ebb (shock) phase there is Inadequate circulation,
decreased insulin level, decreased oxygen consumption, hypothermia (low
body temperature), hypovolemia (low blood volume), and hypotension (low
blood pressure). In the flow phase there is increased levels of
catecholamine, glucocorticoids, and glucagons, normal or elevated
insulin levels, catabolic (breakdown), hyperglycemic (high blood sugar),
increased oxygen consumption/respiratory rate, hyperthermia (high body
temperature) fever sets in, hypermetabolism, increased insulin
resistance, increased cardiac output.

## Premature ventricular contractions (PVC\'s)

Excitation occurs through the SA node to the AV node if there are
abnormalities or drug interference that malfunctions the AV node the
ventricles will not receive the initiating stimuli and the autorhythmic
cells in the bundle branches begin to initiate actions on their own rate
becoming the pacemakers for the ventricles. This in turn will cause
conduction disorder. With conduction that causes problems with the
bundle branches there is the right and the left premature ventricular
contractions. Right is most common and may go untreated. Left is always
a serious problem and must be treated.

## Intrinsic Control of heartbeat

• SA node (located in the right atrium near the entrance of the superior
vena cava)\
• AV node (located at the base of right atrium)\
• AV bundle (located in the intraventricular septum between the two
ventricles that go in two directions right and left bundle branches that
leave the septum to enter the walls of both ventricle)\
• Bundle Branches (the branching off the septum to the walls of the
ventricles that run into the purkinje fibers that then make contact with
ventricular myocardial cells to spread the impulse to the rest of the
ventricles)\
!Animation of a normal ECG
wave.

## Electrocardiogram

• The P is the atrial depolarization\
• QRS is the ventricular depolarization, as well as atrial
repolarization.\
• T is the ventricular repolarization\

!Schematic representation of normal
ECG{width="300"}

## Extrinsic Control of Heartbeat

Autonomic system with two subdivisions: the sympathetic division and the
parasympathetic division. Hormonal control of blood pressure

-   Epinephrine
-   Norepinephrine
-   ANP : Atrial natriuretic peptide
-   ADH: Antidiuretic hormone
-   Renin-Angiotension system

## Case Study

An example of the ever expanding technology for the heart is best
described in this story: In 1955, when I was five years old, I first
learned by my family physician that I had a heart murmur and that it
would eventually need attention. By the time I was 15 in 1965, I had two
cardiac catherizations at Rhode Island Hospital. The tests were
inconclusive and I was told to go on with my life and wait and see if I
had a problem. It wasn\'t until 1975 that I was told by my family
physician that I should have my heart checked again. Dr. David Kitzes of
Mariam Hospital performed another catherization. This time, unlike the
others, I was told that because of new machine technology, Dr. Kitzes
found that I had aortic stenosis, which is a narrowing of the valve
passage by build-up of plaque due to the valve being malformed at birth.
Dr. Kitzes informed me that I could lead a normal life until I was in my
fifties or sixties before I would need corrective surgery. In 1996, I
had an echocardiogram and it was determined that my heart was enlarged.
My family physician said that I should see a cardiologist. I down played
the visit as not being serious after hearing the same thing many times.
This time I entered the office of Jon Lambrecht, I had never met him
before. Within a few minutes my whole life was turned around. After
asking me about my symptoms, which were fatigue, weakness, asthmatic
symptoms, as well as ashen skin color and dizziness, he informed me of
how serious my condition was and the only salvation was immediate
open-heart surgery to replace the aortic valve. I began to cry as I
thought my life was over. Dr. Lambrecht studied my reaction and told me
that this condition is repairable and that I don\'t have a terminal
illness. I didn\'t have a lot of time to think about it. Within 10 days
from that visit, I was the recipient of a Meditronic Hall Prosthetic
heart valve. The operation was performed by Dr. Robert Indeglia at
Miriam Hospital in Providence, R.I. on March 20th, 1996. It has been
almost 3 years since the surgery and I am doing better than I could have
expected. In 1977 my son Kevin was born with Hypoplastic Left-heart
Syndrome and only lived for 2 days because heart surgery wasn\'t
performed like today. I am thankful that I lived at a time when medical
technology paved the way for a second chance because of my new aortic
heart valve. Our goal in this chapter is to take you by the hand and
lead you through each part of the cardiovascular system, so that you too
may learn and come to respect the greatness of this blood pumping
machine we all call the heart.

#### Stroke

Cerebrovascular disease are those that affect blood vessels in the brain
and happen to be the third cause of death in the United States only
behind heart disease and cancer. Stroke (also called cerebrovascular
accident or CVR) is a cerebrovascular disorder caused by a sudden
decrease or stoppage of blood flow to a part of the brain. Decreased
blood flow also known as ischemia is dangerous to any tissue but brain
tissue is even more vulnerable, mainly due to the high rate of its
metabolic reactions. In fact if you stopped blood flow for no more than
three minutes it may be sufficient enough to cause death of most brain
cells. For this reason a stroke can kill people within minutes or leave
them with severe brain damage.

Strokes may be classified as either occlusive or hemorrhagic and may
happen either in the interior of the brain or on its surface. In a
occlusive stroke blood flow through a vessel is blocked. In a
hemorrhagic stroke a blood vessel ruptures causing a hemorrhage.

## Summary

As with all of the body systems, the cardiovascular system plays a part
in maintaining homeostasis. The nervous system regulates the functioning
of the heart based on what the heart is supposed to do. The pumping of
the heart maintains normal blood pressure and proper oxygenation of
tissues. The vascular system forms passageways for the blood, but they
aren\'t simply just a pipeline system. The vessels are not passive
tubes, but rather active contributors to homeostasis. The arteries and
veins help maintain blood pressure, and the capillaries provide sites
for the necessary exchanges of materials between the blood and the
tissues.

## Review Questions

Answers for these questions can be found here

1\. This conducts electricity like nerves

:   A\) Epicardium
:   B\) Pericardium
:   C\) Myocardium
:   D\) Subvalaular Apparatus
:   E\) None of these, only nerves conduct electricity

2\. This carries the most blood at any given time in the body

:   A\) Veins
:   B\) Capillary Beds
:   C\) Veins
:   D\) Aorta
:   E\) Vena Cava

3\. The following contract together to pump blood

:   A\) Right atrium with the right ventricle and left atrium with the
    left ventricle
:   B\) Right atrium with left atrium and right ventricles with left
    ventricle
:   C\) Tricuspid valve and mitral valve
:   D\) Aorta and pulmonary artery
:   E\) Aorta, pulmonary artery and pulmonary vein

4\. This is the pacemaker of the heart

:   A\) AV node
:   B\) Purkinje fibers
:   C\) AV Bundle
:   D\) SA node
:   E\) None of these, a pacemaker is surgically inserted

5\. When reading an EKG, this letter shows the depolarization from the
AV node down to the AV bundle

:   A\) S
:   B\) P
:   C\) U
:   D\) T
:   E\) Q

6\. The T wave in an EKG shows

:   A\) Resting potential
:   B\) Atrial depolarization
:   C\) SA node excitation
:   D\) Ventricle repolarization
:   E\) Purkinje Excitation

7\. Blood pressure is the measure of

:   A\) Pressure exerted by the blood on the walls of the blood vessels
:   B\) Pressure exerted by the blood on the arteries
:   C\) Pressure exerted by the blood on the veins
:   D\) Pressure exerted by the blood on the aorta
:   E\) Pressure exerted by the blood on the capillaries

8\. Systolic Pressure is

:   A\) An average of 120 mm Hg
:   B\) Lowers steadily during ventricle systole
:   C\) The highest when blood is being pumped out of the left ventricle
    into the aorta
:   D\) An average of 80 mm Hg
:   E\) Both A and C
:   F\) Both B and D

9\. The heart has how many chambers?

:   A\) One
:   B\) Two
:   C\) Three
:   D\) Four
:   E\) Five

## Glossary

\
**Acute myocardial infarction (AMI or MI)** commonly known as a heart
attack, is a disease state that occurs when the blood supply to a part
of the heart is interrupted. The resulting ischemia or oxygen shortage
causes damage and potential death of heart tissue. **Aorta**: the
largest of the arteries in the systemic circuit\
**Aortic Valve**: lies between the left ventricle and the aorta\
**Antidiuretic hormone:** Produced in the posterior pituitary ADH
(vasopressin), major function is to regulate blood pressure by water
retention by the kidneys.\
**Arteriole:** a small diameter blood vessel that extends and branches
out from an artery and leads to capillaries\
**Atrial natriuretic peptide:** Produced in the atria of the heart, it
increases urinary excretion of sodium which causes water loss which in
turn the viscosity of the blood is lowered and in turn lowers the blood
pressure.\
**Atrioventricular Node (abbreviated AV node):** the tissue between the
atria and the ventricles of the heart, which conducts the normal
electrical impulse from the atria to the ventricles\
**Atrioventricular valves:** large, multi-cusped valves that prevent
backflow from the ventricles into the atria during systole\
**AV Bundle:** collection of heart muscle cells specialized for
electrical conduction that transmits the electrical impulses from the AV
node\
**Barbiturates:** CNS depressants, sedative-hypnotics\
**Blood Pressure:** the pressure exerted by the blood on the walls of
the blood vessels\
**Capillaries:** the smallest of a body's vessels, they connect arteries
and veins\
**Cardiac Cycle:** term used to describe the sequence of events that
occur as a heart works to pump blood through the body\
**Cerebral Vascular Accident (CVA):** Also known as a stroke, is a
rapidly developing loss of a part of brain function or loss of
consciousness due to an interruption in the blood supply to all or part
of the brain. That is, a stroke involves the sudden loss of neuronal
function due to a disturbance in cerebral perfusion. There are many
different causes for the interruption of blood supply, and different
parts of the brain can be affected. Because of this, a stroke can be
quite heterogeneous. Patients with the same cause of stroke can have
widely differing handicaps. Similarly, patients with the same clinical
handicap can in fact have different causes of their stroke.\
**Chordae Tendinae:** cord-like tendons that connect the papillary
muscles to the tricuspid valve and the mitral valve in the heart\
**Coronary Arteries:** blood vessels that supply blood to, and remove
blood from, the heart muscle itself\
**Continuous Capillaries:** have a sealed epithelium and only allow
small molecules, water and ions to diffuse\
**Deep-vein thrombosis (DVT):** is the formation of a blood clot
(\"thrombus\") in a deep vein. It commonly affects the leg veins, such
as the femoral vein or the popliteal vein or the deep veins of the
pelvis. Occasionally the veins of the arm are affected\
**Diastole:** period of time when the heart relaxes after contraction in
preparation for refilling with circulating blood\
**Diastolic Pressure:** lowest point in blood pressure where the heart
relaxes\
**Edema:** The swelling that forms when too much tissue fluid forms or
not enough taken away\
**Electrocardiogram:** the recording of the heart\'s electrical activity
as a graph\
**Epinephrine:** Produced in the adrenal medulla of the adrenal glands,
major function is vasoconstriction that will in turn increase
respiratory rate and increase cardiac out put.\
**Fenestrated Capillaries:** have openings that allow larger molecules
to diffuse\
**Fibrous Pericardium:** a dense connective tissue that protects the
heart, anchoring it to the surrounding walls, and preventing it from
overfilling with blood\
**Heart Rate:** term used to describe the frequency of the cardiac
cycle\
**Hepatic Veins:** blood vessels that drain de-oxygenated blood from the
liver and blood cleaned by the liver (from the stomach, pancreas, small
intestine and colon) into the inferior vena cava\
**Hypertension or High Blood Pressure:** medical condition wherein the
blood pressure is chronically elevated\
**Inferior Vena Cava (or IVC):** a large vein that carries de-oxygenated
blood from the lower half of the body into the heart\
**Intraventricular Septum:** the stout wall separating the lower
chambers (the ventricles) of the heart from one another\
**Left Atrium:**receives oxygenated blood from the left and right
pulmonary veins\
**Lub:** first heart tone, or S1; caused by the closure of the
atrioventricular valves, mitral and tricuspid, at the beginning of
ventricular contraction, or systole\
**Lumen:** hollow internal cavity in which the blood flows\
**Lymph:** originates as blood plasma that leaks from the capillaries of
the circulatory system, becoming interstitial fluid, filling the space
between individual cells of tissue\
**Mitral valve:** also known as the bicuspid valve; prevents blood
flowing from the left ventricle into the left atrium\
**Myocardium:** the muscular tissue of the heart.\
**Norepinephrine:** Produced in the adrenal medulla of the adrenal
glands, major function is a strong vasoconstrictor that will in turn
increase respiratory rate.\
**Pacemaker Cells:** cells that create these rhythmical impulses of the
heart\
**Plaque: an abnormal inflammatory accumulation of macrophage white
blood cells within the walls of arteries\
**Pulmonary Valve:**lies between the right ventricle and the pulmonary
artery; prevents back-flow of blood into the ventricle\
**Pulse: the number of heartbeats per minute\
**Purkinje Fibers (or Purkinje tissue):** located in the inner
ventricular walls of the heart, just beneath the endocardium;
specialized myocardial fibers that conduct an electrical stimulus or
impulse that enables the heart to contract in a coordinated fashion\
**Renin-Angiotension system:**\
**Right Atrium:** receives de-oxygenated blood from the superior vena
cava and inferior vena cava\
**Serous Pericardium:** functions in lubricating the heart to prevent
friction from occurring during heart activity\
**Semilunar Valves:** positioned on the pulmonary artery and the aorta\
**Sinoatrial Node:** (abbreviated SA node or SAN, also called the sinus
node): the impulse generating (pacemaker) tissue located in the right
atrium of the heart\
**Sinusoidal Capillaries:** special forms of fenestrated capillaries
that have larger opening allowing RBCs and serum proteins to enter\
**Systole:** contraction of the heart\
\'\'Systolic Pressure:**the highest point in blood pressure when the
blood is being pumped out of the left ventricle into the aorta during
ventricular systole\
**Superior Vena Cava (SVC):**a large but short vein that carries
de-oxygenated blood from the upper half of the body to the heart\'s
right atrium\
**Thrombus:**a blood clot in an intact blood vessel\
**Tricuspid Valve:**on the right side of the heart, between the right
atrium and the right ventricle; allows blood to flow from the right
atrium into the right ventricle when the heart is relaxed during
diastole\
**Vasoconstriction:**the constriction of blood vessels\
**Vasodilation: the dilation of blood vessels\
**Veins:**carry de-oxygenated blood from the capillary blood vessels to
the right part of the heart\
**Ventricle:** a heart chamber which collects blood from an atrium\
**Venule:** a small blood vessel that allows deoxygenated blood to
return from the capillary beds to the larger blood vessels called veins

## References

1.  Van De Graaff, Kent M. Human Anatomy. McGraw Hill Publishing, Burr
    Ridge, IL. 2002.
2.  Essentials of Anatomy and Physiology, Valerie C. Scanlon and Tina
    Sanders
3.  Tortora, G. & Grabowski, S. (2000) Principles of Anatomy &
    Physiology. Wiley & Sons. Brisbane, Singapore & Chichester.
4.  Anderson, RM. The Gross Physiology of the Cardiovascular
    System (1993) \<<http://cardiac-output.info>\>.

[^1]: Tortora, G. & Grabowski, S. (2000)Principles of anatomy and
    physiology. Ninth Edition. Wiley page 733.