Cardiac
Structures
• The heart is a four-chambered hollow muscular organ normally the
approximate size of a fist. It lies within the thorax in the mediastinal space
that separates the right and left pleural cavities.
• The heart is composed of three layers: a thin inner lining, the endocardium; a
layer of muscle, the myocardium; and an outer layer, the epicardium.
• The heart is covered by a fibroserous sac called the pericardium. This sac
consists of two layers: the inside (visceral) layer of the pericardium (the
epicardium) and the outer (parietal) layer.
• A small amount of pericardial fluid (approximately 10 to 15 mL) lubricates the
space between the pericardial layers (pericardial space) and prevents friction
between the surfaces as the heart contracts.
• The heart is divided vertically by the septum. The interatrial septum creates a
right and a left atrium, and the interventricular septum creates a right and a
left ventricle. The thickness of the wall of each chamber is different. The atrial
myocardium is thinner than that of the ventricles, and the left ventricular wall
is 2 to 3 times thicker than the right ventricular wall.
Blood Flow of the Heart
• The right atrium receives venous blood from the inferior and superior venae
cavae and the coronary sinus. The blood then passes through the tricuspid
valve into the right ventricle. With each contraction, the right ventricle pumps
blood through the pulmonic valve into the pulmonary artery and to the lungs.
• Blood flows from the lungs to the left atrium by way of the pulmonary veins.
It then passes through the mitral valve and into the left ventricle. As the
heart contracts, blood is ejected through the aortic valve into the aorta and
thus enters the systemic circulation.
Valves
• The four valves of the heart serve to keep blood flowing in a forward
direction.
• The cusps of the mitral and tricuspid valves are attached to thin strands of
fibrous tissue termed chordae tendineae. Chordae are anchored in the
papillary muscles of the ventricles. This support system prevents eversion of
the leaflets into the atria during ventricular contraction.
• The pulmonic and aortic valves (also known as semilunar valves) prevent
blood from regurgitating into the ventricles at the end of each ventricular
contraction.
Coronary Arteries & Veins
• The myocardium has its own blood supply, the coronary circulation.
• Blood flow into the two major coronary arteries occurs primarily during
diastole (relaxation of the myocardium).
• The left coronary artery arises from the aorta and divides into two main
branches: the left anterior descending artery and the left circumflex artery.
These arteries supply the left atrium, the left ventricle, the interventricular
septum, and a portion of the right ventricle.
• The right coronary artery also arises from the aorta, and its branches supply
the right atrium, the right ventricle, and a portion of the posterior wall of the
left ventricle. In 90% of people, the atrioventricular (AV) node and the bundle
of His, part of the cardiac conduction system, receive blood supply from the
, right coronary artery. For this reason, obstruction of this artery often causes
serious defects in cardiac conduction.
• The divisions of coronary veins parallel those of coronary arteries. Most of the
blood from the coronary system drains into the coronary sinus (a large
channel), which empties into the right atrium near the entrance to the inferior
vena cava.
Conduction System
• The conduction system is specialized nerve tissue responsible for creating
and transporting the electrical impulse, or action potential. This impulse
starts depolarization and subsequently cardiac contraction.
• The electrical impulse is normally started by the sinoatrial (SA) node (the
pacemaker of the heart). Each impulse coming from the SA node travels
through interatrial pathways to depolarize the atria, resulting in a contraction.
• The electrical impulse travels from the atria to the AV node through
internodal pathways. The excitation then moves through the bundle of His
and the left and right bundle branches. The left bundle branch has two
fascicles (divisions): anterior and posterior.
• The action potential moves through the walls of both ventricles by means of
Purkinje fibers. The ventricular conduction system delivers the impulse within
0.12 second. This triggers a synchronized right and left ventricular
contraction.
• The result of the cardiac cycle is the ejection of blood into the pulmonary and
systemic circulation. It ends with repolarization, when the contractile fiber
cells and the conduction pathway cells regain their resting polarized
condition.
• Cardiac muscle cells have a compensatory mechanism that makes them
unresponsive or refractory to restimulation during the action potential. During
ventricular contraction, an absolute refractory period occurs, during which
cardiac muscle does not respond to any stimuli. After this period, cardiac
muscle gradually recovers its excitability, and a relative refractory period
occurs by early diastole.
ELECTROCARDIOGRAM
• The electrical activity of the heart can be detected on the body surface using
electrodes and is recorded on an electrocardiogram (ECG). The letters P, QRS,
T, and U are used to identify the separate waveforms.
• The first wave, P, begins with the firing of the SA node and represents
depolarization of the atria.
• The QRS complex represents depolarization from the AV node throughout the
ventricles. Impulse transmission through the AV node is delayed, which
accounts for the time interval between the end of the P wave and the
beginning of the QRS wave.
• The T wave represents repolarization of the ventricles.
• The U wave, if seen, may represent repolarization of the Purkinje fibers, or it
may be associated with hypokalemia.
• Intervals between these waves (PR, QRS, and QT intervals) reflect the length
of time it takes for the impulse to travel from one area of the heart to
another. These time intervals can be measured, and changes from these time
references often indicate pathology.
Mechanical System
• Depolarization triggers mechanical activity.
, • Systole, contraction of the myocardium, results in ejection of blood from the
ventricles.
• Relaxation of the myocardium, diastole, allows for filling of the ventricles.
• Stroke volume is the amount of blood ejected from the ventricle with each
heartbeat.
• Cardiac output (CO) is the amount of blood pumped by each ventricle in 1
minute. It is calculated by multiplying the stroke volume (SV) by the heart
rate (HR): CO = SV X HR.
• For the normal adult at rest, CO is maintained in the range of 4 to 8 L/min.
• Cardiac index (CI) is the CO divided by the body surface area (BSA). The CI
adjusts the CO to the body size. The normal CI is 2.8 to 4.2 L per minute per
meter squared (L/min/m2).
Factors Affecting Cardiac Output
o Numerous factors can affect either the HR or the SV and thus the CO.
The HR, which is controlled primarily by the autonomic nervous
system, can reach as high as 180 beats/minute for short periods
without harmful effects. The factors affecting the SV are preload,
contractility, and afterload. Increasing preload, contractility, and
afterload increases the workload of the myocardium, resulting in
increased oxygen demand.
Preload
Volume of blood in ventricles at the end of diastole
The volume of blood in the ventricles at the end of diastole, before the
next contraction, is called preload. Preload determines the amount of
stretch placed on myocardial fibers. Preload can be increased due to a
number of causes such as myocardial infarction, aortic stenosis, and
hypervolemia.
Contractility
can be increased by epinephrine and norepinephrine released by the
sympathetic nervous system. Increasing contractility raises the SV by
increasing ventricular emptying.
Afterload
Peripheral resistance against which the left ventricle must pump
Afterload is the peripheral resistance against which the left ventricle
must pump. Afterload is affected by size of the ventricle, wall tension,
and arterial blood pressure. If the arterial blood pressure is elevated,
the ventricles will meet increased resistance to ejection of blood,
increasing the work demand. Eventually, this results in ventricular
hypertrophy, an enlargement of cardiac muscle tissue without an
increase in CO or the size of the chambers.
Structures/Functions
Vascular System
o Arteries carry oxygenated blood away from the heart, except for the
pulmonary artery.
o Veins carry deoxygenated blood toward the heart, except for the
pulmonary veins.
, o Small branches of arteries and veins are arterioles and venules,
respectively.
o Blood circulates from the heart into arteries, arterioles, capillaries,
venules, and veins, and then back to the heart.
o The arterial system differs from the venous system by the amount and
type of tissue that makes up arterial walls.
o Arteries and Arterioles. The large arteries have thick walls that are
composed mainly of elastic tissue. This elastic property cushions the
impact of the pressure created by ventricular contraction and provides
recoil that propels blood forward into the circulation. Large arteries
also contain some smooth muscle. Examples of large arteries are the
aorta and the pulmonary artery.
o The innermost lining of the arteries is the endothelium. The
endothelium serves to maintain hemostasis, promote blood flow, and,
under normal conditions, inhibit blood coagulation. When the
endothelial surface is disrupted (e.g., rupture of an atherosclerotic
plaque), the coagulation cascade is initiated and results in the
formation of a fibrin clot.
o Capillaries. The thin capillary wall is made up of endothelial cells, with
no elastic or muscle tissue. The exchange of cellular nutrients and
metabolic end products takes place through these thin-walled vessels.
Capillaries connect the arterioles and venules.
o Veins and Venules. Veins are large-diameter, thin-walled vessels that
return blood to the right atrium. The venous system is a low-pressure,
high-volume system. The larger veins contain semilunar valves at
intervals to maintain the blood flow toward the heart and to prevent
backward flow. The amount of blood in the venous system is affected
by a number of factors, including arterial flow, compression of veins by
skeletal muscles, alterations in thoracic and abdominal pressures, and
right atrial pressure. The largest veins are the superior vena cava,
which returns blood to the heart from the head, neck, and arms, and
the inferior vena cava, which returns blood to the heart from the lower
part of the body.
Regulation of the cardiovascular system
o Autonomic nervous system
system consists of the sympathetic nervous system and the
parasympathetic nervous system.
Stimulation of the sympathetic nervous system increases the
HR, the speed of impulse conduction through the AV node, and
the force of atrial and ventricular contractions. Additionally,
stimulation of α1-adrenergic receptors in vascular smooth
muscle results in vasoconstriction, increasing the blood
pressure.
In contrast, stimulation of the parasympathetic system
(mediated by the vagus nerve) slows HR by decreasing the
impulse from the SA node and thus conduction through the AV
node.
o Baroreceptors
Structures
• The heart is a four-chambered hollow muscular organ normally the
approximate size of a fist. It lies within the thorax in the mediastinal space
that separates the right and left pleural cavities.
• The heart is composed of three layers: a thin inner lining, the endocardium; a
layer of muscle, the myocardium; and an outer layer, the epicardium.
• The heart is covered by a fibroserous sac called the pericardium. This sac
consists of two layers: the inside (visceral) layer of the pericardium (the
epicardium) and the outer (parietal) layer.
• A small amount of pericardial fluid (approximately 10 to 15 mL) lubricates the
space between the pericardial layers (pericardial space) and prevents friction
between the surfaces as the heart contracts.
• The heart is divided vertically by the septum. The interatrial septum creates a
right and a left atrium, and the interventricular septum creates a right and a
left ventricle. The thickness of the wall of each chamber is different. The atrial
myocardium is thinner than that of the ventricles, and the left ventricular wall
is 2 to 3 times thicker than the right ventricular wall.
Blood Flow of the Heart
• The right atrium receives venous blood from the inferior and superior venae
cavae and the coronary sinus. The blood then passes through the tricuspid
valve into the right ventricle. With each contraction, the right ventricle pumps
blood through the pulmonic valve into the pulmonary artery and to the lungs.
• Blood flows from the lungs to the left atrium by way of the pulmonary veins.
It then passes through the mitral valve and into the left ventricle. As the
heart contracts, blood is ejected through the aortic valve into the aorta and
thus enters the systemic circulation.
Valves
• The four valves of the heart serve to keep blood flowing in a forward
direction.
• The cusps of the mitral and tricuspid valves are attached to thin strands of
fibrous tissue termed chordae tendineae. Chordae are anchored in the
papillary muscles of the ventricles. This support system prevents eversion of
the leaflets into the atria during ventricular contraction.
• The pulmonic and aortic valves (also known as semilunar valves) prevent
blood from regurgitating into the ventricles at the end of each ventricular
contraction.
Coronary Arteries & Veins
• The myocardium has its own blood supply, the coronary circulation.
• Blood flow into the two major coronary arteries occurs primarily during
diastole (relaxation of the myocardium).
• The left coronary artery arises from the aorta and divides into two main
branches: the left anterior descending artery and the left circumflex artery.
These arteries supply the left atrium, the left ventricle, the interventricular
septum, and a portion of the right ventricle.
• The right coronary artery also arises from the aorta, and its branches supply
the right atrium, the right ventricle, and a portion of the posterior wall of the
left ventricle. In 90% of people, the atrioventricular (AV) node and the bundle
of His, part of the cardiac conduction system, receive blood supply from the
, right coronary artery. For this reason, obstruction of this artery often causes
serious defects in cardiac conduction.
• The divisions of coronary veins parallel those of coronary arteries. Most of the
blood from the coronary system drains into the coronary sinus (a large
channel), which empties into the right atrium near the entrance to the inferior
vena cava.
Conduction System
• The conduction system is specialized nerve tissue responsible for creating
and transporting the electrical impulse, or action potential. This impulse
starts depolarization and subsequently cardiac contraction.
• The electrical impulse is normally started by the sinoatrial (SA) node (the
pacemaker of the heart). Each impulse coming from the SA node travels
through interatrial pathways to depolarize the atria, resulting in a contraction.
• The electrical impulse travels from the atria to the AV node through
internodal pathways. The excitation then moves through the bundle of His
and the left and right bundle branches. The left bundle branch has two
fascicles (divisions): anterior and posterior.
• The action potential moves through the walls of both ventricles by means of
Purkinje fibers. The ventricular conduction system delivers the impulse within
0.12 second. This triggers a synchronized right and left ventricular
contraction.
• The result of the cardiac cycle is the ejection of blood into the pulmonary and
systemic circulation. It ends with repolarization, when the contractile fiber
cells and the conduction pathway cells regain their resting polarized
condition.
• Cardiac muscle cells have a compensatory mechanism that makes them
unresponsive or refractory to restimulation during the action potential. During
ventricular contraction, an absolute refractory period occurs, during which
cardiac muscle does not respond to any stimuli. After this period, cardiac
muscle gradually recovers its excitability, and a relative refractory period
occurs by early diastole.
ELECTROCARDIOGRAM
• The electrical activity of the heart can be detected on the body surface using
electrodes and is recorded on an electrocardiogram (ECG). The letters P, QRS,
T, and U are used to identify the separate waveforms.
• The first wave, P, begins with the firing of the SA node and represents
depolarization of the atria.
• The QRS complex represents depolarization from the AV node throughout the
ventricles. Impulse transmission through the AV node is delayed, which
accounts for the time interval between the end of the P wave and the
beginning of the QRS wave.
• The T wave represents repolarization of the ventricles.
• The U wave, if seen, may represent repolarization of the Purkinje fibers, or it
may be associated with hypokalemia.
• Intervals between these waves (PR, QRS, and QT intervals) reflect the length
of time it takes for the impulse to travel from one area of the heart to
another. These time intervals can be measured, and changes from these time
references often indicate pathology.
Mechanical System
• Depolarization triggers mechanical activity.
, • Systole, contraction of the myocardium, results in ejection of blood from the
ventricles.
• Relaxation of the myocardium, diastole, allows for filling of the ventricles.
• Stroke volume is the amount of blood ejected from the ventricle with each
heartbeat.
• Cardiac output (CO) is the amount of blood pumped by each ventricle in 1
minute. It is calculated by multiplying the stroke volume (SV) by the heart
rate (HR): CO = SV X HR.
• For the normal adult at rest, CO is maintained in the range of 4 to 8 L/min.
• Cardiac index (CI) is the CO divided by the body surface area (BSA). The CI
adjusts the CO to the body size. The normal CI is 2.8 to 4.2 L per minute per
meter squared (L/min/m2).
Factors Affecting Cardiac Output
o Numerous factors can affect either the HR or the SV and thus the CO.
The HR, which is controlled primarily by the autonomic nervous
system, can reach as high as 180 beats/minute for short periods
without harmful effects. The factors affecting the SV are preload,
contractility, and afterload. Increasing preload, contractility, and
afterload increases the workload of the myocardium, resulting in
increased oxygen demand.
Preload
Volume of blood in ventricles at the end of diastole
The volume of blood in the ventricles at the end of diastole, before the
next contraction, is called preload. Preload determines the amount of
stretch placed on myocardial fibers. Preload can be increased due to a
number of causes such as myocardial infarction, aortic stenosis, and
hypervolemia.
Contractility
can be increased by epinephrine and norepinephrine released by the
sympathetic nervous system. Increasing contractility raises the SV by
increasing ventricular emptying.
Afterload
Peripheral resistance against which the left ventricle must pump
Afterload is the peripheral resistance against which the left ventricle
must pump. Afterload is affected by size of the ventricle, wall tension,
and arterial blood pressure. If the arterial blood pressure is elevated,
the ventricles will meet increased resistance to ejection of blood,
increasing the work demand. Eventually, this results in ventricular
hypertrophy, an enlargement of cardiac muscle tissue without an
increase in CO or the size of the chambers.
Structures/Functions
Vascular System
o Arteries carry oxygenated blood away from the heart, except for the
pulmonary artery.
o Veins carry deoxygenated blood toward the heart, except for the
pulmonary veins.
, o Small branches of arteries and veins are arterioles and venules,
respectively.
o Blood circulates from the heart into arteries, arterioles, capillaries,
venules, and veins, and then back to the heart.
o The arterial system differs from the venous system by the amount and
type of tissue that makes up arterial walls.
o Arteries and Arterioles. The large arteries have thick walls that are
composed mainly of elastic tissue. This elastic property cushions the
impact of the pressure created by ventricular contraction and provides
recoil that propels blood forward into the circulation. Large arteries
also contain some smooth muscle. Examples of large arteries are the
aorta and the pulmonary artery.
o The innermost lining of the arteries is the endothelium. The
endothelium serves to maintain hemostasis, promote blood flow, and,
under normal conditions, inhibit blood coagulation. When the
endothelial surface is disrupted (e.g., rupture of an atherosclerotic
plaque), the coagulation cascade is initiated and results in the
formation of a fibrin clot.
o Capillaries. The thin capillary wall is made up of endothelial cells, with
no elastic or muscle tissue. The exchange of cellular nutrients and
metabolic end products takes place through these thin-walled vessels.
Capillaries connect the arterioles and venules.
o Veins and Venules. Veins are large-diameter, thin-walled vessels that
return blood to the right atrium. The venous system is a low-pressure,
high-volume system. The larger veins contain semilunar valves at
intervals to maintain the blood flow toward the heart and to prevent
backward flow. The amount of blood in the venous system is affected
by a number of factors, including arterial flow, compression of veins by
skeletal muscles, alterations in thoracic and abdominal pressures, and
right atrial pressure. The largest veins are the superior vena cava,
which returns blood to the heart from the head, neck, and arms, and
the inferior vena cava, which returns blood to the heart from the lower
part of the body.
Regulation of the cardiovascular system
o Autonomic nervous system
system consists of the sympathetic nervous system and the
parasympathetic nervous system.
Stimulation of the sympathetic nervous system increases the
HR, the speed of impulse conduction through the AV node, and
the force of atrial and ventricular contractions. Additionally,
stimulation of α1-adrenergic receptors in vascular smooth
muscle results in vasoconstriction, increasing the blood
pressure.
In contrast, stimulation of the parasympathetic system
(mediated by the vagus nerve) slows HR by decreasing the
impulse from the SA node and thus conduction through the AV
node.
o Baroreceptors
