THE CARDIOVASCULAR
SYSTEM
THE HEART
INTRODUCTION
• The cardiovascular system is composed of blood, blood
vessels and the heart.
• Our heart beats nearly 100,000 times daily.
• Blood vessels are fractionated into a pulmonary circuit and
systemic circuit.
• Artery:vessels that carry blood away from the heart.
Usually oxygenated
• vein: vessels that carry blood towards the heart. Usually
deoxygenated.
• Capillaries: a small blood vessel that allow diffusion of
gases, nutrients and wastes between plasma and interstitial
fluid.
HEART ANATOMY
• The human heart, which is about the size of a clenched fist,
is located obliquely within the mediastanium of the thorax.
• The heart is located within a double sac made up of the
outer fibrous pericardium and the inner serous pericardium
(parietal and visceral layers).
• The pericardial cavity contains the thymus, esophagus and
trachea. The pericardium may be divided into the visceral
and parietal pericardium. The cavity is space between the
pericardium and epicardium.
• The cavity contains a fluid called the pericardial fluid.
Layers of the heart wall
• Layers of the heart wall, from the interior out are the
endocardium, the myocardium and the epicardium. Epi.
Lines the heart surface, the myo. Makes up the largest
portion of the heart and endo. The innermost layer of the
heart’s wall.
• The heart has four chambers- two superior atria and two
inferior ventricles. Functionally the heart is a double
pump.
• Entering the right atrium are the superior vena cava, the
inferior vena cava, and the coronary sinus. Four
pulmonary veins enter the left atrium.
• The right ventricle discharges blood into the pulmonary
The Heart
• The heart has four valves, two AV valves :bicuspid and
tricuspid- and two semilunar valves-aortic and pulmonic.
• The tricuspid and bicuspid are located between the atria
and ventricles and these valves are anchored by fibers
called Chordae Tendinae.
• These serve as one way door to keep blood flowing in one
direction.
• The heart cannot pump unless an electrical stimulus
occurs.
• SA--> AV--> Purkinje fibers.
Pathway of blood through the
heart
• The right heart is the pulmonary circuit pump.
Oxygen-poor systemic blood enters the right
atrium, passes into the right ventricle, through the
pulmonary trunk to the lungs, and back to the left
atrium via the pulmonary veins.
• The left heart is the systemic circuit pump.
Oxygen-laden blood entering the left atrium from
the lungs flows into the left ventricle and then into
the aorta, which provides the functional supply of
all body organs. Systemic veins return the
oxygen-depleted blood to the right atrium.
Innervation of the Heart
• The sympathetic and parasympathetic divisions of the
ANSS provide innervation to the heart through the cardiac
plexus.
• The vagus nerve carries PS preganglionic fibers to small
ganglia in the cardiac plexus. Both ANS divisions
innervate the SA and AV nodes and atrial muscle cells.
• Stimulation of the cardioaccedlaratory center activates the
necessary sympathetic neurons; the nearby cardioinhibitory
center governs the activity of the PS neurons.
• The baroreceptors and chemoreceptors innervated by NIX
and NX are monitored by cardiac center.
Coronary Circulation
• The coronary circulation includes the right and left
coronary arteries . These ensure a continuous flow of
blood to meet the demands of active cardiac muscle and
cardiac veins.
• Right cornary artery supplies blood to the rt. Atrium,
portion of both ventricles and portions of the SA and AV
nodes.
• The left coronary artery-to the left ventricle, left atrium
and IV septum.
• Venous blood, collected by the cardiac veins (great, middle
and small), is emptied into the coronary sinus.
Cardiac Veins
• The great cardiac vein begins on the anterior surface of the
ventricles. Then it reaches the level of the atria and then
curves around the left side of the heart within the coronary
sulcus.
• The vein empties into the coronary sinus which then
communicates with the right atrium near the base of the
inferior vena cava.
• Other cardiac veins are:posterior cardiac vein, the middle
cardiac vein and small and anterior cardiac veins. They all
empty into the great cardiac vein or coronary sinus.
Cardiac Muscle Fibers
• Cardiac muscle cells are branching, striated,
generally uninucleate cells. They contain
myofibrils consisting of typical sarcomeres.
• Adjacent cardiac cells are connected by
intercalated discs containing desmososmes and
gap junctions, which convey the force of
contraction from cell to cell and conduct action
potentials.
• Cardiac muscle has abundant mitochondria and
depends primarily on aerobic respiration to form
ATP.
Mechanism and events of
contraction
• Action-potential generation in the contractile cardiac
muscle mimics that of skeletal muscle. Membrane
depolarization causes opening of sodium channels and
sodium entry which is responsible for the rising phase of
the action potential curve.
• Depolarization also opens slow calcium channels.-creates
the plateau.the actin potential is coupled to sliding of the
myofilaments by ionic calcium released by the SR and
entering from the extracellular space.
• Compared to skeletal muscle, cardiac muscle has a longer
refractory period that prevents tetanization.
Heart Physiology
• Certain noncontractile cardiac muscle cells exhibit
automaticity and rhythmicity and can independently
initiate action potentials.
• Such cells have an unstable resting potential called a
pacemaker potential that gradually depolarizes drifting
toward threshold for firing.
• These cells comprise the intrinsic conduction system of the
heart.
• Defects in the intrinsic conduction system can cause
arrhythmia, fibrillation ands heart block.
Electrical Events
• Autorhythmicity:heart contracts without help of hormonal
or neuronal stimulation.
• The conduction or nodal system of the heart consists of the
AV and SA nodes, the AV bundle and bundle branches and
Purkinje fibers.
• This system coordinates the depolarization and ensures the
heart beats as one.
• The SA acts as the heart’s pacemaker and sets the sinus
rhythm.
Impulse pathway
• SA node sends stimulus->reaches AV node-->internodal
pathways->atrial contraction->IV septum->bundle
branches, Purkinje fibers, papillary muscles->Purkinje
relay to ventricular myocardium->ventricular contraction>blood pushed to aortic and pulmonary trunks.
• Conduction deficit problems that happen when the
conducting pathways are damaged. The rhythm of heart
will be disturbed.
• Ectopic pacemaker:an abnormal cell generates action
potential which overrides those of SA or AV nodes.
Disrupts timing of ventricular contraction. Heart efficiency
is reduced.
Electrical events
• The heart is innervated by the ANS.
• Autonomic cardiac centers in the medulla include
the cardioaccelaratory center, which projects to the
T1-T5 region of the spinal cord, which in turn
projects to the cervical and upper thoracic chain
ganglia.
• The PS cardioinhibitory center exerts its influence
via the vagus nerve which project to the heart
wall. Most PS nerves serve the SA and AV nodes.
ECG
• A recording of electrical activities in the heart is an
Electrocardiogram or ECG.
• Important landmarks of an ECG include P wave, QRS
complex and the T wave.
• The P wave reflects atrial depolarization.
• The QRS complex indicates ventricular depolarization.
• T wave indicates ventricular repolarization.
• ECG analysis is especially useful in diagnosing cardiac
arrhythmias, abnormal patterns of cardiac electrical
activity.
Mechanical Events: Cardiac
Cycle
• Cardiac cycle refers to events occurring during
one heartbeat. It contains periods of atrial and
ventricular systole (contraction) and diastole
(relaxation).
• When the heart beats the two ventricles eject equal
volumes of blood.
• The closing of valves and rushing of blood
through the heart causes characteristic heart
sounds which can be heard during auscultation.
Cardiac Cycle
• During mid-to-late diastole, the ventricles fill and atria
contract.
• Ventricular systole consists of the isovolumetric
contraction phase and the ventricular ejection phase.
• During early diastole, the ventricles are relaxed and are
closed chambers until increasing atrial pressure forces the
AV valves open and the cycle begins again.
• At a normal heart rate of 75 beats/min, a cardiac cycle lasts
0.8 s.
• pressure changes promote blood flow and valve opening
and closing.
CARDIAC OUTPUT
• Cardiac output which is typically 5L/min is
the amount of blood pumped out by each
ventricle in one minute.
• Stroke volume is the amount of blood
pumped out by a ventricle with each
contraction.
• Cardiac output=heart rate X stroke volume
CARDIAC OUTPUT
• Stroke volume depends to a large extent on the degree of
stretch of cardiac muscle by venous return.
• This is approx. 70 mL and is the difference between the
EDV(end diastolic volume) and ESV (end systolic
volume). Anything that influences heart rate or blood
volume influences venous return and hence stroke volume.
• Activation of the SNS increases heart rate and
contractility; parasympathetic activation decreases heart
rate and contractility.
CARDIAC OUTPUT
• Chemical regulation of the heart is effected by hormonesepinephrine and thyroxine- and ions (Na, K, Ca).
Imbalances in the ion severely impair heart activity.
• Other factors influencing heart rate are age, sex, exercise
and body temperature.
• Congestive heart failure occurs when the pumping of the
heart is inadequate to provide normal circulation to meet
body needs.
• Right heart failure leads to systemic edema and left to
pulmonary edema.
Factors Affecting Heart Rate
• The cardioaccelaratory center in the medulla
oblongata activates sympathetic neurons;the
cardioinhibitory center governs the activities of
the parasympathetic neurons. These in turn
receive inputs from higher centers and from the
baroreceptors and chemoreceptors.
• The basic heart rate is established by pacemaker
cells of the SA node.
• Sympathetic activity produces more powerful
contractions and parasympathetic stimulation
slows the heart.