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LECTURE 2: THE HEART
Prof. Magidah Alaudi, M.Sc.malaudi@gmail.com
Circuits of the Cardiovascular System
Pulmonary circuit Delivers blood from the right
ventricle of the heart to the lungs and from the lungs to the left atrium of the heart
Systemic circuit Delivers blood from the left
ventricle of the heart to the rest of the body and collects blood from the rest of the body and delivers it to the right atrium of the heart.
The Pericardium
Visceral pericardium or epicardium Parietal pericardium
Pericardial fluid
Pericardial Cavity Visceral Pericardium
Parietal Pericardium
Layers of the Heart Endocardium
inner layersimple squamous epithelium (endothelium)
Myocardiummiddle layercardiac muscle
Epicardiumouter layerloose connective tissue
Superficial Anatomy of the Heart
The heart consists of four chambersTwo upper chamber called atria Two lower chambers called ventricles
The two upper and two lower chambers are separated by atrioventricular valves
Tricuspid ValveBetween RA and RV
Mitral Valve (Bicuspid)Between LA and LV
The Heart Wall
The heart wall is composed of three layers:Epicardium: primarily
composed of Areolar Tissue and epithelium
Myocardium: primarily composed of cardiac muscle tissue
Endocardium: primarily composed of Areolar Tissue and endothelium
Internal Anatomy of the heart: Atria
Right AtriumThin walled chambers that receive blood from
superior and inferior vena cava and pumps blood to the right ventricle
Composed of pectinate muscle Left Atrium
Thin walled chambers that receive blood from pulmonary veins and pumps blood to left ventricle
Internal Anatomy of the heart: Ventricles
Right VentricleThick walled chamber that receives blood
from right atrium and pumps blood to pulmonary artery.
Left Ventricle Thick walled chamber that receives blood
from left atrium and pumps blood to the Aorta.
Both ventricles are composed of trabeculae carne
The two ventricles are separated from the atria by atrioventricular (AV) valves Tricuspid valve separates right atrium from right
ventricle Bicuspid (mitral) valve separates left atrium from left
ventricle Chordae tendineae
Tendinous fibers attached to the cusps of AV valves It attaches the cusps of atrioventricular valves to
papillary muscles It prevents the AV valve from reversing into the atria as
papillary muscles contract Papillary muscle and trabeculae carneae
Muscular projections on the inner wall of ventricles
Blood Flow through the heart Right atria
receives blood from superior and inferior vena cava and pumps it to the right ventricle through the tricuspid valve
Right ventriclereceives blood from right atrium and pumps it
toto the pulmonary artery through the pulmonary semilunar valve
Pulmonary artery -delivers the blood to the lungsAt the lungs gas exchange occurs
○ Oxygen diffuses from the alveoli to the capillary and carbon dioxide diffuses from the capillary to the alveoli.
Pulmonary Vein after the gas exchange at the lungs,
pulmonary veins collect the blood and delivers it to the left atrium.
Left atriareceives blood from pulmonary veins and
pumps it to the left ventricle through the bicuspid valve (mitral valve)
Left ventriclereceives blood from the left atria and
pumps it to the aorta through the aortic semilunar valve
The aorta branches into smaller arteries and delivers the blood to the cells throughout the body.Brachiocephalic Trunk
○ Right Subclavian Artery○ Right Common Carotid Artery
Left Common Carotid ArteryLeft Subclavian Artery
Gas exchange occur between the cell and the capillaries
○ Oxygen diffuses from the capillaries to the cell and carbon dioxide diffuses from the cell to the capillaries.
After the gas exchange the blood is delivered back to the heart by superior and inferior vena cava.
Structural Differences in heart chambers and valves
Compared to the right ventricle the left ventricle is:More muscular and has thicker wallDevelops higher pressure during contractionProduces about 6 times more force during
contractionRound in cross section
Functions of valvesAV valves prevent backflow of blood from the
ventricles to the atriaSemilunar valves prevent backflow of blood from
the pulmonary trunk and aorta to the ventricles.
Sectional Anatomy of the heart
Blood Supply to the Heart Coronary arteries are
the first blood vessels to branch from the aortaArteries include:
○ the right and left coronary arteries
○ marginal arteries○ anterior and posterior
interventricular arteriesLeft Anterior descending
and Posterior descending (LAD, PDA)
○ the circumflex artery
Coronary arteries supply blood to the heart and coronary veins collect the blood from the heartVeins include
○ The great cardiac vein○ anterior and posterior
cardiac veins○ middle cardiac vein○ small cardiac vein
Coronary Circulation
Cardiac Physiology
The Heartbeat
Two classes of cardiac muscle cellsSpecialized muscle cells of the
conducting systemContractile cells
The conducting system
The conducting system includes:Sinoatrial (SA) node - Pacemaker cells are
located in the SA nodeAtrioventricular (AV) nodeAV bundle, bundle branches, and Purkinje fibers
Impulse conduction through the heart
SA node begins the action potential (AP)
Stimulus spreads to the AV node Impulse is delayed at AV node Impulse then travels through
ventricular conducting cells Then distributed by Purkinje fibers
ECG: Electrocardiogram ECG is a recording of the electrical events
occurring during the cardiac cycle Analysis of ECG can reveal:
Condition of conducting systemEffect of altered ion concentrationSize of ventriclesPosition of the heart
Electrocardiogram At an interval of 0.1 second each: (2 ½ small
squares on ECG)P-wave: atrial depolarizationPR interval: conduction delay through the AV
node (~ 200 msec)QRS complex: ventricular depolarization (<120
msec)T wave: ventricular repolarization
○ When inverted, indicates a recent MI At an interval of 0.4 second: (10 squares on ECG)
QT interval: mechanical contraction of the ventricles
An Electrocardiogram
Membrane Potential: difference in electrical impulses between the external and internal environment of a cell.
Depolarization: positive change in a cell's membrane potential that causes the cell to become more (+) or less (-) leads to removal of the charge that developed from all the
negative charges that accumulated on the inner membrane and positive charges on the outer membrane
(outside) + + + + + + + + + + + + - - - - -(inside) - - - - - - - - - - - - - - - - + + + + +
Repolarization: change in membrane potential back to its initial negative state after depolarization of an action potential had changed it to a positive value
Hyperpolarization: change in membrane potential making it MORE negative than its original state.
Contractile cells Resting membrane potential of approximately
–90mV Action potential
Rapid depolarizationA plateau phase unique to cardiac muscle
○ Calcium channels remain open longer than the sodium channels
Repolarization Refractory period follows the action potential
AP in Cardiac Myocytes The action potential in typical cardiomyocytes is composed
of 5 phases (0-4), beginning and ending with phase 4. Phase 4: The resting phase
The resting potential in a cardiomyocyte is −90 mV due to a constant outward leak of K+ through inward channels.
Na+ and Ca2+ channels are closed at resting
transmembrane potential (TMP).
Phase 0: DepolarizationAn action potential triggered in a neighboring cardiomyocyte
or pacemaker cell causes the TMP to rise above −90 mV.Fast Na+ channels start to open one by one
○ Na+ leaks into the cell, causing a rise in TMP.TMP approaches −70mV
○ the threshold potential in cardiomyocytesthe point at which enough fast Na+ channels have
opened to generate a self-sustaining inward Na+ current.
The large Na+ current rapidly depolarizes the TMP to 0 mV and slightly above 0 mV for a transient period of time called the overshoot; fast Na+ channels close (recall that fast Na+ channels are time-dependent).
L-type (“long-opening”) Ca2+ channels open when the TMP is greater than −40 mV and cause a small but steady influx of Ca2+ down its concentration gradient.
Phase 1: Early repolarizationTMP is now slightly positive.Some K+ channels open briefly and an outward
flow of K+ returns the TMP to approximately 0 mV.
Phase 2: The plateau phaseL-type Ca2+ channels are still open and there is a small,
constant inward current of Ca2+. ○ This becomes significant in the excitation-contraction
coupling process described below.K+ leaks out down its concentration gradient through
delayed rectifier K+ channels.These two countercurrents are electrically balanced, and
the TMP is maintained at a plateau just below 0 mV throughout phase 2.
Phase 3: RepolarizationCa2+ channels are gradually inactivated.Persistent outflow of K+, now exceeding Ca2+ inflow,
brings TMP back towards resting potential of −90 mV to prepare the cell for a new cycle of depolarization.
Normal transmembrane ionic concentration gradients are restored by returning Na+ and Ca2+ ions to the extracellular environment, and K+ ions to the cell interior.
The pumps involved include: ○ Na+ -Ca2+ exchanger○ Ca2+ -ATPase ○ Na+ -K+ -ATPase.
Cardiac Cycle
The period between the start of one heartbeat and the beginning of the next
During a cardiac cycleEach heart chamber goes through systole
and diastole○ Systole: ventricular contraction○ Diastole: ventricular relaxation
Correct pressure relationships are dependent on careful timing of contractions
Normal blood pressure: 120/80 mmHg
Cardiac Cycle Sinoatrial (SA) node:
Normal pacemaker of the heart Located in upper wall of RA normally generates the action potential (the electrical impulse that initiates
contraction). excites the right atrium (RA), travels through Bachmann’s bundle to excite left
atrium (LA). The impulse travels through internodal pathways in RA to the atrioventricular
(AV) node. AV node:
Lower wall of RA Sends impulses into lower RA and LA the impulse then travels through the bundle of His and down the bundle
branches○ fibers specialized for rapid transmission of electrical impulses, on either side
of the interventricular septum.
Right bundle branch (RBB): depolarizes the right ventricle (RV).
Left bundle branch (LBB): depolarizes the left ventricle (LV) and interventricular
septum. Both bundle branches terminate in Purkinje fibers
millions of small fibers projecting throughout the myocardium.
An organized rhythmic contraction of the heart requires adequate propagation of electrical impulses along the conduction pathway.
The impulses in the His-Purkinje system travel in such a way that papillary muscle contract before the ventriclesprevents regurgitation of blood flow through
the AV valves.
Heart Sounds Auscultation – listening to heart sound via
stethoscope Four heart sounds
S1 – “lubb” caused by the closing of the AV valvesS2 – “dubb”caused by the closing of the
semilunar valvesS3 – a faint sound associated with blood flowing
into the ventricles○ Prominent in heart murmurs due to backflow of
bloodS4 – another faint sound associated with atrial
contraction
Stroke Volume and Cardiac Output
Stroke volumethe volume of blood ejected with each ventricular
contraction Cardiac output
the amount of blood pumped by each ventricle in one minute○ Average heart pumps:
Males: 5.6 L/min Females: 4.9 L/min.
Heart Rate: heart beats/min.Normal: 72 beats/min.
CO = HR x SV
Abnormal Heart Rates
Bradycardia slow heart rate; less than 60 beats / min
Tachycardia rapid heart rate; more than 100 beats / min
Arrhythmias abnormalities in rhythm Ventricular Fibrillation
○ (ventricles contract at an extremely fast rate and are asynchronous; then stop functioning; can be fatal; can be caused by massive heart attack or electric shock)
Myocardial Infarction (heart attack) usually due to loss of oxygen to the heart can be caused by blocked coronary arteries (plaque – build up of cholesterol;
LDL – bad cholesterol) abnormal QRS complex
Autonomic Activity
The heart is innervated by sympathetic and parasympathetic nerves.Sympathetic stimulation
○ Positive inotropic effect○ Releases NE
Parasympathetic stimulation○ Negative inotropic effect○ Releases ACh
Medulla Oblongata affect autonomic innervation
Cardioacceleratory centeractivates sympathetic neurons what is the action sympathetics on the
heart? Cardioinhibitory center
controls parasympathetic neurons what is the action of parasympathetics on the heart?
Medulla Oblongata centersreceives input and monitors blood pressure and dissolved gas
concentrations which gases?Baroreceptors located in the wall of the aorta and carotid arteries
monitors blood pressure and sends impulse to the medulla ○ Adjusts the sympathetic tone accordingly. ○ The renin-angiotensin-aldosterone (RAS) system is also very important in
maintaining blood pressure Under renal control
Summary: Regulation of Heart Rate and Stroke Volume
Sympathetic stimulation increases heart rate
Parasympathetic stimulation decreases heart rate
Circulating hormones, specifically Epi, NE, and T3, accelerate heart rate
Increased venous return increases heart rate
Clinical View: Heart Murmurs
Most heart murmurs are innocent Caused by blood flowing through healthy valves in a
healthy heart and do not require treatment. Heart murmurs can be caused by blood flowing through
a damaged or overworked heart valve. Heart valve defects may be present at birth or heart
valve disease may result from other illnesses, such as rheumatic fever, heart attacks, heart disease or infective endocarditis.
Clinical View:Types of Heart Valve Diseases
Mitral valve prolapseNormally your mitral valve closes completely when
your left ventricle contracts, preventing blood from flowing back into your left atrium.
If part of the valve balloons out so that the valve does not close properly, you have mitral valve prolapse.
This causes a clicking sound as your heart beats. Often, this common condition is not serious.
However, in rare cases it leads to bacterial endocarditis or mitral regurgitation (backward blood flow through the valve); both can be serious.
Mitral valve or aortic stenosisYour mitral or aortic valves, both on the left side of
your heart, can become narrowed by scarring from infections, such as rheumatic fever, or may be narrow at birth.
Narrowing or constriction is called stenosis.In mitral valve or aortic stenosis, the heart has to work
harder to pump enough blood to satisfy your body's oxygen needs.
If untreated, stenosis can wear out your heart and can lead to heart failure.
Aortic sclerosisOne in three elderly people have a heart murmur
due to the scarring, thickening, or stiffening (sclerosis) of the aortic valve.
This condition is generally not dangerous; typically, the valve can function for years after the murmur is detected.
Aortic sclerosis is usually seen in people with atherosclerosis, or hardening of the arteries.
Mitral or aortic regurgitationRegurgitation (backward flow) of blood can
occur with mitral valve prolapse or mitral valve or aortic stenosis.
To counteract this back flow, the heart must work harder to force blood through the damaged valve.
Over time, this can weaken and/or enlarge the heart and can lead to heart failure.
Congenital heart defectsAbout 25,000 babies are born each year with
heart defects, such as holes in heart walls or misshapen heart valves.
Many congenital heart defects can be corrected by surgery.