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.