The Heart Chapter 19 & 20

• Circulatory System – heart, blood vessels and blood

• Cardiovascular System – heart, arteries, veins and capillaries

• 2 major divisions – Pulmonary circuit - right side of heart

• carries blood to lungs for gas exchange

– Systemic circuit - left side of heart • supplies blood to all organs of the body

• Heart Anatomy

• Cardiac Cycle

• Myocardial Physiology

• Cardiac Conduction System and Innervation

• Coronary Circulation

• Blood Pressure

Cardiovascular System Circuit

• Cardiovascular System consists of: – heart, arteries, veins

and capillaries

• 2 major divisions – Pulmonary Circuit -

right side of heart carries blood to lungs for gas exchange

– Systemic Circuit - left side of heart supplies blood to all organs of the body

Heart Shape and Position

• Heart is located in

the mediastinum,

between lungs

• Base of the heart is

the broad superior

portion of heart

• Apex of the heart is

the inferior end, tilts

to the left, tapers to

point

Heart Position

Pericardium • Pericardium consists of the membranes around the

heart that allows the heart to beat with minimal friction, provides room for the heart to expand but resists excessive expansion.

• Pericardial Sac (also called the parietal pericardium) is the membrane that loosely surrounds the heart. Pericardial sac has 2 layers:

– Fibrous layer: outer, tough, fibrous layer of CT

– Serous layer: inner, thin, smooth, moist membrane

• Pericardial Cavity is the space between heart and pericardial sac that is filled with pericardial fluid that acts as a lubricant.

• Visceral Pericardium (also called the epicardium) is the thin, smooth, moist serous layer that covers heart surface.

Figure 19.3 Pericardium

and

Heart Wall

3 Layers of the Heart Wall

• Epicardium (visceral pericardium)

– serous membrane covers heart

• Myocardium

– thick muscular layer

– fibrous skeleton - network of collagen fibers and elastic fibers

• provides structural support

• attachment for cardiac muscle

• nonconductor important in coordinating contractile activity

• Endocardium

– smooth inner lining

– equivalent to the lining of blood vessels

Figure 19.3

3 layers of the

Heart Wall

Bacterial Endocarditis

Gross pathology of subacute bacterial endocarditis involving mitral valve. Left ventricle of heart has

been opened to show mitral valve fibrin vegetations due to infection with Haemophilus

parainfluenzae. Autopsy. CDC/Dr. Edwin P. Ewing, Jr. http://pathmicro.med.sc.edu/ghaffar/bord-hemo.htm

Cardiac Cycle

• One complete contraction (systole) and

relaxation (diastole) of all 4 chambers of the

heart followed by a brief period of inactivity:

Cardiac Cycle Animation

http://msjensen.cehd.umn.edu/1135/Links/Animations/Flas

h/0028-swf_the_cardiac_cy.swf

Heart Sounds

• Heart sounds as heard through a stethoscope are due to turbulent flow of blood as heart valves close. Auscultation - listening to sounds made by body.

• First heart sound (S1) is louder and longer, sounds like “LUBB”, occurs when the AV valves close.

• Second heart sound (S2), is softer and sharper, sounds like “DUPP”, occurs when the semilunar valves close.

• S3 – rarely audible in people older than 30 – known as triple rhythm or gallop.

Heart Sounds

• S1 and S2 sounds of normal rhythm can be simulated by drumming two fingers on a table.

• If there is a third heart sound (S3) it can be simulated by drumming three fingers on a table, and it is described as a “gallop”.

• The cause of each heart sound is not known with certainty, but they are correlated with specific phases of the cardiac cycle.

http://egeneralmedical.com/egeneralmedical/listohearmur.html

http://www.3m.com/us/healthcare/professionals/littmann/jhtml/sounds/normal_first_and_second_heart_sounds.jhtml

Phases of Cardiac Cycle

1) Quiescent period

2) Ventricular filling

3) Isovolumetric contraction

4) Ventricular ejection

5) Isovolumetric relaxation

1) Quiescent period – all chambers

relaxed

– AV valves open

– blood flowing into ventricles

2) Ventricular Filling – SA node fires, atria

depolarize

– P wave appears on ECG

– atria contract, force additional blood into ventricles

– ventricles now contain end-diastolic volume (EDV) of about 130 ml of blood

3) Isovolumetric Contraction of Ventricles

• Atria repolarize and

relax

• Ventricles depolarize

• QRS complex appears

in ECG

• Ventricles contract

• Rising pressure closes

AV valves - heart

sound S1 occurs

• No ejection of blood

(no change in volume)

4) Ventricular Ejection

• Rising pressure opens semilunar valves

• Rapid ejection of blood

• Stroke volume: amount ejected

– about 70 ml at rest

• End-systolic volume: amount left in heart

• SV/EDV= ejection fraction, at rest ~ 54%, during vigorous exercise as high as 90%, diseased heart < 50%

• Both ventricles must eject same amount of blood

5) Isovolumetric Relaxation of Ventricles

• T wave appears in

ECG

• Ventricles repolarize

and relax (begin to

expand and fill)

• Semilunar valves close

(dicrotic notch of aortic

pressure curve) - heart

sound S2 occurs

• AV valves remain

closed (not shown)

• Ventricles expand but

do not fill so there is

no change in volume

(not shown)

Cardiac Output (CO)

• Amount ejected by each ventricle in 1 minute

• CO = Heart Rate (beats/min) x Stroke Volume (ml/beat)

• Resting values, CO = 75 beats/min x70 ml/beat = 5,250 ml/min, usually about 4 to 6 liters/min

• Vigorous exercise CO to 21 L/min for fit person and up to 35 L/min for world class athlete

• Cardiac reserve: difference between a person’s maximum CO and resting CO

– with fitness, with disease

• Right Ventricle and Left Ventricle eject the same amount of blood.

Preload

• Preload is the amount of tension in ventricular

myocardium before it contracts.

• preload causes contraction strength

– exercise venous return, stretches myocardium

( preload) , myocytes generate more tension

during contraction, CO matches venous return

• Frank-Starling law of heart:

StrokeVolume End Diastolic Volume

– ventricles eject as much blood as they receive;

The more they are stretched ( preload) the

harder they contract.

Unbalanced Ventricular Output

Unbalanced Ventricular Output

Heart Rate • Heart Rate (HR) can be measured from pulse.

• Infants have HR of 120 beats per minute or more

• Young adult females average 72 - 80 bpm

• Young adult males average 64 to 72 bpm

• HR rises again in the elderly

• Indurance trained athletes have resting

heart rates around 40 bpm

– Miguel Indurain, 5 time Tour de France

winner (1991-1995), had a resting

heart rate of 28 bpm and a

cardiac output of 50 liters/minute

Heart Rate

• Tachycardia: persistent, resting adult HR > 100

– can be caused by stress, anxiety, drugs, heart

disease or body temperature

• Bradycardia: persistent, resting adult HR < 60

– common during sleep and also in endurance

trained athletes

Sympathetic Nervous System

• Cardioacceleratory Center

– stimulates sympathetic nerves to SA node, AV

node and myocardium

– these nerves secrete norepinephrine, which

binds to -adrenergic receptors in the heart

– CO normally peaks at HR of 160 to 180 bpm

– Sympathetic n.s. can HR up to 230 bpm,

(limited by refractory period of SA node), but

SV and CO (less filling time)

Parasympathetic Nervous System

• Cardioinhibitory Center

– stimulates vagus nerves • right vagus nerve - SA node

• left vagus nerve - AV node

– secrete ACh (acetylcholine), binds to muscarinic receptors • opens K+ channels in nodal cells, hyperpolarized, fire

less frequently, HR slows down

– vagal tone: background firing rate holds HR to sinus rhythm of 70 to 80 bpm • severed vagus nerves results in the SA node firing at its

intrinsic rate of about 100bpm

• maximum vagal stimulation HR as low as 20 bpm

Structure of Cardiac Muscle • Short, thick, branched cells with central nuclei.

• Sarcoplasmic reticulum is less developed than in skeletal muscle.

– cardiac cells must get more Ca2+ from extracellular fluid during excitation

• T-tubules are invaginations of sarcolemma (muscle cell plasma membrane) that carry depolarization into the cells.

• Intercalated Discs, join cells end to end.

– interdigitating folds - surface area for strength and communication

– mechanical junctions tightly join cells

• adherent junctions and desmosomes

– electrical junctions

• gap junctions form channels that allow ions to flow directly from cell to cell to synchronize contractions

Intercalated Disk

Cardiac Muscle Histology

Metabolism of Cardiac Muscle

• Fatigue resistant aerobic respiration

• Cells have abundant myoglobin

– most tissues extract about 25% of the oxygen

from the blood as it circulates, but cardiac muscle

extracts about 65% of the oxygen

• Abundant, large mitochondria

• Organic fuels: fatty acids, glucose, glycogen,

ketones

Cardiac Conduction System

• Cardiac Conduction System is Myogenic - heartbeat originates within heart

• Cardiac Conduction System is Autorhythmic – heartbeats are regular and spontaneous

Cardiac Conduction System • Cardiac Conduction System Components:

– Cardiac Conduction System is composed of Nodes and Bundles of specialized cardiac cells.

– Sinoatrial (SA) Node is the pacemaker that initiates heartbeat and sets heart rate.

– Atrioventricular (AV) node is an electrical relay to the ventricles

– AV bundle (bundle of His) is a pathway for signals from the AV node to muscles of ventricles.

– Right and left bundle branches are divisions of the AV bundle that enter the interventricular septum and descend to the apex.

– Purkinje Fibers project upward from apex and spread throughout ventricular myocardium.

– Moderator Band is a muscular bundle of heart tissue crossing the ventricular cavity from the interventricular septum to the base of the paillary muscles of the right ventricle.

– fibrous skeleton of the heart is connective tissue that insulates atria from ventricles.

Cardiac Conduction System

Impulse Conduction

• SA node signal travels at 1 m/sec across the atria

• AV node slows signal to 0.05 m/sec

– composed of thinner myocytes with fewer gap junctions

– delays signal 100 msec, allows ventricles to fill

• AV bundle and purkinje fibers speeds up signal to

ventricles at 4 m/sec

• Papillary muscles get the signal first, and their

contraction stabilizes the AV valves

• Ventricular systole begins at apex, progresses up

– spiral arrangement of myocytes twists ventricles slightly

during contraction which increases the efficiency of blood

ejection from the chambers

Cardiac Rhythm • Sinus Rhythm is set by the SA node

– adult sinus rhythm at rest is 70 to 80 bpm

• Nodal Rhythm is set by the AV node

– adult nodal rhythm is 40 to 50 bpm but it is normally overridden by the sinus rhythm

• Ectopic Foci are regions of spontaneous firing outside of the SA node

– ectopic foci can be caused by hypoxia, caffeine, nicotine, electrolyte imbalance, drugs

• Arrhythmia – any abnormal cardiac rhythm

– heart block: failure of conduction system due to disease

• bundle branch block (caused by damage to bundle branches)

• total heart block (caused by damage to AV node)

Physiology of the SA Node

• SA node - no stable resting membrane potential

• Pacemaker potential

– gradual depolarization of SA cells from a slow inflow of Na+ into the SA cells

– membrane potential rises from -60 mV to -40 mV

• Action potential

– occurs at threshold of -40 mV

– depolarizing phase from -40 mV to 0 mV

• Caused by opening of fast Ca+2 channels (lets Ca+2 in)

– repolarizing phase

• Caused by opening of K+ channels (let K+ out)

• at -60 mV K+ channels close, pacemaker potential starts over

• Each depolarization creates one heartbeat

– SA node at rest fires every 0.8 sec or about 75 bpm at rest

Membrane Potentials of SA Node Cells

Contraction of Myocardium

• Myocytes have stable resting potential of -90 mV

• Depolarization (very brief)

– stimulus opens voltage regulated Na+ channels and allows

Na+ to rush in which rapidly depolarizes the membrane

– action potential peaks at +30 mV

– Na+ channels close again quickly

• Plateau of 200 to 250 msec, sustains contraction

– slow Ca+2 channels open, Ca+2 binds to Ca+2 channels on

sarcoplasmic reticulum which releases Ca+2 into cytoplasm

and results in muscle contraction

• Repolarization - Ca+2 channels close, K+ channels

open, rapid K+ outflow returns cell to resting potential

Myocardial Contraction

and Action Potential 1) Voltage gated Na+ channels

open

2) Rapid depolarization due to Na+ inflow

3) Na+ channels close

4) K+ channels and slow Ca+2 channels open. K+ flows out restoring membrane polarity, but Ca+2 flowing in prolongs membrane depolarization

5) Ca+2 channels close and Ca+2 is pumped out

K+ channels still open and re-polarizes as K+ flows out

Electrocardiogram (ECG)

• Composite of action potentials of nodal and myocardial cells

• Detected, amplified and recorded by electrodes on arms, legs

and chest

ECG

• P wave – SA node fires, atrial depolarization

– atrial systole

• QRS complex – AV node fires, ventricular

depolarization

– ventricular systole

– (atrial repolarization and diastole - signal obscured)

• T wave – ventricular repolarization

1) atrial depolarization begins

2) atrial depolarization

complete (atria contracted)

3) ventricles begin to

depolarize at apex;

atria repolarize (atria relaxed)

4) ventricular

depolarization complete (ventricles contracted)

5) ventricles begin to repolarize at apex

6) ventricular repolarization complete (ventricles relaxed)

ECG and the Cardiac Cycle

Diagnostic Value of ECG

• Valuable for diagnosing abnormalities in

conduction pathways, myocardial infarction

(MI), heart enlargement, electrolyte

imbalances and hormone imbalances.

Myocardial Infarction

• Sudden death of heart tissue caused by interruption of blood flow from coronary artery narrowing or occlusion.

• Dead myocardium is replaced with thin, non-contractile connective tissue.

• Anastomoses defend against interruption by providing alternate blood pathways.

– circumflex artery and right coronary artery join and form the posterior interventricular artery.

– anterior and posterior interventricular arteries join at apex of heart.

Coronary Blood

Vessels

Coronary Vessel Atherosclerosis

Plaque

Artery Wall

Major Events of Cardiac Cycle

Chapter 20

Principles of Blood Flow

• Blood Flow: amount of blood flowing through

a tissue in a given time (ml/min)

• Perfusion: rate of blood flow per given mass

of tissue (ml/min/g)

• Important for delivery of nutrients and

oxygen, and removal of metabolic wastes

Variations in Blood Flow in the

Circulatory System

• From aorta to capillaries, blood pressure and

flow DECREASES because:

– friction between blood and vessels

– smaller radii of arterioles and capillaries

– farther from the heart, more vessels, greater total

cross sectional area

• From capillaries to vena cava, rate of flow

INCREASES because:

– large amount of blood is funneled back into fewer

vessels.

Blood Pressure Changes With Distance

More pulsatile

closer to heart

Blood Pressure

• Usually measured at brachial artery of arm

• Systolic pressure: BP during ventricular

systole

• Diastolic pressure: BP during ventricular

diastole

• Normal value, young adult: 120/75 mm Hg

Neural Blood Pressure Control:

Baroreflex • Changes in BP detected by stretch receptors

(baroreceptors) in large arteries above heart in the:

– aortic arch

– aortic sinuses (behind aortic valve cusps)

– carotid sinus (base of each internal carotid artery)

• Autonomic negative feedback response

– baroreceptors send signals to brainstem

– BP increases rate of signals to brainstem which inhibits

the sympathetic nerves to blood vessels causing

vasodilation (causes BP )

– BP causes rate of signals to drop, excites vasomotor

center, sympathetic nerves, causes vasoconstriction and

BP

Baroreceptors

Baroreflex

Negative Feedback Response

Neural Control: Chemoreflex

• Chemoreceptors in aortic bodies and carotid

bodies are located in aortic arch, subclavian

arteries, external carotid arteries

• Chemoreceptors monitor pH

– drop in pH of blood and cerebrospinal fluid due to

increase in CO2 concentration in tissues triggers

an increase in blood pressure and respiration rate

– Carbon dioxide dissolves in water to form carbonic

acid: CO2 + H2O H2CO3

– carbonic acid dissociates in water into H+ (acidic

hydrogen ions) and HCO3- (bicarbonate ions)

Carotid body

Aortic body

Aortic body

Chemoreceptors

Hormonal Control of BP and Flow • Angiotensin II ( BP)

– potent vasoconstictor produced by liver, kidneys and lungs

• Aldosterone ( BP) – Na+ and water retention by the kidneys increases blood volume

• Vasopressin (ADH) ( BP) – causes water retention by the kidneys which increases blood volume

– also causes generalized vasoconstriction

• Atrial Natriuretic Factor ( BP) – produced by atria of heart

– urinary sodium excretion which lowers blood volume

– also causes generalized vasodilation

• Epinephrine and Norepinephrine ( and BP) – binds to -adrenergic receptors on smooth muscle of blood vessels

causing vasoconstriction which increases blood pressure

– binds to -adrenergic receptors on blood vessels in skeletal and cardiac muscle causing vasodilation which decreases blood pressure

Abnormalities of Blood Pressure • Hypertension

– chronic resting BP above 140/90

– can damage capillaries and weaken small arteries

causing aneurysms

• Hypotension is chronic low resting BP

– Blood pressure lower than 120/80 mm Hg is

considered normal. There is no specific number at

which day-to-day blood pressure is considered too

low, as long as no symptoms of trouble are present.

– Hypotension can be caused by blood loss or

dehydration

– Hypotension lowers renal function, causes dizziness

and impairs brain function

http://www.americanheart.org/presenter.jhtml?identifier=4623