Cardiovascular System I: Heart

I. Anatomy of the heart

A. Location

1. Within mediastinun of the medial cavity of the thorax

a. Anterior to vertebral column

b. Posterior to sternum

c. Superior to diaphragm

i. At rest

d. Superior margin

i. 2nd rib

e. Inferior margin

i. 5th intercostals space

f.. 60% of mass to the left of midline

g. Base (posterior surface) faces right shoulder

h. Apex points inferiorly toward left hip

i. Contacts chest wall between 5th and 6th ribs

ii. PMI-point of maximum intensity

B. Pericardium

1. Fibrous pericardium

a. Tense connective tissue

i. Protects heart

ii. Anchors heart to surrounding tissues

iii. Prevents over filling

2. Serous pericardium

a. Two layers

i. Parietal layer

ii. Visceral layer

b. Parietal layer

i. Internal surface of fibrous pericardium

c. Visceral layer-epicardium

i. Part of heart wall

3. Pericardial cavity

a. Between the serous layers

b. Fluid filled

i. Reduces friction between serous membranes

C. Layers of the heart

 

1. Three layers

a. Epicardium

b. Middle myocardium

c. Deep endocardium

2. Epicardium (see above)

3. Myocardium

a. Primarily cardiac muscle

i. Arranged in circular bundles

b. Fibrous skeleton

i. Holds cardiac muscle together

4. Endocardium

a. Inner myocardial surface

b. Lines heart and connective tissues of the valves

c. Squamous epithelium

 

II. Chambers of the Heart

 

A. Four chambers

1. Two atria

2. Ventricles

B. Heart is divided longitudinally

1. Septum

a. Interatrial

b. Interventricular

C. Atria

1. Receive returning blood (i.e., veins)

2. Auricles

a. Appendages

i. Increase atrial volume

3. Fossa ovalis

a. Residual impression of fetal foramen ovale

4. Veins of right atrium

a. Superior vena cava

i. Return flow from regions superior to diaphragm

b. Inferior vena cava

i. Return flow from regions inferior to diaphragm

c. Coronary sinus

i. Drain blood from myocardium

5. Veins of left atrium

a. Four pulmonary veins

i. Lungs back to heart

ii. Most of the posterior surface of the heart

D. Ventricles

1. Blood leaving the heart

2. Most of the mass of the heart

a. Right

i. Anterior surface

b. Left

i. Inferior surface

3. Muscles

a. Trabeculae carneae

i. Crossbars

b. Papillary muscles

i. Valve function

ii. Project into heart cavity

4. Pulmonary trunk

a. Right ventricle

i. Routes blood to lungs

5. Aorta

a. Left ventricle

i. Systemic circulation

 

III. Blood Flow Through the Heart

A. Two circuits

1. Pulmonary: right side of the heart

a. Blood to lungs for gas exchange

b. Right ventricle to left atrium of the heart (*This is a matter of convention. Technically, oxygen poor blood returning to right atrium is the end of the systemic circuit; not the beginning of the pulmonary circuit.)

c. Blood returns from body to right atrium

i. Low O2 concentration

ii. Relatively high CO2 concentration

d. Rt. Atrium to right ventricle

e. Rt. Ventricle to lungs

i. Pick up O2 and drop off CO2

ii. Pulmonary arteries (away from heart, not CO2

f. Lungs to left atrium

i. Pulmonary veins

ii. O2 rich

2. Systemic: left side of the heart

a. Left atrium to left ventricle

b. Left ventricle into aorta

c. Aorta to body through systemic arteries

i. Gases and nutrients are exchanged

d. Systemic veins to right atrium

3. Work loads

a. Equal volumes

b. Unequal work loads

c. Systemic

i. Five times as much resistance to blood flow

ii. Longer route

d. Left ventricle is much larger and thicker to do more work

 

IV. Heart Valves

A. Atrioventricular (AV) valves

1. Valves at atrium-ventricular junction

2. Prevent backflow into atria

3. Closed during ventricular contraction (systole)

B. Right AV valve: tricuspid

1. Three cusps

a. Reinforced endocardium

C. Left AV valve: bicuspid (mitral valve)

1. Two cusps

D. Chordae tendineae (heart strings)

1. Collagen cords attached to the cusps

a. Anchor cusps to papillary muscles

2. During ventricular contraction

a. Intraventricular pressure rises

b. Forces blood against valve flaps

c. Chordae tendinea anchor flaps in closed postion

E. Semilunar (SL) valves

1. Two

a. Aortic

i. Between left ventricle and aorta

b. Pulmonary

i. Between right ventricle and pulmonary trunk

2. Open during ventricular contraction (systole)

a. Intraventricular pressure exceeds the blood pressure in aorta and pulmonary trunk

3. Three crescent shaped cusps

a. Open out against arterial walls

F. Valves between atria and venae cavae and pulmonary veins

1. None

2. Atrial contraction compresses venous entry points

 

V. Blood Flow to the Heart

 

A. Heart requires its own circulatory system

1. Myocardium is too thick to permit diffusion of gases and nutrients

B. Coronary circulation

1. Arterial supply

2. Right and left coronary arteries

a. Arise at base of aorta

3. Left coronary artery

a. Supplies left side of the heart

b. Marginal branches

i. Anterior interventricular artery

ii. Circumflex artery

4. Right coronary artery

a. Supplies right side of the heart

b. Branches

i. Marginal artery

ii. Posterior interventricular artery

5. Anastomoses

a. Collateral routes of blood flow

6. Flow to myocardium occurs only during diastole

 

C. Cardiac veins

1. Follow course of coronary arteries

2. Join to form coronary sinus

a. Empties into right atrium

3. Coronary sinus

a. Tributaries

i. Great cardiac vein

ii. Middle cardiac vein

iii. Small cardiac vein

4. Anterior cardiac veins

a. Empty directly into rt. Atrium

 

VI. Pathology

A. Angina pectoris

1. Temporary deficient blood flow to the myocardium

a. Thoracic pain is symptom

B. Myocardial infarction (MI): heart attack

1. Cardiac cells are amitotic

a. Cells that die are replaced by non-contractile scar tissue

2. O2 deficiency causes necrosis (cell death)

 

VII. Cardiac Muscle

A. Characteristics

1. Branched, short, and interconnected fibers

2. Striated

a. Contract by sliding filament mechanism (see last semesters notes)

3. Cardiac muscle fibers are functionally connected

a. Intercalated discs

i. Anchoring desmosomes

ii. Electrical coupling via gap junctions

4. Functional syncytium

a. Entire myocardium acts as a single unit

i. Result of gap junctions

B. Contraction

1. All cardiac muscle cells contract as a single unit

2. Cardiac muscle is self-excitable (i.e., autorhytmic)

a. Initiate action potentials

i. Independent of nervous innervation

3. Long refractory period

a. Prevents tetanic contractions

C. Autorhythmic fibers

1. Pace maker cells (see below)

a. 1% of heart muscle

b. Depolarize spontaneously

D. Contractile muscle fibers

1. Depolarize in response to pacemaker cell activities

 

VIII. Heart Physiology

A. Intrinsic conduction system

1. Noncontractile cardiac cells that initiate and distribute impulses

a. Sequential distribution from atria to ventricles

B. Autorhythmic cells

1. Unstable resting membrane potential

a. Drift towards threshold

2. Pacemaker potentials

a. Membrane potential changes spontaneously

 

3. Events

a. Na+ influx (slow) offset by K+ efflux (slow)

b. K+ permeability gradually decreases

c. Influx of Na+ depolarizes the cardiac cells

d. Depolarization opens fast CA+ channels

e. Ca2+ influx from extracellular space causes rising phase of action potential

f. Repolarization causes K+ permeability to increase

i. Cardiac cells repolarize

g. K+ channels inactivate

h. Cycle starts again

C. Location of autorhythmic cells

 

1. Sinoatrial (SA) node

a. Pacemaker

i. Fastest rate of depolarization

ii. Characteristic rhythm of the heart: Sinus rhythm

b. Located in right atrial wall

c. After depolarization is initiated

i. Depolarization wave sweeps via gap junctions throughout atria

2. Atrioventricular (AV) node

a. Depolarization wave initiated by SA node reaches AV node

b. AV node is located in interatrial septum

i. Near tricuspid valve

c. Diameter of fibers is smaller

i. Slows impulse conduction (0.1 s)

ii. Permits completion of atrial contraction

d. Impulse passes to bundle of His

3. Atrioventricular bundle (bundle of His)

a. Functional passage of impulse from atria to ventricles

i. No gap junctions between cardiac cells in atria and ventricles

b. Located in inferior interatrial septum

c. Very short

i. Branches to form bundle branches

4. Bundle branches

a. Course interventricular septum toward apex of heart

5. Purkinje fibers

a. Reach apex then branch superiorly into ventricular walls

b. Impulses in fibers moves faster than cell to cell contact

i. Ensures greater pumping efficacy

 

IX. Cardiac action potential

 

Image:Action potential.png

1. Resting membrane potential (Phase 4)

a. Associated with diastole

b. Cells remain in this phase until electrically stimulated

2. Depolarization phase (Phase 0)

a. Due to the opening of the fast Na+ channels

b. Rapid influx of Na+ ions

c. Gates are voltage gated

3. Inactivation of the fast Na+ channels (Phase 1)

a. Transient net outward current

i. Due to the movement of K+ and Cl- ions

4. Plateau phase

a. Balance between inward movement of Ca2+ and outward movement of K+

i. K+ flows through the slow delayed rectifier potassium channels

ii. Ca2+ flows through L-type Ca2+ channels

5. Repolarization (Phase 3)

a. L-type Ca2+ channels close

b. Slow delayed rectifier K+ channels are still open

c. Delayed rectifier K+ channels close when the membrane potential is restored to about -80 to -85 mV

 

X. Pathology of Intrinsic Conductance System

A. Arrhythmias

1. Uncoordinated contractions of atria and ventricles

B. Fibrillation

1. Rapid, irregular contractions

C. Ectopic focus

1. Excitable tissue other than SA node controls heart contractions

D. Heart block

1. Damage to AV node

a. Impulse cannot reach ventricles

 

X. Extrinsic Control of the Heart

A. Brain-based control

1. Cardioaccelatory center

a. Medulla

b. Sympathetic NS control

i. Innervate SA and AV nodes

2. Cardioinhibitory center

a. Vagus nerve

b. Parasympathetic system

i. Innervate SA and AV nodes

ii. Slows heart rate

 

XI. Electrocardiography

A. Electrical changes during heart activity

 

1. ECG (EKG)

B. Deflection waves

1. P wave

a. Depolarization moving from SA node through atria

2. QRS complex

a. Ventricular depolarization

i. Precedes contraction

3. T wave

a. Ventricular repolarization

b. Occurs more slowly than depolarization

i. More spread out than QRS

C. Intervals

1. P-R

a. Interval from beginning of atrial excitation and ventricular excitation

b. Includes

i. Atrial depolarization and contraction

ii. Passage of impulse through intrinsic conduction system

c. Lasts 0.16 s

2. Q-T

a. Ventricular depolarization through repolarization

b. Includes

i. Time of ventricular contraction

 

XII. Mechanical Events during Heart Contraction

A. Cardiac cycle

1. Systole

a. Contraction

2. Diastole

b. Relaxation

3. Length

a. Total

i. 0.8 s

b. Atrial systole

i. 0.1 s

c. Ventricular systole

i. 0.3 s

d. Quiescent period

i. 0.4 s

B. Events

 

1. Start point

a. Atria and ventricles are relaxed

i. Mid-to-late diastole

2. Ventricular filling

a. Mid-to-late diastole

b. AV valves are open

c. Semilunar valves are closed

d. Ventricles begin to fill

i. 70% occurs prior to atrial contraction

e. Atrial systole

i. Atria contract (preceded by P wave)

ii. Increased atrial pressure propels blood from atria into ventricles

f. Atria relax

g. Ventricles depolarize (QRS wave)

3. Ventricular systole

a. As contraction begins, intraventricular blood pressure increases

i. AV valves close

ii. Semilunar valves are also closed

b. Isovolumetric contraction phase (volume constant)

i. Blood pressure in aorta and pulmonary trunk exceeds intraventricular pressure

ii. Pressure in ventricles increases without volume changing

c. Ventricular ejection phase

i. Intraventricular pressure exceeds pressure in large vessels

ii. Semilunar valves open

iii. Blood is propelled out of ventricles

d. Atria begin to fill with blood

4. Isovolumetric relaxation

a. Occurs during early diastole

b. T wave

c. Ventricles relax

d. Intraventricular pressure drops

e. Blood in vessels outside heart begins to flow back into ventricles

i. Semilunar valves close

ii. Aortic pressure increases-dicrotic notch

f. AV valves still closed

i. Isovolumetric relaxation

5. AV valves open when pressure in atria exceeds pressure against AV valves exerted by blood in ventricles

a. Start of cycle

b. Quiescent period

 

XIII. Heart Sounds

A. Associated with closing of heart valves

B. Lub-dub, pause lub-dub, pause,

C. Sound 1

a. AV valves close

b. Onset of systole

c. Louder and longer than sound 2

D. Sound 2

a. Semilunar valves close

b. Beginning of ventricular diastole

c. Short, sharp sound

E. Pause

a. Quiescent period

F. Sounds of separate valves can be differentiated

a. Timing

i. Mitral

ii. Tricuspid

iii. Aortic semilunar

iv. Pulmonary semilunar

b. Location

i. Four corners

 

XIII. Cardiac Output (CO)

 

A. Amount of blood pumped by each ventricle per minute

1. Stroke volume X Heart rate

a. Stroke volume

i. Volume of blood pumped out of each ventricle per beat

2. Increases or decreases with increases or decreases in either stroke volume or heart rate

B. Regulation of stroke volume (SV)

1. SV

a. Difference between EDV and ESV

i. EDV-end diastolic volume

ii. ESV-end systolic volume

2. EDV

a. Determined by:

i. Length of ventricular diastole

ii. Venous pressure

b. Increase either i. or ii., EDV increases

c. 120 ml is normal

3. ESV

a. Determined by:

i. Arterial pressure

ii. Force of ventricular contraction

b. 50 ml is normal

C. Factors that affect stroke volume

1. Preload-degree of stretch prior to contraction

a. Most important factor affecting EDV

b. Greater the stretch of cardiac fibers, the greater the force of contraction

c. Factors increasing stretch

i. Volume and speed of venous return

ii. Heart rate-time for filling

2. Contractility

a. Increase in contractile strength

i. Independent of muscle stretch

b. Increase Ca2+ into cardiac cells

i. Increases contractility and volume ejected from heart

c. Decreases ESV

d. Molecular regulation of contractile events

3. Afterload-arterial blood pressure

a. Pressure ventricular contraction must overcome

i. Back pressure in aorta and pulmonary valves

b. Normal: 80 mm Hg (aorta) and 10 mm Hg (pulmonary trunk)

c. Not normally a factor in healthy individuals

i. May have an adverse effect in individuals with hypertension

 

XIV. Heart Rate regulation

A. Cardiac output is homeostatically regulated

1. Extrinsic factors induce change to cardiac function through

a. Neural mechanisms

b. Chemical mechanisms

c. Physical mechanisms

B. Autonomic nervous system

 

1. Sympathetic nervous system

a. Responds to real or perceived threats

b. 4 F's: flight, fright, fight and sex

2. Sympathetic postganglionic neurons release NE at cardiac targets

a. Mediated by 1 adrenergic receptors

i. Pacemaker cell resting membrane potential is brought closer to threshold (depolarized)

ii. Increases heart rate

b. Increases Ca2+ influx into contractile cells

i. Increases ESV

3. Parasympathetic division

a. Opposes the effects of sympathetic nervous system

i. Decreases heart rate

b. Mediated by acetycholine

i. Hyperpolarizes (inhibits) SA node

4. Vagal tone

a. Sympathetic and parasympathetic divisions are continuously active

i. Effect of parasympathetic division predominates

b. Dominant effect is reduce activity of AV node

i. 25 beats/min reduction in HR

C. Chemical regulation

1. Hormones

a. Adrenal medulla

i. Epinephrine

ii. Sympathetic nervous system

iii. Increases HR and contractility (like NE)

2. Ions

a. Ca2+ concentrations

i. Decreases cause depressed heart function

ii. Increases cause heart irratability

D. Physical factors

1. Age

a. Inverse relation

2. Gender

a. Female faster

3. Exercise

a. Increased HR during exercise

b. Resting rate is lower

i. Bradycardia

ii. SV and muscle mass increased in athlete

4. Body temperature

a. HR lowered when cold