cardiovascular system notes: physiology of the heart

Cardiovascular System Notes:

Physiology of the Heart

Interesting Heart Fact

Capillaries are so small it takes ten of them to equal the thickness of a human hair.

© 2013 Pearson Education, Inc.

Heart Physiology: Electrical Events Heart depolarizes and contracts without

nervous system stimulationRhythm can be altered by autonomic

nervous system


Heart Physiology: Setting the Basic Rhythm Coordinated heartbeat is a function of

Presence of gap junctions

Intrinsic cardiac conduction system○ Network of noncontractile (autorhythmic) cells○ Initiate and distribute impulses coordinated

depolarization and contraction of heart

• PHYSIOLOGY OF THE HEART

• The Conducting System (electric)

1. Sinoatrial Node (SA)

• PACEMAKER OF HEART – starts each heartbeat

• located in wall of right atrium

• made of specialized myocardial cells

• sends impulse to both atria, causing them to contract

• called an intrinsic conduction or “nodal” system of specialized tissue

2. Atrioventricular Node (AV node)

• located at base of right atrium – receives impulse from SA node

SA Node

AV Node

• Atrioventricular Bundle

• Location: middle of septum

• Direction of Heart Beat

• impulse from AV node goes down bundle to base of heart

• it then travels up the sides of the ventricles through the PURKINJE NETWORK causing the ventricles to contract from the bottom up

Atrioventricular Bundle


Figure 18.15a Intrinsic cardiac conduction system and action potential succession during one heartbeat.

The sinoatrial (SA) node (pacemaker)generates impulses.

1

The impulsespause (0.1 s) at theatrioventricular(AV) node.

2

The atrioventricular(AV) bundleconnects the atriato the ventricles.

3

The bundle branches conduct the impulses through the interventricular septum.

4

The subendocardialconducting networkdepolarizes the contractilecells of both ventricles.

5

Superior vena cava Right atrium

Left atrium

Subendocardialconductingnetwork(Purkinje fibers)

Inter-ventricularseptum

Anatomy of the intrinsic conduction system showing the sequence ofelectrical excitation

Internodal pathway

Slide 1

• RATE OF HEART BEAT

• Factors affecting heart rate

• age, sex, physical activity, temperature, thought processes, chemicals (natural and otherwise)

• rate high at birth (100 – 140 bpm) – then declines steadily until average is reached (70 – 80 bpm)

• heart rate faster in females – slower in trained athletes


Extrinsic Innervation of the Heart Heartbeat modified by ANS via cardiac

centers in medulla oblongataSympathetic rate and forceParasympathetic rate Cardioacceleratory center – sympathetic –

affects SA, AV nodes, heart muscle, coronary arteries

Cardioinhibitory center – parasympathetic – inhibits SA and AV nodes via vagus nerves


Homeostatic Imbalances Defects in intrinsic conduction system

may causeArrhythmias - irregular heart rhythmsUncoordinated atrial and ventricular

contractionsFibrillation - rapid, irregular

contractions; useless for pumping blood circulation ceases brain death○ Defibrillation to treat


The vagus nerve(parasympathetic)decreases heart rate.

Dorsal motor nucleusof vagus

Cardioinhibitorycenter

Cardioaccele-ratory center

Medulla oblongata

Sympathetictrunkganglion

Thoracic spinal cordSympathetic trunk

Sympathetic cardiacnerves increase heart rateand force of contraction.

AVnode

SAnode

Parasympathetic fibers Sympathetic fibers Interneurons

Figure 18.16 Autonomic innervation of the heart.

• CARDIAC CYCLE

• the events of one complete heartbeat• length of cycle (heartbeat) is about 0.8 sec

• atria contract at the same time – as they relax, the ventricles contract

• SYSTOLE • contraction of ventricle

• DIASTOLE • relaxation of ventricle

• Cardiac Output

• amount of blood pumped out of each side of the heart in 1 minute

• heart rate X stroke volume


Heart Sounds

Two sounds (lub-dup) associated with closing of heart valvesFirst as AV valves close; beginning of

systoleSecond as SL valves close; beginning of

ventricular diastolePause indicates heart relaxation

Heart murmurs - abnormal heart sounds; usually indicate incompetent or stenotic valves


Aortic valve soundsheard in 2nd intercostalspace at right sternalmargin

Pulmonary valvesounds heard in 2ndintercostal space at leftsternal margin

Mitral valve soundsheard over heart apex(in 5th intercostal space)in line with middle ofclavicle

Tricuspid valve soundstypically heard in rightsternal margin of 5thintercostal space

Figure 18.20 Areas of the thoracic surface where the sounds of individual valves can best be detected.


Mechanical Events: The Cardiac Cycle

• Cardiac cycle– Blood flow through heart during one complete

heartbeat: atrial systole and diastole followed by ventricular systole and diastole

– Systole—contraction – Diastole—relaxation– Series of pressure and blood volume changes


Phases of the Cardiac Cycle

• 1. Ventricular filling—takes place in mid-to-late diastole– AV valves are open; pressure low – 80% of blood passively flows into ventricles– Atrial systole occurs, delivering remaining

20%– End diastolic volume (EDV): volume of

blood in each ventricle at end of ventricular diastole



• 2. Ventricular systole– Atria relax; ventricles begin to contract – Rising ventricular pressure closing of AV

valves– Isovolumetric contraction phase (all valves

are closed)– In ejection phase, ventricular pressure

exceeds pressure in large arteries, forcing SL valves open

– End systolic volume (ESV): volume of blood remaining in each ventricle after systole



• 3. Isovolumetric relaxation - early diastole– Ventricles relax; atria relaxed and filling– Backflow of blood in aorta and pulmonary

trunk closes SL valves• Causes dicrotic notch (brief rise in aortic

pressure as blood rebounds off closed valve)• Ventricles totally closed chambers

– When atrial pressure exceeds that in ventricles AV valves open; cycle begins again at step 1


Electrocardiogram

Heart sounds

Left heartQRS

P T P

1st 2nd

Dicrotic notch120

Aorta

Left ventricle

Left atriumAtrial systole

80

40

0

Pre

ssure

(m

m H

g)

EDV

SV

ESV

120

50

Ventr

icula

rvolu

me (

ml)

Atrioventricular valves

Aortic and pulmonary valves

Phase

Open Closed Open

Closed Open Closed

1 2a 2b 3 1

Left atriumRight atriumLeft ventricleRight ventricle

Ventricularfilling

Atrialcontraction

Isovolumetriccontraction phase

Ventricularejection phase

Isovolumetricrelaxation

Ventricularfilling

Ventricular filling(mid-to-late diastole)

Ventricular systole(atria in diastole)

Early diastole

1 2a 2b 3

Figure 18.21 Summary of events during the cardiac cycle.

• PULSE & BLOOD PRESSURE• Pulse

• expansion & contraction of an artery as the left ventricle contracts

• normal is 70 – 80 bpm (there are exceptions to this)

• pulse can be found at many sites on the body

• Blood Pressure

• pressure blood exerts on the inner walls of blood vessels

• Measuring BP

• BP is a measure of the systolic pressure (ventricles contracting) OVER the diastolic pressure (ventricles relaxing)

• Example: • 110/70 is in normal range

• Factors affecting BP

• nervous system, blood volume (kidneys), temp, chemicals, diet, exercise

• The Electrocardiogram (EKG or ECG)

• amplifies electric current of heart producing distinct wave patterns

• P wave • depolarization of atria

• QRS Complex • depolarization of ventricles

• T wave • repolarization of ventricles

P wave

QRS Complex

T wave


Electrocardiography

Electrocardiogram (ECG or EKG)Composite of all action potentials generated

by nodal and contractile cells at given time

Three waves:P wave – depolarization SA node atriaQRS complex - ventricular depolarization

and atrial repolarizationT wave - ventricular repolarization


Sinoatrialnode

Atrioventricularnode

QRS complex

Ventriculardepolarization

Atrialdepolarization

Ventricularrepolarization

P-RInterval

S-TSegment

Q-TInterval

0 0.2 0.4 0.6 0.8Time (s)

R

Q

S

P T

Figure 18.17 An electrocardiogram (ECG) tracing.


Figure 18.18 The sequence of depolarization and repolarization of the heart related to the deflection waves of an ECG tracing.

Slide 1

SA node

AV node

R

P T

QS

Atrial depolarization, initiated by the SA node, causes the P wave.

1

R

P T

QS

With atrial depolarization complete, the impulse is delayed at the AV node.

2

R

P T

QS

Ventricular depolarization begins at apex, causing the QRS complex. Atrial repolarization occurs.

3

R

P T

QS

R

P T

QS

SQ

P T

R

Ventricular depolarization is complete.4

5

6

Ventricular repolarization begins at apex, causing the T wave.

Ventricular repolarization is complete.

Depolarization Repolarization


Homeostatic Imbalances

Tachycardia - abnormally fast heart rate (>100 beats/min)If persistent, may lead to fibrillation

Bradycardia - heart rate slower than 60 beats/minMay result in grossly inadequate blood

circulation in nonathletesMay be desirable result of endurance

training


Homeostatic Imbalance

Congestive heart failure (CHF)Progressive condition; CO is so low that

blood circulation inadequate to meet tissue needs

Reflects weakened myocardium caused by○ Coronary atherosclerosis—clogged arteries○ Persistent high blood pressure○ Multiple myocardial infarcts○ Dilated cardiomyopathy (DCM)


Homeostatic Imbalance

Pulmonary congestionLeft side fails blood backs up in lungs

Peripheral congestionRight side fails blood pools in body

organs edema

Failure of either side ultimately weakens other

Treat by removing fluid, reducing afterload, increasing contractility


Developmental Aspects of the Heart Embryonic heart chambers

Sinus venosusAtriumVentricleBulbus cordis


Figure 18.24 Development of the human heart.

Day 20: Endothelial tubes begin to fuse.

4a

4

3

2

1

Tubularheart

Day 22:Heart starts pumping.

Arterial end

Ventricle

Ventricle

Venous end

Day 24: Heart continues to elongate and starts to bend.

Arterial end

Atrium

Venous end

Day 28: Bending continues as ventricle moves caudally and atrium moves cranially.

Aorta

Superiorvena cava

Inferiorvena cava

Ductusarteriosus

Pulmonarytrunk

Foramenovale

Ventricle

Day 35: Bending is complete.


Developmental Aspects of the Heart Fetal heart structures that bypass

pulmonary circulationForamen ovale connects two atria

○ Remnant is fossa ovalis in adult Ductus arteriosus connects pulmonary

trunk to aorta○ Remnant - ligamentum arteriosum in adult

Close at or shortly after birth


Developmental Aspects of the Heart Congenital heart defects

Most common birth defects; treated with surgery

Most are one of two types:○ Mixing of oxygen-poor and oxygen-rich blood,

e.g., septal defects, patent ductus arteriosus○ Narrowed valves or vessels increased

workload on heart, e.g., coarctation of aortaTetralogy of Fallot

○ Both types of disorders present


Figure 18.25 Three examples of congenital heart defects.

Occurs inabout 1 in every500 births

Ventricular septal defect.The superior part of theinter-ventricular septum failsto form, allowing blood to mixbetween the two ventricles.More blood is shunted fromleft to right because the leftventricle is stronger.

Narrowedaorta


Coarctation of the aorta.A part of the aorta isnarrowed, increasing theworkload of the left ventricle.


Tetralogy of Fallot.Multiple defects (tetra =four): (1) Pulmonary trunktoo narrow and pulmonaryvalve stenosed, resulting in(2) hypertrophied rightventricle; (3) ventricularseptal defect; (4) aortaopens from both ventricles.


Age-Related Changes Affecting the Heart Sclerosis and thickening of valve flaps Decline in cardiac reserve Fibrosis of cardiac muscle Atherosclerosis

cardiovascular system notes: physiology of the heart

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