Electrical Conduction

• Sinoatrial (SA) node – Sets the pace of the heartbeat at 70 bpm – AV node (50 bpm) and Purkinje fibers (25–40 bpm) can act as pacemakers under some conditions • Internodal pathway from SA to atrioventricular (AV) node – Routes the direction of electrical signals so the heart contracts from apex to base – AV node delay is accomplished by slower conductional signals through nodal cells

© 2016 Pearson Education, Inc. Electrical Conduction

• Purkinje fibers transmit electric signals down the atrioventricular bundle (bundle of His) to left and right bundle branches.

© 2016 Pearson Education, Inc. Figure 14.13 Electrical conduction in myocardial cells

Action potentials of autorhythmic cells Cells of SA node

Electrical Action potentials current of contractile cells

Contractile cell

Intercalated disk with gap junctions

© 2016 Pearson Education, Inc. Cardiac Action Potential

Interactive Physiology® Animation: Cardiovascular Physiology: Cardiac Action Potential

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 1

SA node depolarizes.

SA node Purple shading in steps 2–5 represents AV node depolarization. Electrical activity goes rapidly to AV node via internodal pathways.

Depolarization spreads more slowly across atria. Conduction slows through AV node. THE CONDUCTING SYSTEM OF THE HEART

Depolarization moves rapidly through ventricular conducting system to the SA node apex of the heart.

Internodal pathways Depolarization wave spreads upward from the apex.

AV node

AV bundle Bundle branches Purkinje fibers

FIGURE QUESTION What would happen to conduction if the AV node malfunctioned and could no longer depolarize?

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 2

SA node depolarizes. SA node

AV node

THE CONDUCTING SYSTEM OF THE HEART

SA node

Internodal pathways

AV node

AV bundle Bundle branches Purkinje fibers

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 3

SA node depolarizes.

SA node Purple shading in steps 2–5 represents AV node depolarization. Electrical activity goes rapidly to AV node via internodal pathways.

THE CONDUCTING SYSTEM OF THE HEART

SA node

Internodal pathways

AV node

AV bundle Bundle branches Purkinje fibers

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 4

SA node depolarizes.

SA node Purple shading in steps 2–5 represents AV node depolarization. Electrical activity goes rapidly to AV node via internodal pathways.

Depolarization spreads more slowly across atria. Conduction slows through AV node. THE CONDUCTING SYSTEM OF THE HEART

SA node

Internodal pathways

AV node

AV bundle Bundle branches Purkinje fibers

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 5

SA node depolarizes.

SA node Purple shading in steps 2–5 represents AV node depolarization. Electrical activity goes rapidly to AV node via internodal pathways.

Depolarization spreads more slowly across atria. Conduction slows through AV node. THE CONDUCTING SYSTEM OF THE HEART

Depolarization moves rapidly through ventricular conducting system to the SA node apex of the heart.

Internodal pathways

AV node

AV bundle Bundle branches Purkinje fibers

© 2016 Pearson Education, Inc. Figure 14.14 The conducting system of the heart Slide 6

SA node depolarizes.

SA node Purple shading in steps 2–5 represents AV node depolarization. Electrical activity goes rapidly to AV node via internodal pathways.

Depolarization spreads more slowly across atria. Conduction slows through AV node. THE CONDUCTING SYSTEM OF THE HEART

Depolarization moves rapidly through ventricular conducting system to the SA node apex of the heart.

Internodal pathways Depolarization wave spreads upward from the apex.

AV node

AV bundle Bundle branches Purkinje fibers

FIGURE QUESTION What would happen to conduction if the AV node malfunctioned and could no longer depolarize?

© 2016 Pearson Education, Inc. The Waves of Electrocardiogram (ECG)

• Waves and segments two major components of an ECG • Three waves – P depolarization of the atria – QRS complex: wave of ventricular depolarization – T repolarization of the ventricle – Atrial repolarization is part of QRS

© 2016 Pearson Education, Inc. Figure 14.15f The Electrocardiogram

5 mm 25 mm = 1 sec

An electrocardiogram is divided into waves (P, Q, R, S, T), segments between the waves (the P-R and S-T segments, for example), and intervals consisting of a combination of waves and segments (such as the PR and QT intervals). This ECG tracing was recorded from lead I.

P wave: atrial depolarization

P-R segment: conduction through AV node and AV bundle

ventricular QRS complex: +1 R R depolarization

T wave: ventricular repolarization

P-R S-T

segment segment Millivolts Q S P wave T wave

FIGURE QUESTION 0 1. If the ECG records at a speed of 25 mm/sec, what is the heart rate of the person? (1 little square = 1 mm) PR interval* QT interval QRS complex

*Sometimes the Q wave is not seen in the ECG. For this reason, the segments and intervals are named using the R wave but begin with the first wave of the QRS complex.

© 2016 Pearson Education, Inc. The Electrical Events of the Cardiac Cycle • Mechanical events lag behind electrical events: contraction follows action potential • ECG begins with atrial depolarization, atrial contraction at the end of P wave • P-R segment signal goes through AV node and AV bundle • Q wave end: ventricular contraction begins and continues through T wave • ECG analysis

© 2016 Pearson Education, Inc. The Electrical Events of the Cardiac Cycle • Heart rate: time between two P waves or two Q waves • Rhythm: regular pattern • Waves analysis: presence and shape • Segment length constant

© 2016 Pearson Education, Inc. Figure 14.16 Correlation between an ECG and electrical events in the heart

START P wave: atrial depolarization P

End

R

P-Q or P-R segment: conduction through T P AV node and AV bundle

Q S P

Atria contract

T wave: ventricular repolarization

R ELECTRICAL EVENTS OF THE Ventricular CARDIAC CYCLE T P repolarization

Q S

P Q wave Atrial repolarization S-T segment Q R

R wave P R Q S

R Ventricles contract P

Q

P S wave

Q S © 2016 Pearson Education, Inc. Figure 14.15h The Electrocardiogram

Normal and abnormal ECGs. All tracings represent 10-sec recordings.

10 sec R R P T P T

(1) Normal ECG

R R R R P P P P P P P P P P P P P P

(2) Third-degree block

(3) Atrial fibrillation

FIGURE QUESTIONS 2. Three abnormal ECGs are shown at right. Study them and see if you can relate the ECG changes to disruption of the normal (4) Ventricular fibrillation electrical conduction pattern in the heart. 3. Identify the waves on the ECG in part (5). Look at the pattern of their occurrence and describe what has happened to electrical conduction in the heart.

(5) Analyze this abnormal ECG. © 2016 Pearson Education, Inc. The Mechanical Events of the Cardiac Cycle • Diastole: cardiac muscle relaxes • Systole: cardiac muscle contracts • Beginning of cycle: the heart at rest: atrial and ventricular diastole – The atria are filling with blood from the vein – AV valves open  ventricles fill – Atrial systole: atria contract – Early ventricular contraction and AV valves close  first heart sound

© 2016 Pearson Education, Inc. The Mechanical Events of the Cardiac Cycle – Atrial diastole: all valves shut, isometric contraction of the heart, atria relax and blood flows in the atria – Ventricular systole: ventricles finish contracting pushing semilunar valves open and blood is ejected in arteries: ventricular systole – Ventricular diastole: ventricular relaxation and pressure drops, still higher than atrial pressure – Arterial blood flows back pushing semilunar valves shut  second heart sound

© 2016 Pearson Education, Inc. The Mechanical Events of the Cardiac Cycle – Isovolumic ventricular relaxation, volume of blood in ventricles not changing – AV valves open when ventricular pressure drops below atrial pressure

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 1

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

Isovolumic ventricular relaxation—as ventricles Atrial systole—atrial contraction relax, pressure in ventricles forces a small amount of falls. Blood flows back into additional blood into ventricles. cusps of semilunar valves and snaps them closed.

S1

S2

Isovolumic ventricular contraction— first phase of ventricular contraction Ventricular ejection—as ventricular pressure rises pushes AV valves closed but does and exceeds pressure in not create enough pressure to open the arteries, the semilunar semilunar valves. valves open and blood is ejected.

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 2

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 3

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

Atrial systole—atrial contraction forces a small amount of additional blood into ventricles.

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 4

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

Atrial systole—atrial contraction forces a small amount of additional blood into ventricles.

S1

Isovolumic ventricular contraction— first phase of ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves.

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 5

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

Atrial systole—atrial contraction forces a small amount of additional blood into ventricles.

S1

Isovolumic ventricular contraction— first phase of ventricular contraction Ventricular ejection—as ventricular pressure rises pushes AV valves closed but does and exceeds pressure in not create enough pressure to open the arteries, the semilunar semilunar valves. valves open and blood is ejected.

© 2016 Pearson Education, Inc. Figure 14.17a Mechanical events of the cardiac cycle Slide 6

The heart cycles between contraction (systole) and relaxation (diastole). Late diastole—both sets of chambers are relaxed and ventricles fill passively.

START

Isovolumic ventricular relaxation—as ventricles Atrial systole—atrial contraction relax, pressure in ventricles forces a small amount of falls. Blood flows back into additional blood into ventricles. cusps of semilunar valves and snaps them closed.

S1

S2

Isovolumic ventricular contraction— first phase of ventricular contraction Ventricular ejection—as ventricular pressure rises pushes AV valves closed but does and exceeds pressure in not create enough pressure to open the arteries, the semilunar semilunar valves. valves open and blood is ejected.

© 2016 Pearson Education, Inc. Heart Sounds

• First heart sound – Vibrations following closure of the AV valves – “Lub” • Second heart sound – Vibrations created by closing of semilunar valve – “Dup” • Auscultation is listening to the heart through the chest wall through a stethoscope.

© 2016 Pearson Education, Inc. Cardiac Cycle

Interactive Physiology® Animation: Cardiovascular Physiology: Cardiac Cycle

© 2016 Pearson Education, Inc. Stroke Volume and Cardiac Output

• End diastolic volume (EDV) • End systolic volume (ESV) • Stroke volume – Amount of blood pumped by one ventricle during a contraction – Volume of blood before contraction-volume of blood after contraction = stroke volume – EDV – ESV = stroke volume – Average = 70 mL

© 2016 Pearson Education, Inc. Stroke Volume and Cardiac Output

• Cardiac output (CO) – Volume of blood pumped by one ventricle in a given period of time – Cardiac output = heart rate  stroke volume – Average = 5 L/min

© 2016 Pearson Education, Inc. Stroke Volume

• Force of contraction is affected by – Length of muscle fiber • Determined by volume of blood at beginning of contraction – Contractility of heart – As stretch of the ventricular wall increases, so does stroke volume – Preload is the degree of myocardial stretch before contraction

© 2016 Pearson Education, Inc. Stroke Volume

• Sympathetic activity speeds heart rate

– β1-adrenergic receptors on the autorhythmic cells • Parasympathetic activity slows heart rate

© 2016 Pearson Education, Inc. Figure 14.19a-b Autonomic control of heart rate

© 2016 Pearson Education, Inc. Figure 14.19c Autonomic control of heart rate

© 2016 Pearson Education, Inc. Figure 14.19d Autonomic control of heart rate

© 2016 Pearson Education, Inc. Figure 14.19e Autonomic control of heart rate

© 2016 Pearson Education, Inc. Stroke Volume

• Frank-Starling law states – Stroke volume increases as EDV increases • EDV is determined by venous return • Venous return is affected by – Skeletal muscle pump – Respiratory pump – Sympathetic innervation of veins

© 2016 Pearson Education, Inc. Figure 14.20a Length-tension relationships

© 2016 Pearson Education, Inc. Figure 14.20b Length-tension relationships

© 2016 Pearson Education, Inc. Contractility

• Any chemical that affects contractility is an inotropic agent – Epinephrine, norepinephrine, and digitalis have positive inotropic effects – Chemicals with negative inotropic effects decrease contractility

© 2016 Pearson Education, Inc. Figure 14.20c Length-tension relationships

© 2016 Pearson Education, Inc. Cardiac Output

Interactive Physiology® Animation: Cardiovascular Physiology: Cardiac Output

© 2016 Pearson Education, Inc. Figure 14.21 Catecholamines increase cardiac contraction

© 2016 Pearson Education, Inc. Afterload and Ejection Fraction

• Afterload is the combined load of EDV and arterial resistance during ventricular contraction • Ejection fraction is the percentage of EDV ejected with one contraction – Stroke volume/EDV – Average = 52%

© 2016 Pearson Education, Inc. Figure 14.22 Stroke volume and heart rate determine cardiac output

© 2016 Pearson Education, Inc. Summary

• Overview of the cardiovascular system • Pressure, volume, flow, and resistance • Cardiac muscle and the heart • The heart as a pump

© 2016 Pearson Education, Inc.