Lesson # 6 The Heart-2Chapter 20

Objectives:1- Explain the events of an action potential in cardiac muscle.2- Identify the electrical events associated with the electrocardiogram (ECG).3- Explain the events of the cardiac cycle.4- Defining cardiac output and how it is regulated.

The Conducting System

Sinoatrial node (SA node)

It establishes the heart rate (pacemaker).

Atrioventricular node (AV node)

It delays the impulses to allow the atria to finish contracting before the ventricles start to contract.

It connects electrically the atria to the ventricles.

They conduct the impulse to the Purkinje fibers.

They conduct the impulse to the lateral walls of the ventricles allowing the contraction to spread from the apex to the base.

Right and left bundle branches

Purkinje fibers

Atrioventricular bundle or bundle of His

It is a reduction in the membrane potential because the interior of the cell becomes less negative or more positive.

It is an increase in the membrane potential because the interior of the cell becomes more negative.

0 mV

-90 mV

DEPOLARIZATION

HYPERPOLARIZATION

Depolarization is produced when gated sodium channels are open and sodium ions enter the cell.

Hyperpolarization is produced when gated potassium channels are open and potassium ions exit the cell.

Depolarization

Hyperpolarization

Repolarization

The Sinoatrial (SA) Node K+

Na+

+50

-70

-60

0

-40 -40 mV

+50

-70

-60

0

-40

Resting Potential

+50

-70

-60

0

Action PotentialPacemaker

Potential

-60 mV

SA node does not have a stable resting membrane potential. It starts at -60 mV.

It drifts upward because of a slow inflow of Na+ .

When it reaches a threshold of -40 mV, voltage-gated Ca2+ and Na+ channels open and a faster depolarization occurs peaking at 0 mV.The K+ channels then open and K+ leaves the cell causing repolarization.

Action Potentials: They are changes in the transmembrane potential that, once initiated, affect an entire excitable membrane.

Each depolarization of the SA node sets off one heartbeat. At rest, fires every 0.8 seconds or 75 bpm.

Slow Na+ inflow

Fast Ca+ and Na+ inflow

Fast K+ outflow

Changes in the membrane potential of a pacemakercell in the SA node that is establishing a heart rate of72 beats per minute. Note the presence of a prepotential, a gradual spontaneous depolarization.

Time (sec)

Prepotential(spontaneous depolarization)

Threshold

The Electrocardiogram or ECG (EKG)

An ECG is a composite of all action potentials of nodal and myocardial cells, detected, amplified and recorded by electrodes on arms, legs and chest

+1

0

Mill

ivol

ts

P

Q

R

S

TDepolarization of

atria.

QRS complexDepolarization of

ventricles.

Repolarization of ventricles

PQ segment

Atrial systole

100 msec

ST segment

Ventricular systole

The ST segment represents the time during which the ventricles contract and eject blood.

The Electrocardiogram

Waves and Segments

Extra heart beats produced in any region of spontaneous firing other than the SA node.

It is the failure of any part of the of the cardiac conducting system to transmit signals.

The Cardiac RhythmIt is the normal heartbeat triggered by the SA nodeSinus rhythm:

At rest, the sinus rhythm is about 70 to 80 times per minute (rates from 60 to 100 bpm).

Any region of spontaneous firing other than the SA node. The most common ectopic focus is the AV node, which produces a nodal rhythm. It is the cardiac rhythm produced by the AV node. It is a slower heartbeat of 40 to 50 bpm. If neither the SA nor AV nodes is functioning, an artificial pacemaker is requiredArrhythmia: It is any abnormal cardiac rhythm.Heart block:

Extra-systoles:

Ectopic focus:

Tachycardia: It is a persistent, resting adult heart rate above 100 bpm. Bradycardia: It is a persistent, resting adult heart rate below 60 bpm.

If the SA node is damaged, other part of the myocardium may take over the governance of the heart rhythm.

Nodal rhythm:

During ventricular systole (contraction) the two AV close at the same time and produce the first sound referred as lubb.

Lubb

Dup

p

Dupp

When the ventricles relax (diastole) the two semilunar valves close at the same time and produce the second sound referred as dubb.

Heart Sounds

Lubb

Pressure

Pressure

At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes).

In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed.

The Cardiac Cycle

Pressure

Pressure

At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes).

The Cardiac Cycle

Once pressure in the ventricles exceeds that in the arterial trunks (pulmonary and aortic), the semilunar valves open and blood flows into the pulmonary and aortic trunks. This point marks the beginning of the period of ventricular ejection.

PressurePressure

At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes).

It is the period of isovolumetric relaxation, the ventricles relax and the pressure drops, but blood does not flow because all the valves are closed.

PressurePressure

At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes).

Once pressure in the atria the AV valves open and blood flows into the ventricles. This point marks the beginning of the ventricular filling.

Cardiaccycle

Atrial systole begins:Atrial contraction forces a small amount of additional blood into relaxed ventricles.

Atrial systole ends,atrial diastolebegins

Ventricular systole—first phase: Ventricularcontraction pushes AVvalves closed but doesnot create enoughpressure to opensemilunar valves.

Ventricular systole—second phase: Asventricular pressure risesand exceeds pressurein the arteries, thesemilunar valvesopen and bloodis ejected.

Ventricular diastole—early:As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Bloodflows into the relaxed atria.

Ventriculardiastole—late:All chambers arerelaxed.Ventricles fillpassively.

Start

The Cardiac Cycle The cardiac cycle consist of the events during a complete heart beat.

At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow.

In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed.

At the end of the atrial systole, each ventricle contains a maximum amount of130 mL of blood: End-diastolic volume

(a)

(b)

(c)

(e)

1- Ventricular Filling

2- Isovolumetric Contraction

4- Isovolumetric Relaxation

- Passive (70%)- Active (30%)

This point marks the beginning of the period of ventricular ejection.

(d)

3- Ventricular Ejection

At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow.

(a)

(f)

Atrial Systole

Ventricular Systole

Ventricular Diastole and Atrial Diastole

A small amount of blood (30 %) is forced to the ventricles

Pressure increases and semilunar valves open. Ventricular ejection.

Ventricular contraction closes the AV valves (first sound). Isometric contraction.

Pressure decreases in the ventricles and semilunar valves close (second sound).Atria are also in diastole. Passive blood flow fills the ventricles (70%).

Fist Phase:

Second Phase:

Early:

Late:

ATRIALDIASTOLE

ATRIALSISTOLE ATRIAL DIASTOLE

VENTRICULAR SISTOLE VENTRICULAR DIASTOLEVENTRICULARDIASTOLE

P TDepolarization

of atria

QRS complexDepolarization of

ventriclesRepolarization of

ventricles

ST segment

PQ segment

1- Ventricular Filling

Passive (70%) Active (30%)

2- Isovolumetric Contraction

3- Ventricular Ejection

4- Isovolumetric Relaxation

1- Ventricular Filling

First sound

Isovolumetric contraction.

Second sound

Ventricular filling

Isovolumetric relaxation

Ventricular filling

Ventricular ejection

End-Diastolic Volume (EDV)It is the volume of blood that each ventricle contains at the end of ventricular filling (about 130 mL).

Stroke Volume (SV)It is the volume of blood that each ventricle ejects during ventricular ejection (about 70 - 80 mL).

End-Systolic Volume (ESV)It is the volume of blood left behind in the ventricles after ventricular ejection.

Ejection fractionIt is the percentage of the end-diastolic volume (EDV) that is ejected (about 54%).

EDV – SV = (ESV)

Cardiac Output (CO)The amount of blood pumped by the left ventricle in one minute

Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)

75 bpm x 80 mL/beat = 6000 mL/min (6L/min)

End-Diastolic Volume

End-Sistolic Volume

Stroke Volume