Cardiac Output (Part 1): Electrical events and heart rate

Question 1

Cardiac Output

Answer 1

The heart is the pump that moves the blood (creates a pressure difference) –
its activity is described as the cardiac output
Cardiac output = Heart rate (HR) x Stroke volume (SV)

Changes in cardiac output
Sleep 10% ⇓
Excitement, stress 30% ⇑
Pregnancy 40% ⇑
Exercise 600% ⇑

Question 2

Heart rate is driven by

Answer 2

waves of electrical activity that induce the cardiac muscles to contract

Question 3

Delay at AV node allows

Answer 3

atria to contract before ventricles

Question 4

Spread of action potentials via the purkinje fibres results in

Answer 4

coordinated contraction of ventricles

Question 5

What causes the pacemaker cells of the S-A node to trigger an action potential?

Answer 5

Low resting membrane potential (-60 to -70 mV)

Na+ leakage that leads to depolarization

Question 6

Mechanism underlying the ‘spontaneous’ action potentials in the pacemaker cells (sino-atrial nodes)

Answer 6

Sodium ions “leaking” in through the F-type [funny] channels and calcium ions moving in through the T-type (transient) channels cause a threshold graded depolarization.
The rapid opening of voltage-gated
calcium L-types channels is responsible
for the rapid depolarization phase.
Reopening of potassium channels and
closing of calcium channels are responsible for the repolarization phase.

Question 7

The bigger the heart

Answer 7

the slower the heart beat – elephants 25 beats/min and shrew 600 beat/min

Question 8

Many heart cells demonstrate intrinsic rhythm but the overall driver of heart rate is the

Answer 8

sino-atrial (S-A) node

Question 9

Mechanism of contraction of the ventricular cardiomyocytes

Answer 9

The rapid opening of voltage-gated sodium channels is responsible for the rapid depolarization phase.
The prolonged “plateau” of depolarization (contraction) is due to the slow but prolonged opening of voltage-gated calcium channels and closure of potassium channels.
Opening of potassium channels results in the repolarization phase.

Question 10

How does calcium produce contraction of the cardiac muscles?

Answer 10

Excitation-Contraction Coupling in Cardiac Muscle
Calcium ions regulate the contraction of cardiac muscle:
The entry of extracellular calcium ions causes the release of calcium from the sarcoplasmic reticulum, the source of about 95% of the calcium in the cytosol.

Question 11

Refractory period

Answer 11

time required before it is possible to re-stimulate muscle contraction
Cardiac muscle (250ms)
Skeletal muscle (1-2ms)
Prevents tetanus

Question 12

Cardiac muscle has a prolonged refractory period which allows for

Answer 12

ventricles to fill with blood prior to pumping

Question 13

Sino-atrial is regulated by the

Answer 13

Autonomic Nervous System

Question 14

Parasympathetic innervates just the

Answer 14

SA and AV nodes
Sympathetic system innervates the WHOLE heart
Both systems are tonically active but parasympathetic dominants a rest

Question 15

Vagus nerve

Answer 15

it is apart of the parasympathetic system and reduces heartbeat

Question 16

sympathetic ganglia

Answer 16

part of the sympathetic system increases the heart beat ( also regulates force/stroke volume)

Question 17

Parasympathetic and sympathetic nervous system regulate the

Answer 17

rate of depolarisation in the sino-atrial node

Question 18

what does the parasympathetic neurons do i heart rate

Answer 18

1.Parasympathetic neurons -
release acetylcholine
2.m2 muscarinic receptors
of SA node
3.↑ K+ efflux, ↓ Ca2+ influx
4.Hyperpolarizes cell and
↓ rate of depolarization
5.↓HR

bradycardia
Both the parasympathetic and sympathetic
nerves are tonically active

At rest – parasympathetic dominates and heart rate reduced from 100 to 70

Question 19

what does the sympathetic neurons do in heart rate

Answer 19

1.Sympathetic neurons -
release noradrenaline
2.b1 adrenergic receptors
of SA node
3.↑ Na+ and Ca2+ influx
4.↑ Rate of depolarization
5.↑ HR
tachycardia
Adrenaline = epinephrine

Question 20

Electrocardiogram was discovered by

Answer 20

Willem Einthoven

Measured the ELECTRICAL activity of the heart
Assigned the letters P, Q, R, S and T to the various deflections
Described the electrocardiographic features of a number of cardiovascular disorders
Awarded the Nobel Prize in Medicine for his discovery in 1924
Einthovenb’s electrocardiogram recorded using a string galvanometer
This ECG opened the modern era of cardiac physiology

Question 21

ECG is a summation of the spread of

Answer 21

action potentials through the various sections of the heart

Question 22

P

Answer 22

Atrial depolarization

Question 23

QRS

Answer 23

Ventricular depolarization

Question 24

T

Answer 24

Ventricular repolarization

Question 25

PQ segment

Answer 25

Atrial contraction

Question 26

QT segment

Answer 26

Ventricular contraction

Question 27

Electrical events of the Cardiac Cycle

Answer 27

1. p wave :atrial depolarization
2. PQ or PG segment : conduction through AV node and A-V bundles
3. Q wave
4. R wave
5. S wave
6. ST segment ventricals contract
7. T wave: ventricular repolarization

Question 28

ECG is a powerful tool for

Answer 28

diagnosing heart disease
Provides valuable information concerning the electrical but NOT mechanical activity of the heart

Heart rate
Heart rhythm
Disturbances of rhythm and conduction
(arrhythmia, pacemaker)
Conduction velocity
Anatomical orientation of the heart
Relative size of chambers
Condition of tissue within the heart
Damage to the myocardium
Influence of certain drugs

Question 29

sinus arrhythmia

Answer 29

normal
inspiration (faster – vagal parasympathetic activity ⇓ > heart rate ⇑)
expiration (slows – vagal parasympathetics activity ⇑ > heart rate ⇓)

Question 30

Ventricular ectopic beat

Answer 30

heart missed a beat’
Aberrant firing of myocytes in ventricles – ill synchronised and fails to eject blood
Common after heart attacks

Question 31

Third Degree

Answer 31

Completely failure of conduction from atria to ventricles
Atria and ventricles beat independently
HEART PACEMAKER

Question 32

Ventricular fibrillation

Answer 32

Result:
Random Firing of hearts
Fibrillating ventricles cannot pump blood
Fatal after a few minutes
450,000 deaths/year in the USA
Caused by:
Myocardial Infarction (heart attack)
Electrical shock
Drug intoxication
Impaired cardiac metabolism
Treatment: CPR followed by electronic defibrillator

Question 33

Cardiac output (I)
Electrical events, heart rate and its measurement using the electrocardiogram (ECG)

Answer 33

Cardiac Output = Heart rate x Stroke volume

Spontaneous depolarisation of the sino-atrial (S-A) node is the driver of heart rate (100 beats/min) and this is tonically reduced by the parasympathetic nervous system (70 beats/min)

3. Sino-atrial node is regulated by the parasympathetic (reduce) and sympathetic (increase) nerves
4. Electrical activity flows from sino-atrial node – atrioventricular node – purkinje fibres - cardiomyocytes

Electrical activity in the heart can be measured using the electrocardiogram (ECG) and is a powerful tool for diagnosis of disease