M3 Topic 3: Cardiac Cycle

Question 1

What is the cardiac cycle?

Answer 1

The complete sequence of contraction and relaxation during a single heart beat

Question 2

Systole

Answer 2

Refers to the period of cardiac contraction

• Blood forced out of the heart and into circulation
• Pressure in chamber increases

Question 3

Diastole

Answer 3

Refers to period of cardiac relaxation

• Heart chambers refill with blood
• Pressure in chamber decreases

Question 4

Cardiac muscle cells

Answer 4

• Involuntary muscle
• Striated (striped) muscle where cells are branched and interconnected
• Like neurons, cardiac myocytes are excitable cells that can fire AP’s
• Electrical activity generated by cardiac myocytes that causes contraction and relaxation of the heart which makes up cardiac cycle

Question 5

Features of cardiac myocytes essential for heart function

Answer 5

• Intercalated discs
• Desmosomes
• Gap junctions
• Mitochondria

Question 6

Intercalated discs in cardiac myocytes

Answer 6

It is where the PM’s of two adjacent cardiac muscle cells join together

• Contains desmosomes and gap junctions that connect cardiac myocytes

Question 7

Desmosomes in cardiac myocytes

Answer 7

Are anchoring proteins that link adjacent cardiac muscle cells

• Provide mechanical connection between cells so they don’t separate during muscle contraction

Question 8

Gap junctions in cardiac myocytes

Answer 8

Provides electrical connection between adjacent myocytes critical for getting heart to contract in coordinated matter

• Form a pore between adjacent cells that allow ions to flow from one cell to another, transmitting electrical signal throughout heart
• When cardiac muscle cell depolarise, it spreads from cell to cell

Question 9

Mitochondria in cardiac myocytes

Answer 9

Cardiac myocytes rely almost exclusively on aerobic metabolism for energy

• Heart cannot operate effectively if deprived of oxygen for any period of time

Question 10

What are the 2 types of cardiac myocytes?

Answer 10

• Contractile myocytes
• Non-contractile autorhythmic cells (AKA pacemaker cells)

Question 11

Contractile myocytes

Answer 11

Cells that contract & generate force, responsible for pumping action of heart

• Make vast majority of cardiac myocytes

Question 12

Non-contractile autorhythmic cells (pacemaker cells)

Answer 12

Ability to spontaneously depolarise & generate own AP’s

• Have unstable RMP, once reaches threshold, AP fires
• These cells allow heart to beat on its own
• Make up ~1% of cardiac myocytes

Question 13

How do AP’s work in pacemaker cells?

Answer 13

1. Pacemaker potential. Slow, spontaneous depolarisation due to slow entry of Na+
2. Depolarisation. At threshold, Ca2+ channels open, rapid entry of Ca2+ causing depolarisation, rising phase of AP
3. Repolarisation. Ca2+ channels inactivate, repolarisation due to opening of K+ channels and exit of K+ from cell

Question 14

Can the heart be completely independent from the nervous system?

Answer 14

No, despite having pacemaker cells allow heart to contract w/o neural signal or input

• ANS important regulator of HR

Question 15

What happens to the AP’s that occur in pacemaker cells?

Answer 15

AP’s and electrical activity generated by pacemaker cells can be spread to adjacent cells via gap junctions

• When AP’s generated in contractile cardiac myocytes, causes contraction of these cells
• Coordinated spread of electrical activity ensures heart contracts in coordinated manner

Question 16

Cardiac conduction system

Answer 16

Network of specialised cells that generate and spread electrical activity throughout heart

• Coordinated spread ensures that atria and ventricles contract in correct order to maintain proper blood flow through the heart

Question 17

How is HR determined?

Answer 17

Pacemaker cells located in the sinoatrial (SA) node generate electrical impulses that travel through heart

• The rate at which these autorhythmic pacemaker cells depolarise and generate AP’s determines HR

Question 18

Components of the cardiac conduction pathway

Answer 18

1. Sinoatrial (SA) node
2. Atrioventricular (AV) node
3. AV bundle (Bundle of His) and bundle branches
4. Purkinje fibres

Question 19

Sinoatrial (SA) node

Answer 19

Cluster of autorhythmic pacemaker cells that are located in the right atrium that spontaneously fire AP’s (depolarisation begins)

Question 20

Atrioventricular (AV) node

Answer 20

Depolarisation/electrical activity spreads to AV node, where it is delayed

• Delay allows atria to depolarise and contract before ventricles begin contracting
• Located between atria and ventricles

Question 21

AV bundle (Bundle of His) and bundle branches

Answer 21

Helps depolarisation move through septum to heart apex

• No direct connection between atria and ventricles

Question 22

Purkinje fibres

Answer 22

Helps spread depolarisation from apex to ventricles

• As electrical activity spreads upwards from apex of heart, means contraction of heart also begins at apex and moves upwards
• Drives blood towards the great arteries for blood to exit ventricles

Question 23

Cardiac conduction pathway

Answer 23

1. Depolarisation begins in SA node, spreads through atria
2. Depolarisation spreads to AV node via internodal pathways
3. Depolarisation moves through septum to heart apex via AV bundle (Bundle of His) and bundle branches
4. Depolarisation spreads from apex to ventricles via Purkinje fibres

Question 24

Electrocardiogram (ECG)

Answer 24

Uses recording electrodes placed on body surface to measure electrical activity of heart

• Important diagnostic tool for cardiac pathologies
• Represents overall electricity in heart
• Cannot detect individual AP’s

Question 25

3 components in the ECG representing one heart beat

Answer 25

- P wave - QRS complex - T wave

Question 26

What does the P wave represent?

Answer 26

Atrial depolarisation - Smaller than QRS complex because atrial muscle mass < ventricular muscle mass

Question 27

What does the QRS complex represent?

Answer 27

Ventricular depolarisation - Atrial repolarisation masked by QRS complex in an ECG

Question 28

What does the T wave represent?

Answer 28

Ventricular repolarisation

Question 29

Phases of the cardiac cycle

Answer 29

1. Atrial/ventricular diastole 2. Atrial systole 3. Isovolumic ventricular contraction 4. Ventricular ejection 5. Isovolumic ventricular relaxation

Question 30

Atrial/ventricular diastole

Answer 30

Heart at rest, both atria and ventricles relaxed & passively filling with blood

Question 31

Atrial systole

Answer 31

- Depolarisation in atria = atrial contraction - Additional blood is pushed out of atria, into ventricles - Ventricles still relaxed

Question 32

Isovolumic ventricular contraction

Answer 32

- Depolarisation in ventricles = begin of ventricular contraction - Increases pressure inside ventricles - When pressure in ventricles > atria, AV valves close - Pressure in ventricles < aorta/pulmonary artery, SL remain closed

Question 33

Ventricular ejection

Answer 33

- Ventricles continue to contract, pressure continues to increase - Once pressure in ventricles > aorta/pulmonary artery, SL valves open - Blood flows out of ventricles and into these arteries

Question 34

Isovolumic ventricular relaxation

Answer 34

- Ventricles repolarise, relax - Pressure in ventricles decrease - Once pressure in ventricles < aorta/pulmonary artery, SL valves close - Once pressure in ventricles < atria, AV valves open again, ventricular filling begins (back to stage 1)

Question 35

Heart sounds

Answer 35

Using a stethoscope, 2 sounds usually associated with each heartbeat First sound ('lubb') usually louder, longer - Associated with closure of AV valves Second sound ('dup') usually softer, shorter - Associated with closure of SL valves