Cardiac cycle

Question 1

Why is the sino-atrial node known as the heart’s “natural pacemaker?”

Answer 1

Sets the rhythm of the heart

Question 2

How does the SA node set the rhythm of the heart?

Answer 2

It spontaneously produces action potentials that travel through the heart via the electrical conduction system causing the heart to contract

Question 3

What is the function of the atrio-ventricular node?

Answer 3

To delay the impulses coming from the SA node so that atria have time to eject their blood into ventricles before ventricular contraction.

Question 4

What is a pacemaker potential?

Answer 4

The depolarisation of the membranes of pacemaker cells (cells of SA node) that cause these cells to produce an action potential

Question 5

What is the first phase involved in the production of pacemaker potentials?

Answer 5

Phase 4: Once membrane reaches resting membrane potential (-60 mV) it repolarises causing funny current to activate. Funny current leads to further depolarisation which causes an Na+ channel to become activated, leading to Na+ influx. Na+ influx causes membrane potential to reach If threshold (-40mV).

Question 6

What is the second phase involved in the production of pacemaker potentials?

Answer 6

Phase 0: Once If threshold reached it causes opening of voltage gated Ca2+ channels leading to Ca2+ influx and rapid depolarisation

Question 7

What is the third phase involved in the production of pacemaker potentials?

Answer 7

Phase 3: Voltage gated Ca2+ channels switch off leading to activation of voltage-gated K+ channels. This causes K+ to leave cell leading to repolarisation and eventually the resting potential being reached.

Question 8

Why is the resting potential of the SA node unstable?

Answer 8

Because the membrane potential of the pacemaker cells are almost never at the resting potential due to the funny current causing spontaneous depolarisation of these cells

Question 9

What is the first phase involved in contraction of the atria/ventricles?

Answer 9

Phase 0 (Depolarisation) – Muscle receives depolarisation stimulus from SA node causing activation of ligand gated sodium channels causing them to open leading to slight depolarisation. This causes the voltage-gated Na+ channels to open leading to large depolarisation and action potential to be produced.

Question 10

What is the second phase involved in contraction of the atria/ventricles?

Answer 10

Phase 1 (Falling of action potential/early repolarisation) - Once top of action potential is reached it causes voltage gated Na+ channels to become inactivated – plugged by inactivation loop thus stopping further flow of Na+ ions.

Question 11

What is the third phase involved in contraction of the atria/ventricles?

Answer 11

Phase 2 (Plateau phase) – Opening of voltage-gated potassium channels causes K+ ions to flow out of cell causing slight depolarisation. There is also opening of chloride ion channels causing influx of Cl- ions. There is also opening of calcium channels causing influx of Ca2+ into the cell. Net flow of charge doesn’t change that much so membrane potential stays around the same. 
Influx of Ca2+ leads to calcium-induced calcium release from sarcoplasmic reticulum which leads to muscle contraction

Question 12

What is the fourth phase involved in contraction of the atria/ventricles?

Answer 12

Phase 3 (Rapid repolarisation) – Calcium and chloride channels close and voltage gated potassium channels become fully open thus leading to K+ leaving the cell.

Question 13

What is the fifth phase involved in contraction of the atria/ventricles?

Answer 13

Phase 4 (Diastole) – heart is at rest (voltage gated Na+ and K+ channels closed but sodium and potassium “leak” channels open). Sodium-potassium pump works to get membrane potential back to resting potential. 
Will only go back to phase 0 if muscle receives depolarisation signal from SA node

Question 14

Why is the resting potential of cardiac cells of the heart considered stable compared to the resting potential of the pacemakaer cells?

Answer 14

Because within normal cardiac cells there’s no funny current so depolarisation will only occur if muscle receives depolarisation signal from SA node meaning the cell will spend more time at its resting potential

Question 15

Describe the flow of an action potential through the electrical conduction system

Answer 15

1. Electrical activity generated in SA node spreads out via gap junctions into atria as well as AV node – This causes the contraction of the atria
2. At AV node, conduction is delayed allowing the atria to fully contract and fill the ventricles before they contract
3. After atrial contraction the action potential travels through the fibres to the bundle of His
4. Action potential then travels through left left and right bundle branches down to the apex of the heart
5. Once at the apex the action potential will travel through the further divisions of left and right bundle branches (the purkinje fibres) into the walls of the left and right ventricle causing them to contract from the apex upwards

Question 16

What does the P wave of an ECG represent?

Answer 16

P wave = Atrial depolarisation.

Question 17

What does the PR segment represent?

Answer 17

PR segment = Time taken for depolarisation to spread out from the SA node to the ventricles (atrial contraction also occurs)

Question 18

What does the QRS complex represent?

Answer 18

QRS complex = Ventricular depolarisation (atria repolarising simultaneously).

Question 19

What does the ST segment represent?

Answer 19

ST segment = Time during which ventricles are contracting and emptying.

Question 20

What does the T wave represent?

Answer 20

T wave = Ventricular repolarisation.

Question 21

What does the TP interval represent?

Answer 21

TP interval = Time during which ventricles are relaxing and filling.

Question 22

Describe the flow of blood through the body

Answer 22

Venae Cavae -> Right atrium -> Tricuspid valve -> Right ventricle -> Pulmonary valve -> Pulmonary artery -> pulmonary circulation -> pulmonary vein -> left atrium -> mitral valve -> Left ventricle -> Aortic valve -> Aorta -> systemic circulation

Question 23

What are the stages of the cardiac cycle?

Answer 23

Ventricular filling/atrial contraction;

Isovolumetric contraction; Ejection and Isovolumetric relaxation

Question 24

What occurs during ventricular filling/atrial contraction?

Answer 24

Ventricular filling/atrial contraction: Both atria and ventricles in diastole so blood enters atria and moves through open atrioventricular valves into ventricles. Although ventricular volume increases, initially ventricular pressure decreases. This is because the ventricles are still recovering from last systole phase and so the elastic recoil of the ventricles causes suction of blood into them. Eventually, filling of ventricles causes them to stretch and causes increase in ventricular pressure. Atria then contract and push more blood into the ventricles thus causing them to stretch even further.

Question 25

What occurs during isovolumetric contraction?

Answer 25

Isovolumetric contraction: Ventricular pressure rises above atrial pressure so both atrioventricular valves close. Both the aortic valves and the pulmonary valves are also closed due to ventricular pressure not being high enough to overcome the resistance of these valves. All this means that initially as the ventricles contract the blood has nowhere to go causing ventricular pressure to rise rapidly but ventricular volume to remain the same.

Question 26

What occurs during Ejection?

Answer 26

Ejection: Rise in ventricular pressure eventually overcomes resistance of aortic and pulmonary valves causing them to open. Blood is ejected into the aorta and pulmonary artery – initially blood is ejected rapidly into the 2 arteries, so it causes the arterial pressure to rise rapidly to the max. or systolic level. However, ejection rate then begins to slow down causing arterial pressure to decrease as blood is flowing through the arteries faster than it is being ejected into them. Blood re-enters atria

Question 27

What occurs during isovolumetric relaxation?

Answer 27

Isovolumetric relaxation: Eventually arterial pressure becomes lower than ventricular pressure so aortic and pulmonary valves close. Relaxed myocardium of ventricles causes ventricular pressure to fall below atrial pressure causing opening of atrioventricular valves. Blood flows in from atria causing cycle to start over.

Question 28

What is the end-diastolic volume?

Answer 28

Volume of blood within ventricles at the end of its filling phase. EDV = 120ml.

Question 29

What is the end-systolic volume?

Answer 29

Volume of blood within ventricles at the end of contraction. ESV = 40-50ml

Question 30

How can you calculate the stroke volume form the EDV and the ESV?

Answer 30

Stroke volume = EDV - ESV

Question 31

What does the area within the ventricular pressure-volume loop represent?

Answer 31

Amount of stroke work done

Question 32

What is the right atrial cycle?

Answer 32

Cycle showing the changes in pressure that the right atrium experiences during the cardiac cycle

Question 33

What are the different stages of the right atrial cycle?

Answer 33

A wave: Atrium contracting
X descent: Atrium relaxing; tricuspid closed
V wave: Atrium full and tense; tricuspid closed
Y descent: atrium emptying; tricuspid open

Question 34

What causes the S1 “Lub” heart sound?

Answer 34

Closure of tricuspid/mitral values at beginning of ventricular systole.

Question 35

What causes the s2 “dub” heart sound?

Answer 35

Closure of aortic/pulmonary valves (semilunar valves) at end of ventricular systole.