Cardiac Cycle

• Functional syncytium


  A network of cells that are electrically connected to each other and therefore contract together as a single unit
• Once one cell depolarises, current will flow to the adjacent cell via intercalated discs. This is how depolarisation flows through the heart; it is not directionally dependent, that is set by the refractory period
• Cardiac Muscle

  • Cardiomyocytes are myogenically active (involuntary), contraction initiated in the muscle itself and not dependent on neural stimulation, have irregular Y-shaped fibres, mostly single nucleated, striated in appearance, connected by intercalated discs (gap junctions)
• Cardiac Electrophysiology


  Cardiac myocytes are electrically active and are connected by intercalated discs, depolarisation quickly spreads from one cell to the next, The sinoatrial node in the right atrium is the ‘pacemaker’ – it determines heart rate, The SA node spontaneously generates an action potential and it spreads through both atria, this causes the atria to contract, Depolarisation spreads to atrioventricular (AV) node and is delayed (~100 msec) before entering the ventricles
• Cardiac muscle is refractory (i.e., does not respond) to restimulation during an action potential – similar to nerve cells
• The normal refractory period of a ventricle muscle is approximately 0.25-0.30 seconds
• The intercalated discs mean that the myocardium is a “functional syncytium”
• There is a relative refractory period of approximately 0.05 seconds during which the muscle is more difficult than normal to excite but can be excited by a very strong excitatory signal, resulting in the early “premature” contraction
• The Conduction Pathway: Depolarisation starts in the right atrium at the SA node, the ‘funny current’ gives this the fastest depolarisation rate. Via intercalated discs, depolarisation spreads through the right and left atria. The depolarisation reaches the AV node and is slowed, helping faci
• The refractory period of atrial muscle is much shorter at about 0.15 seconds
• Sino-Atrial (SA) Node

  • The pacemaker because it has a leaky membrane
  • Resting potential is around -55 mV
  • At rest, Na+ (and some K+) leak into the cell slowly depolarising it
  • Via 'funny' Na/K hyperpolarisation activated, cyclic nucleotide gated channels (HCN)
  • Around -40 mV, voltage-gated L-type Ca2+ channels open and the cell depolarises
  • Some Na+ involvement via a Na+/Ca2+ exchanger
  • Time-dependent: after ~200 ms, Ca2+ channels close and K+ open, leading to K+ efflux
  • Cell repolarises and the process starts again
• Conduction Pathway & Velocities

  • Conduction speed depends on the diameter and the number of gap junctions
  • The more gap junctions, the quicker the spread
  • The larger the diameter, the quicker the spread
  • Through muscle is relatively slow
  • Atria ~0.3 m/sec
  • Ventricle 0.3 m/sec
  • Through pathways/fibres much quicker
  • Internodal pathway ~1.0 m/sec
  • Purkinje fibres ~3.0 m/sec
  • Atria 0.3 m/sec
  • Purkinje fibres 3.0 m/sec (large diameter)
  • AV node 0.03 m/sec (small diameter)
  • Internodal pathway 1.0 m/sec
  • Ventricles 0.3 m/sec
• Conduction Pathway

  1. Depolarisation starts in right atrium at the SA node

  2. 'Funny current' gives this the fastest depolarisation rate

  3. Depolarisation spreads through the right and left atria via intercalated discs

  4. Depolarisation reaches the AV node and is slowed, helping facilitate ventricular filling

  5. Atria acts as primer pump

  6. From the AV node, depolarisation spreads down the Bundle of His, onto the Purkinje fibres, and out to the ventricular muscle

  7. From endocardium to epicardium
• Ventricular Action Potential

  • AP largely similar to that in nerves
  • During repolarisation, Ca2+ channels open causing a plateau phase
  • Plateau is due to the influx of Ca2+ at the same time as the efflux of K+
  • Allows the ventricle to fully contract and relax before the next action potential arrives
  • Prevents the heart from tetanic contraction because the twitch is over before the cell can depolarise again
  • The plateau and refractory period means the heart must contract rhythmically
• Ventricular Action Potential

  1. AP occurs when depolarisation spreads from adjacent cells via the intercalated discs

  2. Ca2+ channels open causing a plateau phase

  3. Efflux of K+

  4. Influx of Ca2+
• SA Node Action Potential

  1. Ca2+ entry (ms)

  2. K+ efflux

  3. Na+ leak influx

  4. Ca2+ channels open

  5. K+ channels open and Ca2+ channels close
• SA Node

  • It is the pacemaker because it has a leaky membrane
  • Resting potential is around -55 mV
  • At rest, Na+ (and some K+) leak into the cell slowly depolarising it
  • Voltage-gated L-type Ca2+ channels open around -40 mV and cell depolarises
  • After ~200 ms, Ca2+ channels close and K+ open leading to K+ efflux and cell repolarises
• Cardiac Cycle

  1. Complete cycle of heart activity from the start of one beat to the start of the next

  2. Consists of a period of contraction (systole) and a period of relaxation (diastole)

  3. Heart rate at rest is ~70 bpm

  4. Duration of a cycle is approximately 850 ms

  5. During exercise heart rate can increase to 200 bpm

  6. Cycle of about 300 ms (200 ms systole, 100 ms diastole)
• K. Craven, W. N. Zagotta: 'CNG and HCN channels: two peas, one pod. Annual Review of Physiology 2006'
• You can estimate atrial pressure indirectly by the venous pressure
• Cardiac Cycle

  • Period of contraction (systole)
  • Period of relaxation (diastole)
• Most blood entering the atria flows directly into the ventricles (~75%)
• Pulmonary capillary wedge pressure is a measure of left atrial pressure
• Jugular venous pressure (JVP) is a measure of right atrial pressure
• Ventricular Pressure - Diastole

  1. Pressure drops to 0 mmHg, enlarging the chamber as it relaxes more than counteracts filling from atrium

  2. As chamber fills from the atrium, pressure starts to rise slowly

  3. The atrium has filled during ventricular systole
• Atrial pressure estimation


  1. a wave: Atrial systole, 5-7 mmHg increase in pressure

  2. c wave: AV valves close and cusps bulge into atria

  3. x descent: Atria relax (diastole), valves no longer bulge

  4. v wave: Ventricular systole, atria fill with venous return

  5. y descent: Blood enters the ventricles (atria pressure drops); ventricular diastole
• Arterial Pressure

  1. During systole, aortic pressure rises from diastolic pressure to peak systolic pressure

  2. During diastole, ventricular pressure drops below arterial pressure but backflow is prevented by aortic valve

  3. Blood is driven forward by the elastic recoil of the large arteries and aortic pressure drops to diastolic pressure

  4. The same thing happens in pulmonary circulation, though the pressures are much lower
• Ventricular Pressure - Systole

  1. Ventricle contract, AV valves close and pressure starts to rise. This is isovolumetric contraction

  2. No ejection until semi-lunar valves open when pressure exceeds arterial pressure

  3. Once pressure exceeds arterial pressure there is the period of ejection as blood is expelled

  4. As ventricle relaxes, pressure drops and there is isovolumetric relaxation
• Atrial contraction only adds ~25%
• Ventricular Volume


  1. Ventricle fills during diastole

  2. Atrial contraction adds another ~25% (up to ~120 ml)

  3. Isovolumetric contraction until ventricular pressure exceeds arterial pressure then ejection phase

  4. Ejection fraction is approximately 60% of the end-diastolic volume
• Cardiac Valves

  1. Pressure in the atria, ventricles and arteries changes with respect to each other over the cycle

  2. There are four valves in the heart that prevent backflow and keep blood moving forward

  3. Atrioventricular (AV) valves between the atria and the ventricles, on the left is the mitral valve, on the right is the tricuspid valve

  4. Semi-lunar valves
• Quiziology: Ventricular ejection occurs in Early systole
• Outline of Lecture

  1. Cardiac Electrophysiology: the cardiac conduction system, action potentials of the SA node and ventricular muscle

  2. Pressure and Volume Changes of the Cardiac Cycle: pressure changes, volume changes

  3. Valves and heart sounds: AV valves, aortic and pulmonary valves, phonocardiogram
• There are four valves in the heart that prevent backflow and keep blood moving forward
• AV valves

  • Mitral valve (left), Tricuspid valve (right)
• Heart valves

  • Atrioventricular (AV) valves, Semi-lunar valves
• The cardiac cycle is the rhythmical contraction and relaxation of the heart, triggered by depolarisation in the SA node
• Heart Sounds

  1. 1st heart sound: closing of the AV valves as the ventricles contract causes vibrations, known as "Lubb"

  2. 2nd heart sound: rapid snap closing of the aortic and pulmonary valves causing vibrations, known as "Dub"

  3. 3rd heart sound: transition between rapid filling and slow filling of the ventricle

  4. 4th heart sound: rapid filling of a stiff ventricle
• Semi-lunar valves

  • Aortic valve (left), Pulmonary valve (right)
• The contraction of the chambers leads to pressure changes that drive blood flow through the heart and circulation