Cardiac Cycle Flashcards
What allows cardiac contraction to be coordinated?
- Heart has pacemaker potential and so generates its own electrical activity
- IMPORTANT - there are systems to coordinate this electrical activity
What is the effect of the sinoatrial node?
- Pacemaker activity is generated here
- Allows depolarisation of the atria
What is the purpose of the atrioventricular node?
- Delays electrical signal to allow ventricles to fill
- This is before the signal is passed to to apex of ventricles - causing ventricular contraction
The atrioventricular node also has pacemaker potential. Suggest why.
- Allows pumping of blood by ventricles even when there is damage to to conduction system upstream of AVN
RECAP : What is membrane potential?
Potential difference across the plasma membrane
Potassium ion concentration is higher inside the cell, and sodium ion concentration is higher outside the cell.
Suggest a reason for this.
- Membranes have sodium-potassium pumps
- These transport 3 sodium ions outside of the cell and 2 potassium ions inside.
Outline what occurs during phase 4 (first step of the pacemaker potential) PART 1
- Cell is at an unstable resting potential (-40mV to -60mV)
- Voltage-gated sodium ion channels open - influx of sodium ions
Outline what occurs during phase 4 (first step of the pacemaker potential) PART 2
- Membrane potential becomes more positive and cells depolarise
- Once a threshold potential is met, VGCCs open
Outline what occurs during phase 0 (second step of pacemaker potential)
- Rapid influx of calcium ions
- Further depolarisation - membrane potential becomes more positive (around +20mV)
Outline what occurs during phase 3 (final step of pacemaker potential)
- VGCCs close but potassium ion channels open
- Efflux of potassium ions and cell repolarises
- Hyperpolarisation will trigger opening of sodium ion channels again. Cycle repeats
What do the pacemaker potentials cause?
Action potentials in contractile cells in atria and ventricles
RECAP: What are the cardiac muscle cells known as and how are they connected?
CARDIOMYOCYTES
- Connected by intercalated discs containing gap junctions
RECAP: What is the purpose of the gap junctions in cardiomyocytes?
- Allows inducing of action potential events in adjacent cells
- Allows spread of wave of depolarisation - contraction is coordinated
Outline what occurs during phase 4 (first phase of atrial/ventricular action potential)
- Resting membrane potential
- Cells don’t generate their own electrical activity - rely on external stimuli e.g depolarisation of adjacent cells
Outline what occurs during phase 0 (second phase of atrial/ventricular action potential)
- Depolarisation from SAN stimulates opening of voltage-gated sodium ion channels
- Influx of sodium ions. Causes depolarisation of the cell
Outline what occurs during phase 1 (third phase of atrial/ventricular action potential)
- Sodium ion channels close
- Slight repolarisation due to Na/K exchanger
- VGCCs open
Outline what occurs during phase 2 (fourth phase of atrial/ventricular action potential)
- Plateau phase
- Calcium influx - intracellular [Ca2+] rises
- CICR occurs
Outline what occurs during phase 3 (final phase of atrial/ventricular action potential)
- VGCCs close and potassium ion channels open
- Efflux of potassium ions - causes repolarisation
- Membrane potential becomes more negative - falls back to resting potential
What are the two ways spreading of the cardiac action potential is coordinated?
- Passing of action potential to adjacent myocytes
- Coordinated sequence of contraction and relaxation across WHOLE heart
Outline the first step in electrical conduction throughout the heart.
- Action potential initiated at SAN with pacemaker potentials generated by sodium influx
- Action potential spreads across atria through gap junctions present in myocytes
- Atria become depolarised and contract simultaneously.
Outline the second step in electrical conduction throughout the heart.
- Wave of depolarisation reaches AVN
- AVN delays conduction to ventricles to allow proper filling from atria
- Important to allow proper stroke volume and cardiac output
Outline the third step in electrical conduction throughout the heart.
Depolarisation signal passed to apex of ventricles through the Bundle of His
Outline the final step in electrical conduction throughout the heart.
- Ventricular contraction begins at apex
- Depolarisation spreads across ventricular walls through the Purkinje fibres
- Contraction is simultaneous
What does an ECG measure?
Electrical activity of the heart
What is on the y- and x-axis of an ECG?
X-AXIS is time
Y-AXIS is amplitude of depolarisation
What do the following represent on an ECG?
- P-wave
- PR segment
P-WAVE: Atrial depolarisation
PR-SEGMENT: AVN delay
What do the following represent on an ECG?
- QRS complex
- ST segment
QRS-COMPLEX: Ventricular depolarisation
ST-SEGMENT: Ventricles contracting and emptying
What do the following represent on an ECG?
- T-wave
- TP segment
T-WAVE: Ventricular repolarisation
TP-SEGMENT: Ventricles relaxing and filling
How does SAN activity trigger muscle contraction?
- Wave of depolarisation induced
- Causes opening of VGCCs - causes influx of calcium
- Calcium ions used for muscle contraction
How does contraction of a chamber of the heart affect blood pressure and movement of blood?
- Contraction will increase blood pressure
- Blood will naturally move from an area of high pressure to an area of low pressure
What causes valves to open?
Pressure differences
Why is blood not able to move into the atria from the ventricles?
Valves only open one way
Which side of the heart pumps at lower pressure?
Right
Outline what occurs during cardiac diastole.
- Relaxation of all chambers of heart
- Blood returns to heart through venae cavae (on right) and pulmonary vein (on left)
- Ventricular filling occurs
- Due to low ventricular pressure, mitral and tricuspid valves open
Outline what occurs during atrial systole
- Atrial contraction causes filling of ventricles
- Ventricular pressure rises
- Mitral and tricuspid valves close
- Ventricular pressure not high enough to trigger opening of aortic/pulmonary valves
Outline what occurs during ventricular systole
- Ventricles contract - ventricular pressure increases
- Once ventricular pressure > aortic/pulmonary pressure, valves open and blood is ejected
- Atria refill
- Ventricles relax
VENTRICULAR SYSTOLE - What is isovolumetric contraction?
Occurs when aortic pressure>ventricular pressure
- Energy is used for contraction but no blood is ejected out of the heart since pressure not high enough
- Characterised by a rise in pressure but no change in volume
VENTRICULAR SYSTOLE - What is isovolumetric relaxation?
- Occurs whilst ventricles are relaxing
- When the volume of the ventricles don’t change significantly when relaxing
- Causes a pressure drop - allows mitral and tricuspid valves to open and allows cycle to continue
Left ventricular pressure changes can be plotted on a graph with a green line and a red line. What do these lines represent?
GREEN LINE - Left ventricular pressure changes
RED LINE - Aorta pressure changes
LABELLED DIAGRAM CAN BE FOUND ON SLIDES
Outline what occurs at point 1 on the pressure change graph. (FOUND ON SLIDES)
- Contraction of left atrium
- Ventricular pressure begins to rise, but still relatively low
- Ventricular pressure > atrial pressure, so mitral valve closes
Outline what occurs at point 2 on the pressure change graph. (FOUND ON SLIDES)
- Ventricular systole
- Aortic valve won’t open until ventricular pressure>aortic pressure
- Isovolumetric contraction occurs
Outline what occurs at point 3 on the pressure change graph. (FOUND ON SLIDES)
- Ventricular pressure>aortic pressure so aortic valve opens
- Ejection of blood from ventricles into aorta
- Ventricular pressure begins to reduce
Outline what occurs at point 4 on the pressure change graph. (FOUND ON SLIDES)
- Ventricular pressure < aortic pressure so aortic valve closes
- Isovolumetric relaxation occurs
Left ventricular volume changes can be plotted on a graph. What is on the y-axis of this graph?
Y-AXIS: Left ventricular volume
LABELLED DIAGRAM is on the slides
Outline what occurs at point 1 on the volume change graph. (FOUND ON SLIDES)
- Atrial contraction so ventricular volume rises
- Ventricles are still relaxed
- Volume of blood that enters before ventricular systole known as EDV - end diastolic volume (around 120ml)
Outline what occurs at point 2 on the volume change graph. (FOUND ON SLIDES)
- Ventricular pressure increases due to filling
- Mitral valve closes
- Ventricles contract but volume stays constant - ISOVOLUMETRIC CONTRACTION
Outline what occurs at point 3 on the volume change graph. (FOUND ON SLIDES)
- Ventricular pressure > aortic valve so ejection of blood from ventricles. Ventricular volume increases
- Ejection is initially rapid but over time rate of ejection decreases
Outline what occurs at point 4 on the volume change graph. (FOUND ON SLIDES)
- Ventricular pressure decreases, so aortic valve closes
- ISOVOLUMETRIC RELAXATION - pressure drops but no change in volume of ventricles
- Volume of blood remaining in ventricles post-ejection is ESV (end systolic volume) - around 40ml
What is the equation linking stroke volume, ESV and EDV.
STROKE VOLUME = EDV - ESV
Outline the importance of the ventricular pressure-volume loop
Provides insight into the amount of energy the heart expends during contraction and relaxation
What is on the x-axis and y-axis of the ventricular pressure-volume loop?
X-AXIS: Left ventricular volume
Y-AXIS: Left ventricular pressure
LABELLED DIAGRAM CAN BE FOUND ON SLIDES
Outline what occurs at point A on the pressure-volume loop. (FOUND ON SLIDES)
- Mitral valve opens
- Ventricles begin to fill and volume begins to increase
- Ventricles relaxed - not too much change in pressure
Outline what occurs at point B on the pressure-volume loop. (FOUND ON SLIDES)
- Mitral valve closes and ventricles contract
- Undergoes period of isovolumetric contraction
- Pressure rises but no volume change(since aortic and mitral valves are closed)
Outline what occurs at point C on the pressure-volume loop. (FOUND ON SLIDES)
- Ventricular pressure > aortic pressure so aortic valves open
- Blood ejected from ventricles
- Ventricular volume decreases. Pressure slightly rises due to contraction of ventricles
Outline what occurs at point D on the pressure-volume loop. (FOUND ON SLIDES)
- Aortic valve closes
- Ventricles relax so pressure of ventricles decrease
What does area inside the pressure-volume loop tell you?
Work done to eject volume of blood
What are the two characteristic of a healthy heart? How would these characteristic change during heart failure?
- A high stroke volume with low energy consumption
- In heart failure, low stroke volume with high consumption
What causes S1?
- Closure of tricuspid and mitral valves at beginning of ventricular systole/after ventricular filling ends
What causes S2?
- Closure of aortic valves when ventricular pressure < aortic pressure
- Ejection of blood occurs at end of ventricular systole
What causes S3?
Turbulent blood flow into ventricles
What causes S4?
Forceful atrial systole against stiff ventricles
