The Cardiac Cycle Flashcards
Describe the Sinoatrial Node (SAN) and its functions.
The SA node is a group of cells located in the walls of the right atrium.
It has the ability to spontaneously produce action potentials that travel through the heart via the electrical conduction system.
It sets the rhythm of the heart, and so is known as the heart’s natural pacemaker.
The rate of action potential production (and therefore the heart rate) is influenced by nerves that supply it.
Describe the Atrioventricular Node (AVN) and its functions.
The AV node is a part of the electrical conduction system of the heart that coordinates the top of the heart.
It electrically connects the right atrium and the right ventricle, delaying impulses so that the atria have time to eject their blood into ventricles before ventricular contraction.
Describe the phases of the SAN pacemaker potentials
PHASE 4: PACEMAKER - the “funny current (If)”
The membrane repolarises below the If threshold (40mV). This is not a genuine resting potential because it is unstable.
At around 50mV, a Na+ channel is activated, causing Na+ influx and slow depolarisation
PHASE 0: VOLTAGE-GATED CA2+ CHANNELS
As the cell depolarises, it reaches a threshold for voltage-gated Ca2+ channels leading to a Ca2+ influx. RAPID depolarisation
PHASE 3: REPOLARISATION
Ca+ channels switched off at max depolarisation.
Activation of voltage-gated K+ channels; K+ leaves, causing repolarisation
Describe the resting potential in SAN pacemaker potentials
- There is a resting -ive voltage in the cell interior compared to the cell exterior ranging from -40mV to -80 mV
- Normally, high K+ inside Na+ Cl- outside. Na+ K+ pump uses ATP to transport 3 Na+ out of the cell and 2 K+ in
Describe the phases of atrial & ventricular muscle action potentials
PHASE 0: RAPID DEPOLARISATION
Recieves depolarisation stimulus from SA node causing voltage gated Na+ channels open and Na+ influx. Voltage-gated Ca2+ channels start to open very slowly
PHASE 1: EARLY REPOLARISATION
Na+ channels close and cells begin to repolarise
PHASE 2: PLATEAU PHASE
Voltage-gated calcium channels are now fully open - Ca2+ influx halts the repolarisation. Voltage-gated K+ channels start to open slowly
PHASE 3: RAPID REPOLARISATION
Calcium channels close & K+ channels open fully, causing a K+ efflux
PHASE 4: RESTING PHASE
Stable - Na+/K+ pump - 3Na+ out & 2K+ in. The membrane is slightly impermeable to Na+ but slightly permeable to K+
Describe the electrical conduction through the heart
1) Electrical activity generated in the SA node spreads out via the gap junctions into the atria
2) At the AV node, conduction is delayed to allow the correct filling of the ventricles
3) Conduction occurs rapidly through the bundle of His into the ventricles
4) Conduction occurs through the Purkinje fibres and spreads quickly throughout the ventricles.
Ventricular contraction begins at the apex
There are many parts to an ECG. List what is represented by the P wave, PR segment, QRS complex, ST segment, T wave and TP interval
P WAVE: atrial depolarisation & contraction
PR SEGMENT: AV nodal delay
QRS COMPLEX: ventricular depolarisation & contraction (atria repolarising simultaneously)
ST SEGMENT: ventricles contracting and emptying
T WAVE: ventricular repolarisation
TP INTERVAL: ventricles are relaxing and filling
What are some general principles of the cardiac cycle regarding electrical conductivity, pressure and valves
- Electrical activity is generated at the SA node and conducted throughout the heart
- Electrical activity is converted into myocardial contraction which creates pressure changes within chambers
- Blood flows from an area of high pressure to low pressure, unless the flow is blocked by a valve
- Valves open and close depending on the pressure changes in the chambers
- Events of the right and left sides of the heart are the same, but pressures are lower on the right
Describe cardiac diastole
- All chambers are relaxed and blood flows into the heart
- Blood returns to the heart and begins to fill the atria and ventricles
- Low pressure in the ventricles allows the mitral and tricupsid valves to open and the ventricles fill with blood
Describe atrial systole
- Atrial contraction causes blood to move into relaxed ventricles
- As the ventricles fill, the increase in pressure in the ventricles forces mitral and tricuspid valves to close
Describe ventricular systole
- After the atria relax, ventricles begin to contract, pushing blood out of the heart
- After a period of isovolumetric contraction, pressure rises sufficiently to force open aortic and pulmonary valves and blood is ejected from the ventricles
Describe ventricular diastole
- Ventricle empties and once its pressure is less than the aorta the aortic valves close. This is followed by isovolumetric relaxation and large pressure drop below that of atrium causing mitral valve to open
- Blood flows into the relaxed heart in preparation for another atrial systole
Describe left ventricular pressure changes
- Contraction of the left atrium pushes blood into the relaxed ventricle. Once the ventricle is full its pressure rises slightly and forces the mitral valve to close
- Pressure rises during isovolumetric contraction of ventricle
- When ventricle pressure is higher than the aorta, the aortic valve is pushed open and blood is ejected from the ventricle
- Ventricle empties and once its pressure is less than the aorta the aortic valve closes. This is followed by isovolumetric relaxation and large pressure drop below that of atrium causing mitral valve to open
- Blood flows into the relaxed heart in preparation for another atrial systole
Describe left ventricular volume changes
- Filling ventricle contraction of atria. EDV 120ml
- Full ventricle higher pressure closes mitral valve. Systole begins isovolumetric contraction
- Ventricular pressure overcomes aortic valve and blood ejected
- When ventricular pressure falls the aortic pressure closes aortic valve, isovolumetric ventricular relaxation.
Stroke volume (SV) = EDV - ESV
Describe the ventricular pressure-volume loop, and include the equation learnt for work
Work = change in ventricle pressure x change in volume
The loop relates to the amount of energy consumption during the cardiac cycle. The area inside the loop is equal to the amount of stroke work done.
