Cardiac Cycle Flashcards
What are the sinoatrial node and the atrioventricular node?
SA node is a group of cells located in the wall of the right atrium
The AV node is a part of the electrical conduction system of the heart.
What do the SA and AV nodes do?
SA:
Ability to spontaneously produce action potential that travels through the heart via the electrical conduction system
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
AV:
It electrically connects the right atrium and right ventricle delaying impulses so that atria have time to eject their blood into ventricles before ventricular contraction
Describe the phases of SA pacemaker potentials
Phase 4- Pacemaker, the funny current, If
The membrane repolarises below the If threshold (approx. -40mV) This is not a genuine resting potential because it is unstable
At around -50mV an 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 Ca2+ influx.
RAPID depolarisation
Phase 3- Repolarisation
Ca2+ channels switched off at max depolarisation
Activation of voltage-gated K+ channels K+ leaves causing repolarisation
What is the usual resting membrane potential?
There is a resting negative voltage in the cell interior as compared to the cell exterior ranging from -40mV to -80mV
Normally high K+ inside high Na+ Cl- outside
Recall and describe the phases of atrial and ventricular muscle action potentials.
These contractile cardiac muscles have a slightly different action potential to the SA node
Phase 0: rapid depolarisation
Receives depolarisation stimulus from SA node causing…
Voltage-gated Na+ channels open, Na+ influx.
Voltage-gated Ca2+ channels start to open very slowly.
Phase 1: early repolarisation
Na+ channels close cells beginning to repolarise.
Phase 2: plateau phase
Voltage gated calcium channels fully open - Ca2+ influx halts the repolarisation. Voltage-gated K+ channels start to open slowly
Phase 3: rapid repolarisation
Ca2+ channels close & K+ channel open fully so K+ efflux
Phase 4: resting phase
Stable - Na+/K+ pump – 3Na+ out & 2K+ in
Membrane slightly impermeable to Na+ slightly permeable to K+
How is electricity conducted through the heart? (steps)
- Electrical activity generated in SA node spreads out via gap junctions into atria
- At AV node, conduction is delayed to allow correct filling of ventricles
- Conduction occurs rapidly through bundle of His into ventricles
- Conduction through Purkinje fibres spreads quickly throughout the ventricles
Ventricular contraction begins at the apex
What are the general principles of the cardiac cycle?
Electrical activity is generated at SA node and conducted throughout heart
Electrical activity is converted into myocardial contraction which creates pressure changes within chambers
Blood flows from an area of high pressure to an area of low pressure- unless flow is blocked by a valve
Valves open and close depending on pressure changes in chambers
Events on the right and left sides of the heart are the same, but pressures are lower on the right
What is cardiac diastole? What happens during diastole?
Relaxation of all heart muscles
Blood returns to the heart and begins to fill the atria and ventricles
Low pressure in the ventricles allows the mitral and tricuspid valves to open and the ventricles fill with blood
What is atrial systole? What happens during 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
What is ventricular systole? What happens?
The ventricles enter systole and begin contracting
After a period of isovolumetric contraction pressure rises sufficiently to force open aortic and pulmonary valves and blood is ejected from the ventricles
How does ventricular pressure change?
- 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 the atrium causing mitral valve to open
- Blood flows into the relaxed heart in preparation for another atrial systole
How does ventricular volume change?
- 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 aortic valve, isovolumetric ventricular relaxation
Stroke volume (SV)= EDV - ESV
Describe the ventricular pressure-volume loop.
A- Diastole, ventricle relaxed and filling so the pressure remains low but volume increases
B- Mitral valve closes and ventricle contracts, the volume doesn’t change as both mitral valves and aortic valves are closed, but pressure increases
C- Pressure becomes high enough to force aortic valve open and blood begins to leave the ventricle, The volume decreases as pressure continues to rise because the heart is still contracting
D- As the ventricle empties the aortic valve closes and the ventricle relaxes so the pressure falls dramatically
How do heart sounds relate to the cardiac cycle?
S1- Lub
Closure of tricuspid/mitral values at beginning of ventricular systole
S2- Dub
Closure of aortic/pulmonary valves (semilunar valves) at end of ventricular systole
S3- Occasional
Turbulent blood flow into ventricles, detected near end of first 1/3 diastole, especially in older people
S4- Pathological in adults
Forceful atrial contraction against a stiff ventricle less so in young people
Vibrations induced by the closure of cardiac valves
Vibrations in ventricular chambers
Turbulent blood flow through valves
