cardiac cycle Flashcards
what are the general principles of the cardiac cycle ?
- electrical activity is generated at SA node and conducted throughout the heart
- electrical activity is converted into myocardial contraction which creates pressure changes in the chambers
- blood flows from an area of high pressure to an area of low pressure unless blocked by a valve
- valves open and close depending on pressure changes in chambers
- events on the R and L sides of the heart are the same but pressure is lower on R
what is the SA node and what is its function ?
- group of cells located in the wall of the R atrium
- spontaneously produces action potential that travels through the heart via electrical conduction system
- sets rhythm of heart - natural pacemaker
- rate of AP production and heart rate is influenced by nerves that supply it.
what is the AV node and what is its function ?
- part of the electrical conduction system
- electrically connects R atrium and ventricle
- delays impulses so atria have time to eject blood into ventricles before contraction
describe atrial and ventricular muscle action potentials
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+
describe Sinoatrial node (SA) pacemaker potentials
- resting -ve voltage inside cell compared to outside cell
- normally high K+ inside , high Na+ , Cl- outside.
- Na K pump uses ATP to transport 3 Na out and 2 K in
- PHASE 4 (funny current) - membrane repolarises below funny current threshold. not genuine resting potential as it is unstable. At around - 50mV Na + channel activated causes Na+ influx and slow depolarisation.
- PHASE 0 - as cell depolarises it reaches a threshold for voltage-gated ca channels so ca influx and rapid depolarisation
- PHASE 3 - ca channels shut at max depolarisation. K channel activated k leaves causing repolarisation.
describe electrical conduction through the heart
- electrical activity generated in SA node and spreads out via gap junctions intro 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 spread quickly throughout ventricles
ventricle contraction begins at apex.
describe the different stages of electrical activity on the ECG
describe cardiac diastole
- relaxation of all heart muscles
- blood returns to the heart and begins to fill the atria and ventricles
- low pressure in ventricles allows mitral and tricuspid valves to open and ventricles fill with blood
describe atrial systole
- atrial contraction
- causes blood to move into relaxed ventricles
- as ventricles fill the increase in pressure causes mitral and tricuspid valves to close
describe ventricular systole
- ventricles contract
- after a period of isovolumetric contraction ( using energy to contract but not pumping any blood) pressure rises to force open aortic and pulmonary valves so blood is ejected
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.
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.
name some heart sounds and describe them
S1 – Lub
Closure of mitral valve at beginning of ventricular systole.
S2 – Dub
Closure of aortic valve 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.
label this heart
what is the end-diastolic volume(EDV) ?
the maxiumum amount of blood that is in the heart
what is the end-systolic volume (ESV) ?
the amount of blood that’s left in the heart after a beat.
