Cardiac cycle Flashcards
what is the cardiac cycle
repetitive sequence of events with each heartbeat
what are the events of the cardiac cycle
flow into atria - continuous except when they contract, inflow leads to pressure rise
opening of AV valves - flow to ventricles
atrial systole - completes ventricle filling
ventricular systole (atrial diastole) - pressure rise closes AV valves, opens aortic and pulmonary valves
ventricular diastole - causes closure of aortic and pulmonary valves
what are the heart sounds
1st - closing of AV valves (lub)
2nd - closing of semilunar valves (dub)
3rd - early diastole of young and trained athletes, normally absent after middle age, termed the ventricular gallop, emergence in later life indicates abnormality
4th - caused by turbulent blood flow, due to stiffening of walls of left ventricle, occurs prior to 1st heart sound, atrial gallop
during tachycardia 3 and 4 are indistinguishable, known as a summation gallop
what does the elastic arterial tree do
stores pressure energy
helps maintain pressure in arterial system during diastole (pressure only drops about 1/3 from systolic BP)
what is the stroke volume
volume of blood pumped by each ventricle per beat
around 75ml, but may double during exercise
what is the ejection fraction
% volume pumped out
at rest around 55-60%, exercise 80%, in heart failure may be 20%
during the cardiac cycle the chambers never fully empty
why does systemic arterial pressure remain high
due to elasticity of the vessel walls and peripheral resistance
what is the cardiac output
volume of blood pumped per minute (by each ventricle)
cardiac output = heart rate x stroke volume
at rest CO is usually 5l/m
during exercise it can be more than 25l/min as heart rate increase 2-3 fold and stroke volume increases 2 fold
what is the effect of heart rate on cardiac output
normally an increased heart rate is associated with increased CO
but if filling time is decreased then so is stroke volume
venous return determines CO
what us stoke volume dependent on
contractility - the force of contraction ( adrenaline increases force, therefore increases SV)
end diastolic volume - volume of blood in ventricle at the end of diastole
force is stronger the more muscle fibres are stretched, within limits ( frank-starling mechanism)
stroke volume is proportional to diastolic filling
what is the frank-starling mechanism
also known as preload
important in ensuring the heat can deal with wide variations in venous return and balancing the output of the two sides of the heart
what is peripheral resistance
afterload
resistance to blood flow away from the heart
altered by dilation or constriction of blood vessels (mainly pre-capillary resistance arteries)
CO = BP/PR
sum of afterload and end diastolic volume determine force
effect of peripheral resistance
normally small changes in peripheral resistance have little effect on cardiac output
with increased PR expect decreased SV and CO
decreased stroke volume leads to an increased end systolic volume, increased end diastolic volume and increased stoke volume (via frank-starling mechanism) so cardiac output does not decrease much
