4. The cardiac cycle Flashcards
What are the two sequential phases of one heart beat?
- Systole
- Diastole
What is systole?
Period of cardiac contraction and emptying
Describe systole
Time of ejection
Length varies according to HR (0.3sec @65bpm; 0.16sec @200bpm)
Myocardium contracts & repolarises faster at high HR (Ca return to SR)
What is diastole?
A period of cardiac relaxation (length varies with varying heart rate)
Describe diastole
Time of refilling
Length varies according to HR (0.62sec @65bpm; 0.14sec @200bpm)
What are the sequence of events in the cardiac cycle?
Atrial diastole → ventricular diastole → atrial systole → ventricular systole
When do AV valves open?
When P atria > P ventricles
When do semilunar valves open?
When P ventricles > P arteries
Mechanical phases of the cardiac cycle
- Late diastole: both sets of chambers are relaxed and ventricles fill passively
- Atrial systole: atrial contraction forces a small amount of additional blood into ventricles
- Isovolumic ventricular contraction: first phase of ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves.
- Ventricular ejection: as ventricular pressure rises and exceeds pressure in the arteries the semilunar valves open and blood is ejected.
- Isovolumic ventricular relaxation: as ventricles relax pressure in ventricles falls, blood flows back into cups of semilunar valves and snaps them closed.
EDV =
End diastolic volume = volume of blood in ventricle at end of diastole
ESV =
End systolic volume = volume of blood in ventricle at end of systole
SV =
Stroke volume = volume of blood ejected from ventricle in each cycle
SV = (eqn)
SV = EDV - ESV
Efection fraction =
% EDV ejected with each stroke (ranges 50-75%) - A good index of ventricular function
Pressure- Volume curve/loop
Wigger’s Diagram
