THE CARDIAC CYCLE
I. General Principles.
A. Contraction of the myocardium generates
pressure changes which result in the orderly
movement of blood.
B. Blood flows from an area of high pressure
to an area of low pressure, unless flow
is blocked by a valve.
C. Events on the right and left sides of the
heart are the same, but pressures are lower on
A. Systole = Period of ventricular contraction.
B. Diastole = Period of ventricular relaxation.
diastole is longer than systole.
III. Events of the Cardiac Cycle.
1. Both atria and ventricles are relaxed.
2. AV valves are open.
3. Semilunar valves are closed.
4. Blood is flowing from veins into atria and into ventricles. Passive filling.
NOTE: About 80% of ventricular filling occurs at rest, before the atria contract.
B. Late diastole.
1. Wave of depolarization
begins at SA node, spreads across both atria.
P-wave of the ECG.
2. Atria contract. This increases the pressure in the atria.
3. More blood
4. End-diastolic volume = Volume of blood in each ventricle at the end of diastole.
NOTE: This is when the ventricular volume is greatest.
5. Arterial blood pressure is lowest. Diastolic blood pressure = 80 mmHg.
6. Wave of depolarization reaches AV node.
1. Action potential
is conducted through AV node, down bundle of His, across both
ventricles -- ventricular depolarization. QRS-complex of the ECG.
contract. This increases the pressure in the ventricles above the
in the atria.
3. AV valves close, preventing backflow into atria -- first heart sound ("lub").
4. Semilunar valves open.
5. About 2/3
of blood in the ventricles is ejected into arteries.
Stroke volume = Volume of blood ejected from each ventricle in a single beat.
6. Arterial blood
pressure rises to its highest point.
Systolic blood pressure = 120 mmHg.
D. Early diastole.
1. Ventricular repolarization. T-wave of the ECG.
2. Ventricles relax.
3. Ventricular pressure falls below arterial BP.
4. Semilunar valves close -- second heart sound ("dub").
5. AV valves open.
begin to fill because ventricular pressure is lower than atrial pressure.
This returns us to mid-diastole,
Return to Top of Page