The Cardiac Cycle
The heart pumps in a pulsatile fashion, and a complete cardiac cycle involves the contraction of the two atria followed by the contraction of the two ventricles. The period of contraction is termed systole, while diastole describes the period of relaxation. The contractility of the heart, multiplied by the heart rate, results in the measure of cardiac output.
The cardiac cycle is controlled by the electrical signals and pathways of the heart and is manifest in the rhythmic contractions of the atria and ventricles.
Systole
Systole is the contraction of the ventricles. Atrial systole lasts about 0.1 second, and ventricular systole lasts about 0.3 seconds. The remaining 0.4 seconds represent diastole.
S1 and S2 are normally heard during systole.
The duration of systole remains constant from beat to beat, while the length of diastole varies with heart rate.
Right heart function can be estimated by the jugular venous pulse (JVP).
Diastole
Diastole is the relaxation and filling of the ventricles. Blood passes from the atria into the ventricles across the open tricuspid and mitral valves, causing a gradual increase in ventricular pressure. In late diastole, atrial contraction pushes a further amount of blood into the ventricles
Stroke Volume
Stroke volume, or EDV - ESV, is close to 70 ml.
Left-sided heart failure will lead to decreased SV, which will lead to increased pulmonary congestion, which leads to decreased right sided stroke volume, which leads to venous congestion.
Atrial systole adds perhaps 5% to ventricular stroke volume.
Stroke volume is increased by contractility and preload, and decreased by afterload.
Preload is the ventricular wall tension at the end of diastole, approximated by EDV.
Afterload is the vetricular wall tesion during contraction, equalling the resistance that the heart must overcome. It is approximated by systolic pressure.
The ejection fraction is the percentage of blood ejected; ie 70/120. It is normally 55-75%
Pressures throughout the Heart
While pressures are different between the right and left heart are very different (6x), the volumes they pump are the same.
The LV sees are huge change in pressure throughout its action, ranging from 10 - 120 mm Hg.
The increase in volume just before closure of the mitral valve is due to atrial contractrion, which contributes 5% of ventricular volume. This therefore corresponds to the P wave.