The cardiac cycle is the sequence of events that occurs during a heart beat (Figure 16a). The amount of blood ejected by the ventricle in this process is the stroke volume (SV), ∼70 mL, and cardiac output is the volume ejected per minute (SV × heart rate).
Towards the end of diastole (G) all chambers of the heart are relaxed. The valves between the atria and ventricles are open (AV valves: right, tricuspid; left, mitral), because atrial pressure remains slightly greater than ventricular pressure until the ventricles are fully distended. The pulmonary and aortic (semilunar) outflow valves are closed, as pulmonary artery and aortic pressure are greater than the respective ventricular pressures. The cycle begins when the sinoatrial node initiates the heart beat (see Chapter 11).
Atrial Systole (A)
Contraction of the atria completes ventricular filling. At rest, the atria contribute less than 20% of ventricular volume, but this proportion increases with heart rate, as diastole shortens and there is less time for ventricular filling. There are no valves between the veins and atria, and some blood regurgitates into the veins. The a wave of atrial and venous pressure traces reflects atrial systole. Ventricular volume after filling is known as end-diastolic volume (EDV), and is ∼120–140 mL. The equivalent pressure (end-diastolic pressure, EDP) is <10 mmHg, and is higher in the left ventricle than in the right due to the more muscular and therefore stiffer left ventricular wall. EDV is an important determinant of the strength of the subsequent contraction (Starling’s law; see Chapter 17). Atrial depolarization causes the P wave of the ECG.
Ventricular Systole
Ventricular contraction causes a sharp rise in ventricular pressure, and the atrioventricular (AV) valves close once this exceeds atrial pressure. Closure of the AV valves causes the first heart sound (S1; see below). Ventricular depolarization is associated with the QRS complex of the ECG. During the initial phase of ventricular contraction pressure is less than that in the pulmonary artery and aorta, so the outflow valves remain closed. This is isovolumetric contraction
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