17.1 Cardiac cycle
17.2 Methods of studying the cardiac cycle
17.3 Energy support of the heart
17.4 Cardiac output assessment
17.5 Heart sounds
17.6 Mechanical manifestations of cardiac activity
17.7 Electrocardiography
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17.1. Cardiac cycle
Pumping of the heart is an integral manifestation of its basic physiological functions: automatism, excitation, conduction, and contraction, and this is the function of the cardiac cycle. The cardiac cycle includes atrial and ventricular systole and diastole (Fig. 17.1, 17.2).
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Fig. 17.1. The circuit of cardiac cycle. See the text for more details
Atrial systole (0.1 s; P wave on the ECG at a heart rate of 75 per minute).
Myocardial contraction begins from the sinoatrial nodearea (mouth of the vena cava), which blocks the flow of blood from the atria to the vena cava.
Contraction of atrial walls produces the fourth heart sound (recorded on a phonocardiogram). Atrial pres-
sure increases: from 0 to 5 mm Hg on the right, from 5 to 12 mm Hg on the left.
State of the valves: atrioventricular valves open, semilunar valves closed.
Hemodynamics: the last portion (≈30 ml) of blood passes into the ventricles, and the main amount (8090 ml) is already in the ventricles by the time of atrial systole.
The rest of the cycle time (0.7 s) of the atrium is based in the diastole and it is filled with blood.
Ventricular systole (0.33 s, Q, R, S, T waves on the ECG) has two phases - contraction and ejection of blood.
The voltage period (0.08 s) has two phases - asynchronous contraction and isometric contraction.
Phase of asynchronous contraction (despite the rapid conduction of AP along the bundle of His and Purkinje fibers, in the first 0.05 s some cardiomyocytes are in the contraction phase, others are in the relaxation phase):