Contraction mechanism and atrial systole
Myocardial contraction occurs due to the interaction of actin and myosin proteins involving calcium ions. Unlike skeletal muscle, cardiomyocytes receive $Ca^{2+}$ not only from the sarcoplasmic reticulum but also from the extracellular environment. This phenomenon is called the «calcium burst».
The cycle begins with atrial systole, which lasts 110 ms on the right and 100 ms on the left. At this moment, blood is ejected into the ventricles. The auricles play an important role: they provide the necessary atrial stretching and act as an auxiliary pump. The right and left halves of the heart contract almost simultaneously, with minimal asymmetry due to the specifics of excitation propagation.
Tension period and blood ejection
Ventricular systole begins with the tension period, which consists of two stages:
- Asynchronous contraction.
- Isometric contraction: cardiomyocytes develop tension but do not shorten. The volume of the chambers remains unchanged because blood is incompressible and all valves are closed.
As soon as the pressure in the right ventricle reaches 15 mm Hg and in the left ventricle 80 mm Hg, the semilunar valves open, and blood rushes into the vessels.
At this time, the atrioventricular (mitral and tricuspid) valves are shut. To prevent the cusps from everting back into the atria (prolapse), they are anchored by chordae tendineae attached to contracting papillary muscles. These cords also act as shock absorbers, dampening the hydraulic shock.
Relaxation and diastole
When the ventricles begin to relax, the pressure within them drops below that in the aorta and pulmonary trunk. Blood rushes back, fills the pockets of the semilunar valves, and snaps them shut. The aortic wall contains elastic and collagen fibers: they cushion the impact of the blood and return the valves to their initial position (this function is taken into account when designing artificial valves).
Next comes the isometric relaxation phase (60 ms for the right ventricle and 80 ms for the left ventricle). The length of the muscle fibers is constant, the valves are closed, and the pressure drops to 5–10 mm Hg.
Ventricular filling
Blood enters the heart from the veins only during diastole. Ventricular filling is divided into several phases:
- Rapid filling (80 ms). The atrioventricular valves open. Blood rapidly fills the chambers due to the pressure gradient and sharp relaxation of the walls. The filling of the coronary arteries creates the so-called «hydraulic framework», which physically helps the ventricles expand. Pressure drops almost to zero.
- Slow filling (160–170 ms). The blood flow velocity decreases due to a diminishing pressure gradient and the elastic resistance of the ventricular walls.
Filling is completed by the next atrial systole.
Heart function and hemodynamics
The physical work of the heart is equal to the product of the change in pressure and the change in blood volume ($ \Delta P \times \Delta V $). On a pressure-volume loop, this work is represented by the area of the loop.
- Increased venous return (Starling's law): leads to an increase in stroke volume and cardiac work, while arterial pressure does not rise.
- Increased vascular resistance (afterload): forces the heart to perform more work and increases arterial pressure, while stroke volume remains unchanged.
Briefly: answers to topical questions
What are the phases of the ventricular blood ejection period?
The ventricular blood ejection period consists of two consecutive phases.
- Rapid blood ejection — lasts 120 ms, accompanied by the opening of the aortic valve and maximum output.
- Slow blood ejection — lasts 130 ms, characterized by a decrease in flow velocity before valve closure.
During both phases, ventricular pressure exceeds the pressure in the pulmonary trunk and aorta.
What heart sounds (acoustic phenomena) occur during different phases of the cardiac cycle?
During the cardiac cycle, the first and second heart sounds normally occur, and the 3rd and 4th sounds are also registered on a phonocardiogram.
- 1st sound — systolic, appears at the beginning of ventricular systole (isometric contraction period) when the atrioventricular valves snap shut.
- 2nd sound — diastolic, occurs at the beginning of diastole due to the closure of the aortic and pulmonary trunk valves.
How does atrial pressure change during the cardiac cycle?
Atrial pressure changes during the cardiac cycle: low-amplitude oscillations are noted on the left atrial pressure curve. When atrial pressure rises, blood flows into the ventricles. After the tricuspid valve opens, pressure in the atria and veins drops as blood rushes from the right atrium into the right ventricle. During right atrial diastole, pressure in it and in the venae cavae decreases, facilitating blood inflow from inferior veins.
The sequence of blood flow in diastole is easy to remember: Veins → Atria → Ventricles. During systole, blood entry into the heart is completely blocked.
Class and exam questions
Why does ventricular volume not change during isometric contraction?
During this phase, the atrioventricular and semilunar valves are tightly closed, and blood is an incompressible fluid. Therefore, the muscle fibers tense up but cannot change the total length and volume of the chamber.
What is the "hydraulic framework of the heart"?
This is a physical effect that occurs at the onset of myocardial relaxation when the coronary vessels fill with blood. Filled vessels impart rigidity to the ventricular walls, helping them expand and draw in blood.
What ensures the durability of heart valves?
Durability is achieved through shock absorption: the atrioventricular valves are secured by the tension of elastic chordae tendineae, and the semilunar valves by a complex of collagen and elastic fibers in the vessel walls.
What else to study on this topic
- Excitation-contraction coupling (the role of the T-system)
- Graphs of pressure changes in the heart chambers
- Mechanism of heart sound generation on the PCG
- Frank-Starling law and regulation of stroke volume
- Hemodynamic physics: calculation of cardiac work
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