The function of the heart is to force blood from the veins into the arteries and ensure its constant movement. This is achieved by the rhythmic contractions of the heart. Contraction of the heart (systole) alternates with its relaxation (diastole). The heart shrinks automatically. In his conducting system, an excitation (impulse) arises, which spreads through the myocardium and causes its contraction. Normally, these impulses occur in the sinus node at almost equal intervals of 60-80 times in 1 minute, from where they spread throughout the atrial myocardium and cause their contraction. Atrial systole lasts 0.1 sec. Next, the impulse passes through the node of Ashoff - Tavara, reaches the bundle of His and extends along his legs to the ventricular myocardium, causing their contraction.
When the impulse passes to the ventricles, the rate of its spread decreases, therefore the atrial systole has time to finish before the excitation of the ventricles ends and their contraction begins. Ventricular systole lasts about 0.3 sec. and is replaced by their diastole, the duration of which depends on the heart rate (about 0.5 sec on average). The automatism function is characteristic of almost the entire conducting system; therefore, in certain diseases (see Arrhythmias of the heart ), impulses to contraction may occur in the His bundle and Purkinje fibers.Go
The sequence of the heart is as follows. During ventricular systole, blood pressure rises in their cavities, resulting in atrial ventricular valves slamming and the valves of the aorta and pulmonary trunk open; ventricles expel blood contained in them into the aorta and pulmonary trunk. With the onset of ventricular diastole, the blood pressure in their cavities begins to fall and, when it becomes less than in the aorta and pulmonary trunk, the semilunar valves of the aorta and pulmonary trunk close, preventing blood flow from the aorta and pulmonary trunk back to the ventricles. At the same time, atrioventricular valves open, and blood from the atria, the diastole of which begins shortly before the diastole of the ventricles, and then from the venous system through the relaxed atria enters the ventricles. In about 0.15 seconds. until the next ventricular systole, atrial systole begins, as a result of which blood in the atria is injected into the ventricles; then the heart cycle starts over.
Since the pressure in the aorta is 4-5 times higher than in the pulmonary trunk, the left ventricle during systole has to develop much more stress than the right. Intracardiac pressure in the cavities of the heart is also unequal (for example, in the left ventricle during systole it reaches 110–130 mm Hg., In the right ventricle - 25–30 mm Hg.). In diseases of the heart, the pressure in its cavities may increase. In these cases, the myocardium of that part of the heart, where the pressure is increased, must be reduced with greater force, which leads to a compensatory increase in the muscle mass of this part of the heart (compensatory myocardial hypertrophy). During systole, about 70 ml of blood is expelled from the ventricles of the heart (stroke, or systolic, volume). For 1 minute each ventricle ejects from 3 to 5 liters of blood (minute volume). With intensive work, the minute volume can exceed 25 liters. Although the heart itself produces impulses for contraction, its activity is controlled by the nervous system. The increased influence on the heart of the vagus nerve slows down the production of impulses in the sinus node, inhibits their conduction, reduces the number of heart contractions. The increased influence on the heart of the sympathetic nervous system increases the excitability of the myocardium, accelerates the production of impulses in the sinus node and their conduction, increases the number of contractions of the heart.