Heart cycle. Cardiac cycle phases

The heart muscle performs such important functions for the human body as the ability to be excited, carry out excitation and contract.

Excite all the cells of the myocardium. But the rhythm is determined by the organ contractions of the cells of the sinus node, since only they are able to be excited more times per minute than the rest. This happens usually 60-80 times in quiet conditions.

Electrochemical interaction of cells betweenit ensures the conductivity of excitation: it spreads throughout the myocardium. With the help of an electrocardiograph, this process can be fixed and analyze the work of the muscle.

And the ability of the heart to contract is realized,as is known, in the form of a cycle consisting of several phases. It is regulated by the nervous system. The heart cycle and its phases are diastole and systole of the heart.

The cardiac cycle begins from the moment of excitation insinus node. At this time, both the atrium and the ventricles are relaxed and filled with blood. When the signal arrives, the atria begin to decrease actively. As a result, the ventricles receive an additional portion of blood and are also excited. At the same time, the first wave, the phase of asynchronous contraction, also spreads. The pressure in the ventricles becomes stronger than the pressure in the atria, and the valvular valves are immediately closed. From this second, the cardiac cycle enters the phase of isometric contraction. During this period, the volume of the ventricles does not change, and the pressure in them continues to grow. When it becomes stronger than that which comes from the vessels of the heart, the semilunar valves open and the isometric contraction phase ends. After that, blood from the heart is pushed into the blood vessels. The isometric and asynchronous phases of the cardiac cycle constitute the systole of the heart.
Then the heart is in a relaxed state. The cardiac cycle enters the phase of isometric relaxation. By the closing of the semilunar valves and further relaxation of the organ, already with all the closed valves, the pressure in the vessels results. At this time, blood gradually accumulates in the atria. As soon as her pressure becomes higher than the pressure in the ventricles, the valvular valves will open and the ventricles will fill with blood. Then the next cardiac cycle begins in a circle. Both the time of filling the ventricles, and the contraction of the atria, and the phase of isometric relaxation all together constitute the diastole of the heart.

At the time of discharge of blood in portions into the vesselsa pressure not exceeding 120 mm Hg is created. Art. In large vessels it does not change. During the cessation of blood in them from the heart, it decreases, but not less than 80 mm Hg. Art. Within one minute, a full cycle of five to six liters of blood occurs in the body of an adult healthy person. If any organ needs a larger blood flow, the arteriolar lumen will first expand, a lot of capillaries will open and only the heart will increase productivity in the last place. It will be the higher, the larger the volume of blood will be ejected in one reduction (the volume per heartbeat) and the more often the organ will shrink. The amount of blood that the heart overtakes in one minute (minute volume) depends not only on the physical and emotional state, but also on the height and weight of the person. Therefore, in order to individually evaluate the activity of the heart, a special cardiac index is calculated.

Also, the heart can change the frequency of cuts and theirforce, speed with which excitation is carried out, under the influence of impulses, which sends the central nervous system. For example, the signals of the vagus nerve make the contractions weak, their frequency becomes less frequent, the excitation decreases and is much slower. But the impulses sent by sympathetic nerves act directly opposite.

The further path of blood expelled from the heart continues along the arteries.