When resting, the human heart beats about 60-85 times per minute, or 100 thousand times a day, pumping with almost 10 tons of blood! Throughout the y

Cardiac cycle

When resting, the human heart beats about 60-85 times per minute, or 100 thousand times a day, pumping with almost 10 tons of blood! Throughout the year it contracts for 40 million times, pumping 300-400 thousand tons of blood!

All four chambers of the heart periodically contract. In addition to contractions, there are periods of rest. The sequence of contractions and relaxation of the heart is called the cardiac cycle. The blood flows simultaneously in relaxed atriums: in the right — the venous of vena cava, the left — enriched with oxygen arterial blood from the pulmonary veins. Atria gradually stretch by blood which flows in them. As the blood fills them, the pressure increases, and some blood flows into the ventricles. At the same time spontaneously main pacemaker excites in the right atrium, and the pulses transmit to both atria. They excite and at the same time contract (contraction of the heart called systole — from the Greek “contraction”, “compression”), pushing the rest of the blood into the ventricles.

Excitation of atrial rapidly transmits via conduction system to the ventricles and excites them.

The ventricles simultaneously reduce (systole occurs), blood pressure in them is growing, and this leads to the closing of the atrioventricular valves and the opening of the semilunar valves of the aorta and pulmonary artery. Under significant pressure and a high velocity, blood ejects from the heart and flows into these vessels and moves further along the pulmonary and systemic circulation laps. After contraction, the ventricular muscles need rest to resume stock of energy spent on the contraction. Moreover, right during heart relaxation — diastole (from the Greek “stretching”) — the coronary arteries fill with blood. Taken from http://worldofschool.org

Diagram 62. Cardiac cycle: 1 — blood filling the atria; 2 — atrial contraction (the ventricles are relaxed); 3 — ventricular contractions; 4 — overall relaxation of the atria and ventricles

When the heart is at rest, particularly in early diastole, pressure in the ventricles is low, and in the aorta and pulmonary arteries — high. Because of this, the blood flows on the laws of physics, where the pressure is lower, that is, into the ventricles. At the same time it fills pockets of semilunar valves; they rise and close tightly, hermetically separating the ventricles of the vessels. At this time, the atrium fills with blood again and the cardiac cycle is restarted. Normally in resting, the length of the cardiac cycle is about 0.7-0.8 seconds. The contraction time of the ventricles is about 0.3-0.35 seconds, the relaxation time is 0.4-0.5 seconds.

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Questions:
  • Explain the mechanism of the cardiac cycle.

  • Give the definition of “cardiac cycle”.

  • Look precisely at Diagram 62 and determine the characteristics of the heart cycle.

  • Explain the meaning of the terms “systole” and “diastole”.

  • What provides continuous work of the heart?

  • What processes take place in the period of systole and diastole?

  • What will happen to the person if the atria and ventricles reduce at the same time?

  • What process leads to the closure of the atrioventricular valves?

  • Explain the function of the heart valves in the different phases of the cardiac cycle.