Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Cardiovascular System
Heart
Cardiac Activity

The Heart Muscle contracts rhythmically, unlike Skeletal Muscle. The ability to contract rhythmically under METABOLISM/18.html">The Influence of impulses originating within the heart itself is a hallmark of the myocardium. This property is known as cardiac automatism.

Cardiac automatism is associated with the function of atypical muscle Cells within the conduction system. The impulses responsible for rhythmic heart contractions initially arise in the sinoatrial node.

Excitation occurs in the cardiac conduction system due to changes in electrical potentials On the surface of Cell membranes. The resulting excitation rapidly spreads from the sinoatrial node to the atrial myocardial cells and into the atrioventricular node, from which impulses travel to the ventricular myocardium. The conduction of impulses through the atrioventricular node is delayed, meaning excitation reaches the ventricular myocardium slower than that of the atria. Consequently, the atria contract first, followed by the ventricles. During ventricular contraction (systole), the heart muscle is unable to respond to new stimuli. This period of myocardial inexcitability is called the refractory phase. This property of the heart muscle—not responding with additional contractions during systole—allows the myocardium to contract more fully and rhythmically without fatigue.

The cardiac cycle consists of the contraction of the atria and ventricles followed by their relaxation. In a healthy resting person, the heart beats 60–70 times per minute. Contraction of the heart muscle is called systole, and its relaxation is called diastole. The cardiac cycle comprises three phases: atrial systole, ventricular systole, and the general pause. The total duration of a human cardiac cycle is approximately 0.8 s.

Each cycle begins with atrial systole, lasting 0.1 s. At this moment, the ventricular myocardium is relaxed, the atrioventricular Valves are open, and the semilunar valves are closed. During atrial contraction, all Blood from the atria enters the ventricles. Upon completion of atrial systole, ventricular systole begins, lasting 0.3 s. By the time the ventricles contract, the atria are already relaxed, and the bicuspid and tricuspid valves connecting the atria to the ventricles close. As the ventricular musculature contracts, blood is forced out into the aorta and pulmonary trunk. The semilunar valves at the Base of the aorta and pulmonary trunk open, their cusps pressed against the inner walls of these vessels, resulting in the ejection of blood from the ventricles.

Ventricular contraction is followed by relaxation, known as ventricular diastole. Driven by the high pressure built up in the aorta and pulmonary trunk, the semilunar valves of these vessels close, preventing blood from flowing back into the ventricles. This is followed by a resting period for all heart chambers, or general relaxation—the general pause. The overall cardiac pause lasts 0.4 s. This interval between contractions is sufficient for the heart to fully recover its working capacity.

During each ventricular contraction, a specific portion of blood is pushed into the vessels. Its volume, known as the stroke or systolic volume, is 70–80 ml. In one minute, the heart of a resting adult pumps 5–5.5 liters of blood into the Blood Vessels. During physical exertion, the volume of blood pumped by the heart per minute in a healthy person increases to 15–20 liters.

Heart sounds are acoustic phenomena produced during cardiac activity. They can be heard by placing an ear against the anterior chest wall. There are two main sounds: the first sound is systolic, and the second sound is diastolic. The systolic sound is lower-pitched and longer. It occurs at the beginning of ventricular systole and is associated with the contraction of the ventricular wall musculature, the vibration of taut chordae tendineae, and the oscillation of atrioventricular valve cusps at the moment of closure. The diastolic sound is short and high-pitched, occurring at the onset of diastole when the cusps of the aortic and pulmonary semilunar valves close.

Specific locations on the chest wall are known where heart sounds are heard most distinctly. The first sound, produced by the closure of the left atrioventricular (mitral) valve, is auscultated at the apex of the heart in the fifth intercostal space, slightly to the left of the Sternum (Fig. 87). The first sound, resulting from the closure of the right atrioventricular valve and the contraction of the right ventricular myocardium, is heard at the junction of the body of the sternum with its xiphoid process. The second sound of aortic valve closure is auscultated In the second intercostal space to the right of the sternum, while the closure of the pulmonary trunk valve is heard in the second intercostal space to the left of the sternum.

The apical impulse can be felt with the hand at the level of the fifth intercostal space to the left of the sternum. This impulse is produced by a shift in THE POSITION OF the heart during systole, when the left ventricle presses against the anterior chest wall and "strikes" it.

ELECTRICAL PHENOMENA IN the heart. Electrical processes occur in the heart when an excited region becomes electronegative relative to an unexcited one.

Since The Human Body acts as a liquid conductor, the bioelectric currents of the heart propagate in various directions and can be recorded using an instrument called an electrocardiograph. Electrocardiography is used to detect changes in heart rhythm, conduction disorders, as well as the Location and nature of myocardial damage.

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Fig. 87. Projection of heart orifices, atrioventricular and semilunar valves onto the anterior chest surface (diagram). Arrows indicate the Auscultation points for heart sounds:

1 — orifice of the pulmonary trunk; 2 — left atrioventricular valve; 3 — apex of the heart; 4 — right atrioventricular valve; 5 — aortic orifice

REVIEW AND SELF-Control Questions:

1. Describe the Structure of Blood vessel walls.

2. Name the vessels belonging to the microcirculatory bed.

3. Name the Chambers of the heart and the orifices through which these chambers communicate.

4. Describe the internal surface of the atria and ventricles.

5. Discuss the PROJECTION OF THE heart borders and valves onto the anterior chest wall.

6. What is the Pericardium? Describe its structure.

7. Characterize cardiac automatism. Where does it originate, and how is it implemented within the heart walls?

8. Describe the cardiac cycle, its onset, phases, and duration.

9. What are systole and diastole? What processes occur in the heart during systole and diastole?

10. Describe the heart sounds. What causes them, and at which locations on the anterior chest wall are they auscultated?