Development and age-related characteristics of the heart

In human ontogenesis, the heart develops from the mesoderm as a paired rudiment at the stage of 1-3 somites (approximately on the 17th day of embryonic development). From this paired rudiment, a simple tubular heart is formed, which is located in the neck area. Anteriorly, it passes into the primitive bulb of the heart, and posteriorly - into the expanded venous sinus. The anterior (head) end of the simple tubular heart is arterial, and the posterior - venous. The middle section of the tubular heart intensively grows in length, therefore it bends in the form of an arc in the ventral direction (in the sagittal plane). The apex of this arc is the future apex of the heart. The lower (caudal) section of the arc is the venous section of the heart, the upper (cranial) is the arterial section.

Next, the simple tubular heart, which had the appearance of an arc, bends counterclockwise in an S-shape and turns into a sigmoid heart. On the outer surface of the sigmoid heart, the atrioventricular groove (the future coronary groove) appears.

The common atrium grows rapidly, embraces the arterial trunk from behind, on the sides of which two protrusions are visible (from the front) - the rudiments of the right and left auricles. The atrium communicates with the ventricle by a narrow atrioventricular canal. In the walls of the canal, ventral and dorsal thickenings arise - atrioventricular endocardial ridges, from which valves then develop on the border of the heart chambers - bicuspid and tricuspid.

At the mouth of the arterial trunk, four endocardial ridges are formed, which later turn into the semilunar valves (valves) of the beginning of the aorta and pulmonary trunk.

In the 4th week, the primary (interatrial) septum appears on the inner surface of the common atrium. It grows toward the atrioventricular canal and divides the common atrium into right and left. From the side of the upper posterior wall of the atrium, the secondary (interatrial) septum grows, which fuses with the primary and completely separates the right atrium from the left.

At the beginning of the 8th week of development, a fold appears in the posteroinferior part of the ventricle. It grows forward and upward toward the endocardial ridges of the atrioventricular canal, forming the interventricular septum, completely separating the right ventricle from the left. At the same time, two longitudinal folds appear in the arterial trunk, growing in the sagittal plane toward each other, and also downward - toward the interventricular septum. Connecting with each other, these folds form a septum that separates the ascending part of the aorta from the pulmonary trunk.

After the interventricular and aortopulmonary septa have formed, a four-chambered heart is formed in the human embryo. The small oval opening (the former interatrial opening), through which the right atrium communicates with the left, closes only after birth, i.e. when the small (pulmonary) circulation begins to function. The venous sinus of the heart narrows, turning together with the reduced left common cardinal vein into the coronary sinus of the heart, which flows into the right atrium.

Due to the complexity of heart development, congenital malformations occur. The most common are incomplete closure (defect) of the interatrial (less often interventricular) septum; incomplete division of the arterial trunk into the ascending aorta and pulmonary trunk, and sometimes narrowing or complete closure (atresia) of the pulmonary trunk; non-closure of the arterial (Botallo's) duct between the aorta and pulmonary trunk. In one and the same individual, 3 or even 4 defects are sometimes observed simultaneously in a certain combination (the so-called triad or tetrad of Fallot). For example, narrowing (stenosis) of the pulmonary trunk, formation of the right aortic arch instead of the left (dextroposition of the aorta), incomplete infection of the interventricular septum and significant enlargement (hypertrophy) of the right ventricle. Malformations of the bicuspid, tricuspid and semilunar valves are also possible due to abnormal development of the endocardial ridges. The causes of heart defects (as well as other organs) are considered to be primarily harmful factors affecting the parents' body and especially the mother's body in the early stages of pregnancy (alcohol, nicotine, drugs, some infectious diseases).

The heart of a newborn is round. Its transverse size is 2.7-3.9 cm, the length is on average 3.0-3.5 cm. The atria are large compared to the ventricles, the right one is significantly larger than the left. The heart grows especially quickly during the first year of a child's life, and its length increases more than its width. Individual parts of the heart change differently at different age periods. In the first year of life, the atria grow faster than the ventricles. From 2 to 5 years of age and especially at 6 years of age, the growth of the atria and ventricles occurs equally intensively. After 10 years, the ventricles increase faster. The total mass of the heart in a newborn is 24 g. At the end of the first year of life, it increases approximately 2 times, by 4-5 years - 3 times, by 9-10 years - 5 times and by 15-16 years - 10 times. The mass of the heart up to 5-6 years is greater in boys than in girls; at 9-13 years, on the contrary, it is greater in girls. At 15 years of age, the heart mass is again greater in boys than in girls.

The volume of the heart increases by 3-3.5 times from the neonatal period to the age of 16, and increases most intensively from 1 year to 5 years and during puberty.

The myocardium of the left ventricle grows faster than the myocardium of the right ventricle. By the end of the second year of life, its mass is twice that of the right ventricle. At 16 years of age, these ratios are maintained. In children of the first year of life, fleshy trabeculae cover almost the entire inner surface of both ventricles. Trabeculae are most strongly developed in adolescence (17-20 years). After 60-75 years, the trabecular network is smoothed out, its mesh-like character is preserved only in the region of the apex of the heart.

In newborns and children of all age groups, the atrioventricular valves are elastic, the cusps are shiny. At 20-25 years of age, the cusps of these valves become thicker, their edges become uneven. In old age, partial atrophy of the papillary muscles occurs, due to which the function of the valves may suffer.

In newborns and infants, the heart is located high and lies transversely. The transition of the heart from a transverse position to an oblique position begins at the end of the first year of life. In 2-3 year olds, the oblique position of the heart predominates. The lower border of the heart in children under 1 year is located one intercostal space higher than in adults. The upper border is at the level of the second intercostal space, the apex of the heart is projected into the fourth left intercostal space (outward from the midclavicular line). The right border of the heart is most often located corresponding to the right edge of the sternum, 0.5-1.0 cm to the right of it. As the child ages, the relationship of the sternocostal (anterior) surface of the heart to the chest wall changes. In newborns, this surface of the heart is formed by the right atrium, the right ventricle and most of the left ventricle. The ventricles are mainly in contact with the anterior chest wall. In children over 2 years old, in addition, part of the right atrium is adjacent to the chest wall.

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