Heart development in embryogenesis
The development of the heart muscle begins in the third week of intrauterine development. The initial material is the mesenchyme. The process goes through several key stages:
- Tube formation: Initially, two paired endocardial tubes are laid down in the region of the head gut. Soon they fuse into a single structure with a double-layered wall.
- Layer differentiation: The inner layer of this tube gives rise to the endocardium. The outer layer, called the myoepicardial plate (epimyocardium), differentiates into the myocardium and epicardium.
- Morphogenesis and chamber separation: The heart tube actively elongates, acquiring a characteristic S-shape. Septa begin to grow inside it. The common atrium is established, which is then divided into right and left. A similar process occurs with the common ventricle. Atrioventricular valves and cusps are formed from endocardial thickenings.
- Topography: Initially, the heart is located in the cervical region (ventrally in the ventral mesogastrium), but as development proceeds, it gradually descends into the thoracic cavity, becoming covered by the serous membrane.
Fetal placental circulation
Before birth, the lesser (pulmonary) circulation does not participate in gas exchange because the lungs are not functioning and vascular resistance in them is high. Gas exchange is carried out entirely in the placenta (placenta).
- Arterial blood supply: Oxygen-enriched blood from the placenta travels via the single umbilical vein (v. umbilicalis), which passes as part of the umbilical cord through the umbilical ring to the liver.
- Distribution in the liver: Part of the blood passes through the liver parenchyma, joining the portal system. However, the bulk is shunted via the ductus venosus (ductus venosus) directly into the inferior vena cava, where it mixes with venous blood from the lower part of the body.
- Intracardiac shunts: In the right atrium, mixed blood from the inferior vena cava is directed by a special valve through the foramen ovale (foramen ovale cordis) directly into the left atrium. This allows the most oxygen-rich blood to enter the left ventricle, and then into the ascending aorta and aortic arch to supply the vessels of the heart, head, neck, and upper extremities.
- Venous blood shunt: Purely venous blood from the superior vena cava enters the right ventricle, and from there into the pulmonary trunk (truncus pulmonalis). Since the lungs do not function, this blood is shunted via the ductus arteriosus (ductus arteriosus) into the descending aorta, supplying mixed blood to the lower half of the fetus's body.
- Return to the placenta: Blood returns to the placenta for oxygenation via two umbilical arteries (aa. umbilicales dextra et sinistra), branching from the internal iliac arteries.
Age-related features and growth dynamics
The topography and weight of the heart undergo significant changes as the child grows older.
- Position in the chest cage: In newborns, the heart is positioned transversely. This is due to the high standing of the diaphragm caused by the large size of the liver, as well as the fact that the heart is displaced posteriorly by the enlarged thymus. The oblique position characteristic of adults is formed only by the end of the first year of life.
- Apex projection: In a newborn, it is located at the level of the 4th intercostal space, by five years of age it descends to the 5th intercostal space, and by 10 years it corresponds to the adult level.
- Growth dynamics of heart compartments: In the first months of life, the atria grow faster than the ventricles. In the second year, their growth equalizes. After 10 years, the ventricles begin to develop more intensively, especially the left one.
- Heart weight: From 17–20 g at birth, the weight increases to 52–55 g by 1–2 years, reaches 95–111 g by 10 years, and 190–193 g by 16 years. In boys, heart weight is usually greater, except for the puberty period (12–13 years), when girls temporarily surpass boys in this indicator.
Lymphatic system of the neck and eye drainage
General angiology also covers lymphatic drainage pathways and fluid circulation in specialized organs.
Eye drainage: The outflow of aqueous humor occurs from the anterior and posterior chambers of the eyeball, as well as the zonular spaces (spatia zonularia). Fluid passes through the spaces of the iridocorneal angle and enters the venous sinus of the sclera (sinus venosus sclerae), from where it drains into the veins of the eye.
Lymphatic vessels of the neck (vasa lymphatica colli):
- Lateral group: Superficial vessels carry lymph toward the external jugular vein and superficial cervical nodes. Deep vessels drain lymph from internal organs (pharynx, larynx, thyroid gland, trachea, esophagus). They head toward the deep nodes (including the jugulodigastric node), whose efferent vessels form the right and left jugular trunks (trunci jugulares dexter et sinister).
- Anterior group: Drains lymph from the isthmus of the thyroid gland and larynx, terminating in the paratracheal and prelaryngeal nodes.
Briefly: answers to topical questions
Which heart structures are formed from endocardial cushions (endocardial thickenings) during embryogenesis?
During embryogenesis, atrioventricular valves and heart cusps are formed from endocardial thickenings. These structures develop at the morphogenesis stage, when septa appear inside the heart tube to divide it into four chambers.
- Atrioventricular valves — each cusp is a fold (duplication) of the endocardium into which the connective tissue of the epicardium subsequently grows.
What do the umbilical vein, ductus venosus, and ductus arteriosus of the fetus obliterate into after birth?
After birth, these vessels undergo obliteration and transform into ligaments:
- Umbilical vein (v. umbilicalis) — into the round ligament of the liver (lig. teres hepatis).
- Ductus venosus (ductus venosus) — into the ligamentum venosum (lig. venosum).
- Ductus arteriosus (ductus arteriosus) — into the ligamentum arteriosum (lig. arteriosum).
Where do the right and left jugular lymphatic trunks empty?
The right jugular lymphatic trunk empties into the right venous angle, whereas the exact termination site of the left trunk is not described in sources.
- Right jugular trunk (truncus jugularis dexter) — merges with the right subclavian trunk, forming the right lymphatic duct (ductus lymphaticus dexter), which empties into the right venous angle.
- Left jugular trunk (truncus jugularis sinister) — is formed from the efferent vessels of the lower deep cervical nodes, however, its further pathway is not specified.
Which branches arise from the aortic arch to supply blood to the head, neck, and upper extremities?
Three large trunks arise from the convex surface of the aortic arch, providing blood supply to the head, neck, and upper extremities.
- Brachiocephalic trunk (truncus brachiocephalicus) — arises first (on the right) and divides into the right common carotid (a. carotis communis dextra) and right subclavian (a. subclavia dextra) arteries.
- Left common carotid artery (a. carotis communis sinistra) arises directly from the aortic arch.
- Left subclavian artery (a. subclavia sinistra) runs upward and to the left toward the cervical region.
Sometimes, the thyroid ima artery (a. thyroidea ima) may additionally arise from the brachiocephalic trunk.
Class and exam questions
What type of blood flows through the umbilical vein in the fetus?
Arterial (most oxygen-rich) blood flows through the umbilical vein from the placenta to the fetus.
What is the function of the ductus arteriosus (Botallo's duct)?
It connects the pulmonary trunk with the aortic arch, allowing venous blood from the right ventricle to be shunted into the systemic circulation, bypassing the non-functioning lungs.
Why does the newborn's heart have a transverse orientation?
The transverse position is due to the high standing of the diaphragm because of a very large liver and the pressure of the large thymus, which displaces the heart posteriorly.
Where does lymph drain from the lateral deep group of neck vessels?
It drains into the deep cervical lymph nodes (e.g., the jugulodigastric node), which ultimately form the right and left jugular trunks.
What else to study on this topic
- Formation of the arterial trunk and sinus venosus in embryogenesis.
- Mechanisms of obliteration of fetal shunts (foramen ovale, ductus venosus, and ductus arteriosus) after birth.
- Anatomic prerequisites for sex differences in heart mass during puberty.
- Topography of the paratracheal and prelaryngeal lymph nodes.
- Hemodynamic differences between the ascending and descending aorta in the fetus.
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