Embryonic Development of the Heart

The heart begins as a simple tube formed by the fusion of two endocardial tubes during lateral folding of the embryo. This primitive heart tube elongates and undergoes looping to establish the basic structure of the heart. By the fourth week, the heart tube differentiates into distinct regions:

• Sinus venosus: Inflow region

• Primitive atrium: Future atria

• Primitive ventricle: Future ventricles

• Bulbus cordis: Outflow tract

• Truncus arteriosus: Future great arteries

The septation process divides the heart into four chambers, while the development of valves ensures unidirectional blood flow.

Fetal Circulation: A Unique System

Fetal circulation is distinct from postnatal circulation due to the presence of specialized structures that bypass the lungs, which are non-functional in the fetus. Oxygenation occurs in the placenta, and the oxygen-rich blood is transported to the fetus via the umbilical vein.

Key components of fetal circulation include:

1. Umbilical Vein: Carries oxygenated blood from the placenta to the fetus.

2. Ductus Venosus: A shunt that allows blood to bypass the liver and flow directly into the inferior vena cava.

3. Foramen Ovale: An opening between the right and left atria that enables oxygenated blood to bypass the lungs.

4. Ductus Arteriosus: Connects the pulmonary artery to the descending aorta, diverting blood away from the lungs.

Pathway of Blood Flow

1. Oxygen-rich blood enters the fetus through the umbilical vein.

2. Blood bypasses the liver via the ductus venosus and enters the inferior vena cava.

3. From the right atrium, most blood flows through the foramen ovale into the left atrium, then to the left ventricle, and out to the systemic circulation via the aorta.

4. Deoxygenated blood returns to the placenta through the umbilical arteries for reoxygenation.

Changes at Birth

At birth, the fetal circulation undergoes dramatic changes as the newborn begins to breathe:

• The foramen ovale closes, forming the fossa ovalis.

• The ductus arteriosus constricts and eventually becomes the ligamentum arteriosum.

• The ductus venosus closes and becomes the ligamentum venosum.

• Umbilical vessels degenerate, leaving remnants such as the ligamentum teres in the liver.

These changes establish the adult pattern of circulation, where the lungs take over the role of oxygenation.

Clinical Relevance

Understanding fetal circulation is crucial for recognizing congenital heart defects, such as:

• Patent ductus arteriosus (PDA)

• Atrial septal defect (ASD)

• Ventricular septal defect (VSD)

Congenital heart defects (CHDs) are structural abnormalities of the heart present at birth. Here are some common types:

1. Atrial Septal Defect (ASD): A hole in the wall separating the two atria, allowing blood to mix between the right and left atrium.

2. Ventricular Septal Defect (VSD): A hole in the wall between the ventricles, leading to mixing of oxygenated and deoxygenated blood.

3. Patent Ductus Arteriosus (PDA): Persistence of the ductus arteriosus after birth, causing abnormal blood flow between the aorta and pulmonary artery.

4. Tetralogy of Fallot: A combination of four defects: VSD, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta.

5. Coarctation of the Aorta: Narrowing of the aorta, which obstructs blood flow and increases workload on the heart.

6. Transposition of the Great Arteries (TGA): The positions of the pulmonary artery and aorta are reversed, leading to inadequate oxygenation.

7. Hypoplastic Left Heart Syndrome (HLHS): Underdevelopment of the left side of the heart, affecting its ability to pump blood