Development of the Heart

Core Concepts

Week 3: from mesoderm to a single heart tube

The heart arises from splanchnic mesoderm in the cardiogenic area at the embryo's head end, where blood islands and a pair of endocardial tubes appear; they fuse at the midline into a single primitive heart tube in the third week (~day 21–23, commonly taught). The tube differentiates into five regions in flow order: sinus venosus → primitive atrium → primitive ventricle → bulbus cordis → truncus arteriosus. The sinus venosus receives the returning veins; the truncus becomes the aorta and pulmonary trunk. It begins beating around day 22–28 (week 4), long before it is four-chambered.

Days 23–28: cardiac looping

The straight tube cannot fit in the chest, and its regions are in the wrong order. Cardiac looping solves both: the tube bends into a C-shape, then an S-shape, the bulbus cordis and ventricle swinging right and the atrium swinging up and behind. This places the right ventricle on the right, the left on the left, the atria above, and brings the inflow and outflow ends together at the base. Defects here are rare but show how fundamental this step is.

Weeks 4–7: septation — four chambers from one tube

The single-chambered tube is divided by walls (septa) that grow inward and fuse:

• Atrioventricular canal: the endocardial cushions divide it into right and left AV openings guarded by the tricuspid and mitral valves.
• Interatrial septum: the septum primum grows downward; before it fuses, the septum secundum grows to its right, together leaving a valved opening, the foramen ovale, that lets blood pass from right atrium to left atrium during fetal life.
• Interventricular septum: a muscular wall grows upward from the ventricle's floor, finished by connective tissue.
• Conotruncal septation: ridges in the bulbus cordis and truncus grow, fuse, and twist in a spiral, dividing the outflow tract into the aorta and pulmonary trunk.

By the end of the embryonic period (~week 8), the four-chambered heart is essentially formed.

Fetal circulation: three shunts around the lungs

In the fetus, the lungs do not exchange gases — the placenta does — so three shunts bypass the lungs:

• Foramen ovale — the opening between the atria. Fetal pulmonary resistance is high, so right atrial pressure exceeds left atrial pressure and most blood crosses to the left atrium.
• Ductus arteriosus — a vessel from the pulmonary trunk to the aorta, shunting blood past the lungs into the descending aorta.
• Ductus venosus — a vessel from the umbilical vein to the inferior vena cava, bypassing the liver so oxygenated blood reaches the heart quickly.

Changes at birth: closing the shunts

Birth reverses the pressure relationships: the first breaths expand the lungs, pulmonary resistance falls, and left atrial pressure rises above right atrial pressure. The foramen ovale is pushed shut and later fuses, leaving the adult remnant called the fossa ovalis. The ductus arteriosus constricts into the ligamentum arteriosum; the ductus venosus becomes the ligamentum venosum; the umbilical vessels become ligaments. If the ductus arteriosus fails to close, the result is patent ductus arteriosus (PDA) — a commonly taught congenital condition.

Common congenital variations (educational overview)

Congenital heart defects are among the most commonly taught birth-defect categories, mostly incomplete septation or abnormal connections. Ventricular septal defect (VSD) is commonly taught as the most frequent; atrial septal defect (ASD) is an opening between the atria; patent ductus arteriosus (PDA) is failure of the ductus to close; and tetralogy of Fallot is classically taught as four features — pulmonary stenosis, VSD, overriding aorta, right ventricular hypertrophy. Whether a defect causes cyanosis depends on whether deoxygenated blood reaches the systemic circuit.

Common Confusions