Mammalian heart development is a remarkable example of embryonic precision, beginning around the third week of gestation. It starts with mesodermal cells forming two endocardial heart tubes, which fuse into a single primitive heart tube. This early tube then elongates and undergoes rightward looping by the fourth week, creating a bulboventricular loop that establishes the spatial arrangement of the future chambers, including the bulbus cordis, ventricle, and atrium.

Septation soon follows, dividing the heart into left and right chambers. In the atria, the septum primum grows downward, followed by the septum secundum, forming the foramen ovale, which allows blood to bypass the fetal lungs. In the ventricles, the interventricular septum rises upward to separate left and right chambers. The truncus arteriosus, the outflow tract, is partitioned into the aorta and pulmonary artery by the spiral aorticopulmonary septum. Endocardial cushions in the atrioventricular region give rise to the mitral and tricuspid valves, ensuring unidirectional blood flow.

By the eighth week, the heart has a fully formed four-chambered structure capable of supporting independent circulation, though the foramen ovale remains functional until birth. Any errors during these tightly regulated steps, such as incomplete septation or valve malformations, can lead to congenital heart defects like septal holes or transposition of the great arteries. The process highlights the complex orchestration of genetic, cellular, and morphogenetic signals essential for normal cardiac development.