Cardiovascular system

Embryologic formation of the cardiovascular system The heart is the first functional organ of the embryo and forms between the second and eighth weeks of life. On day 15, the mesoderm differentiates from the ectoderm and gives rise to the straight heart tube, which will form the atria, ventricles, bulbus cordis, and truncus arteriosis. The primitive heart tube begins to beat by day 23 and gives rise to embryonic circulation by day 30. Various factors, including differential growth rates, constrictions at sites of future structures, genetic factors, cell differentiation, and influence from the surrounding pericardium and neural crest cells chaperone ongoing formation of the heart over the next several weeks. Over the next 4–6 weeks, the structure of the heart matures almost completely into that seen at the end of gestation. Cardiac looping on day 21 causes the primitive heart tube to fold on to itself, migrating the eventual cardiac chambers and great vessels into their relative positions. Subsequent development includes septation and heart chamber formation, formation of heart valves, formation of the pulmonary vascular system, formation of the conduction and coronary artery system, and septation and formation of the outflow tracts and great arteries. Abnormal cardiac development that leads to congenital heart disease likely occurs through a complex relationship of genetic and environmental factors in utero. Further discussion of embryology is beyond the scope of this chapter, but excellent reviews are available for the interested reader.