when does the heart form in a fetus
Heart development At around 18 to 19 days after fertilisation, the heart begins to form. This early development is critical for subsequent embryonic and prenatal development. The heart is the first functional organ to develop and starts to beat and pump blood at around day 21 or 22.[1] The heart begins to develop near the head of the embryo in the cardiogenic area.[1] Following cell signalling, two strands or cords begin to form in the cardiogenic region[1] As these form, a lumen develops within them, at which point, they are referred to as endocardial tubes.[1] At the same time that the tubes are forming other major heart components are also being formed.[8] The two tubes migrate together and fuse to form a single primitive heart tube, the tubular heart which quickly forms five distinct regions.[1] From head to tail, these are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus.[1] Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus.[1] The truncus arteriosus will divide to form the aorta and pulmonary artery; the bulbus cordis will develop into the right ventricle; the primitive ventricle will form the left ventricle; the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venous will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus.[1]