<Tr> <Td_colspan="2"> Development of the human heart during the first eight weeks (top), and the formation of the heart chambers (bottom). In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail / atria to the ventricles / head, a very different pattern from that of an adult . </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Heart development refers to the prenatal development of the human heart . This begins with the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube, that loops and septates into the four chambers and paired arterial trunks that form the adult heart . The heart is the first functional organ in vertebrate embryos, and in the human, beats spontaneously by week 4 of development . </P> <P> The tubular heart quickly differentiates into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus . The truncus arteriosus splits into the ascending aorta and pulmonary artery . The bulbus cordis forms part of the ventricles . The sinus venosus connects to the fetal circulation . </P>

When does an embryo's cardiovascular system become functional