Function and form in the developing cardiovascular system.

Function of the developing heart is dictated by changes in its morphology. For simplicity, we distinguish four stages with different contraction mechanics and conduction parameters. Straight or looped tubular hearts, similar to those of invertebrates such as Drosophila or Ciona, operate as suction pumps and are characterized by a caudally localized pacemaker and slow, peristaltoid conduction and contraction. There is a complete occlusion of the lumen during systole. When the atrial and ventricular chambers appear, the preseptation heart is in many functional aspects similar to the adult heart, but the same function is achieved by different means. There are parallels in design among the hearts of lower vertebrates, such as a spongy ventricle without coronary vasculature and a myocardial atrioventricular canal. Even after septation, considerable maturation of cardiac morphology and function occurs during the foetal and early postnatal period.