Circulation begins early in fetal life.
It is estimated that a given portion of the blood completes its course of circulation in approximately 30 seconds. When the right auricle contracts, it forces the blood through an opening into the right ventricle. C ontraction of this ventricle drives the blood to the lungs. Blood is prevented from returning into the auricle by the tricuspid valve, which completely closes during contraction of the ventricle. W hen this chamber contracts, blood is forced into the left ventricle and thence by ventricular contraction into the aorta.