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. The bicuspid, or mistral, valve prevents the blood from flowing back into the auricle, and the seminar valves at the beginning of the aorta stop it from flowing back into the ventricle.
Similar valves are present in the pulmonary artery.
The aorta divides into a number of main branches, which in turn divide into smaller ones until the entire body is supplied by an elaborately branching series of blood vessels. The capillaries then unite to form small veins. The veins, in turn, unite with each other to form larger veins until the blood is finally collected into the superior and inferior venae cavae from which it goes to the heart, completing the circuit. In addition to the pulmonary and systemic circulations described above, a subsidiary to the venous system exists, known as portal circulation. A certain amount of blood from the intestine is collected into the portal vein and carried to the liver.