The terminal =pulmonary veins= (Vv. pulmonales), usually seven or eight
in number, return the aërated blood from the lungs and open into the
left atrium of the heart. They are destitute of valves. Their
tributaries arise in the capillary plexuses in the lobules of the lungs,
and unite to form larger and larger trunks which accompany the branches
of the bronchi and pulmonary arteries. A very large vein is formed by
the union at an acute angle of a trunk from each lung, where the latter
are adherent to each other.