THE PULMONARY CIRCULATION
The blood reaching the alveoli is that of the pulmonary circulation . Blood that has circulated through the body, with most of the oxygen removed, enters the right ventricle via the superior and inferior vena cava. Contraction of the right ventricle pumps this blood into the pulmonary trunk. The pulmonary trunk divides into right and left branches, which take the blood to the right and left lungs, respectively. As in all organs, the arteries divide to form arterioles and capillaries. It is this network of capillaries that sur-round the alveoli and participate in gas exchange. The capillaries join and rejoin to form venules and veins. Ultimately, four pulmonary veins transport the oxygenated blood into the leftatrium, where they enter the left ventricle and then are pumped into the aorta and distributed to the rest of the body. Bronchial arteries—branches from the aorta—bring oxygenated blood to the bronchi and other lung tissue. Most blood from the bronchial ar-teries returns to the heart via the pulmonary veins. Some reach the heart through veins that ultimately reach the heart via the superior vena cava.
The blood vessels in the lungs, unlike those in other tissue, constrict when there is less oxygen in the surrounding region. This is advantageous be-cause it helps direct blood to better-ventilated parts of the lung.