Presented below is a photomicrograph of bovine pulmonary artery cells. This digital image was captured with the DXM 1200 ACT-1 control software in single-image acquisition mode utilizing fluorescence illumination.

In mammalian physiology, arteries are the vessels that carry oxygenated blood from the heart to the tissues of the body. The exception to this is the pulmonary artery, which carries oxygen-depleted blood from the heart to the lungs where carbon dioxide is removed and replaced with oxygen.

The pulmonary artery emerges above the right chamber of the heart, separating into two branches that enter the right and left lungs. In the lungs, the arteries further subdivide into smaller and smaller branches until they reach the capillaries in the pulmonary air sacs (alveoli). In the capillaries, blood takes up oxygen from the air breathed into the air sacs and releases carbon dioxide. It then flows into larger and larger vessels until it reaches the pulmonary veins. These veins open into the left atrium of the heart, which then pumps the freshly oxygenated blood to the rest of the body.

Pulmonary circulation forms a closed-circuit between the heart and the lungs, as distinguished from the systemic circulation between the heart and all other body tissues. Pulmonary circulation first occurs in lungfishes and amphibians, the first animals to acquire a three-chambered heart. It becomes a separate circulatory pattern in crocodilians, birds, and mammals, when the ventricle is divided into two chambers, producing a four-chambered heart.