In pulmonary circulation, deoxygenated blood returning from the organs and tissues of the body travels from the right atrium of the heart to the right ventricle. From there it is pushed through the pulmonary artery to the lung. In the lung, the pulmonary artery divides, forming the pulmonary capillary region of the lung.

 At this site, microscopic vessels pass adjacent to the alveoli, or air sacs of the lung, and gases are exchanged across a thin membrane: oxygen crosses the membrane into the blood while carbon dioxide leaves the blood through this same membrane. Newly oxygenated blood then flows into the pulmonary veins, where it is collected by the left atrium of the heart, a chamber that serves as collecting pool for the left ventricle. The contraction of the left ventricle sends blood into the aorta, completing the circulatory loop. On average, a single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.