<P> The pulmonary arteries carry deoxygenated blood to the lungs, where carbon dioxide is released and oxygen is picked up during respiration . Arteries are further divided into very fine capillaries which are extremely thin - walled . The pulmonary vein returns oxygenated blood to the left atrium of the heart . </P> <P> The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle . This blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle . From the left ventricle, the blood passes through the aortic valve to the aorta . The blood is then distributed to the body through the systemic circulation before returning again to the pulmonary circulation . </P> <P> From the right ventricle, blood is pumped through the semilunar pulmonary valve into the left and right main pulmonary arteries (one for each lung), which branch into smaller pulmonary arteries that spread throughout the lungs . </P> <P> The pulmonary circulation loop is virtually bypassed in fetal circulation . The fetal lungs are collapsed, and blood passes from the right atrium directly into the left atrium through the foramen ovale: an open conduit between the paired atria, or through the ductus arteriosus: a shunt between the pulmonary artery and the aorta . When the lungs expand at birth, the pulmonary pressure drops and blood is drawn from the right atrium into the right ventricle and through the pulmonary circuit . Over the course of several months, the foramen ovale closes, leaving a shallow depression known as the fossa ovalis . </P>

Where does blood go from the pulmonary artery