Final answer:
Blood leaves the right side of the heart through the pulmonary artery, as part of the pulmonary circulation system.
Step-by-step explanation:
The blood leaves the right side of the heart on its way to the lungs through the pulmonary artery. Specifically, it travels from the right ventricle into the pulmonary trunk, which then bifurcates into the right and left pulmonary arteries. These arteries carry oxygen-depleted blood to the right and left lungs for gas exchange. The pulmonary trunk and its branches are part of the pulmonary circulation system, where blood is oxygenated in the lung alveoli.