The circulatory system pumps blood around the body, and the respiratory system facilitates the gas exchange that keeps the blood fresh. According to HowStuffWorks, the heart is connected to the lungs via the pulmonary arteries and veins.
Oxygen-poor blood enters the heart through the right atrium. From there, according to HowStuffWorks, it flows through the tricuspid valve into the right ventricle. When the heart contracts during the diastolic phase, this blood is pumped out through the pulmonary arteries that run toward the lungs. At the lungs, the blood is circulated through a series of progressively smaller arterioles until it flows through capillaries lining the lungs' alveolar sacs. It is here that gas exchange takes place as oxygen is taken up by the blood, and carbon dioxide is released into the waste air.
After oxygenation, the fresh blood is circulated back through the bronchial veins and into the pulmonary veins. These run from the lungs and drain into the heart's left atrium. During the systolic phase of the heartbeat, the mitral valve under the left atrium opens and permits blood to pass into the left ventricle. This chamber is heavily muscled, according to HowStuffWorks, and it has the power to pump the oxygen-rich blood out through the aorta and into the rest of the body.