The cardiovascular system refers to the heart and blood vessels. The respiratory system refers to the trachea, bronchial tubes, bronchi, bronchioles and alveoli. The bronchioles and alveoli are in the lungs. When a person breathes, oxygen passes into the respiratory system through the nose, to the trachea and eventually to the alveoli, the tiny air sacs in the lungs. They are thin and moist which is a requirement for the diffusion of oxygen into tiny blood vessels called capillaries, which are associated with the alveoli. Once in the capillaries, tiny blood vessels which are part of the cardiovascular system, hemoglobin, a transport pigment in the red blood cells binds with the oxygen and transports it to the pulmonary veins. These veins return oxygenated blood to the left side of the heart. Eventually, the left ventricle pumps oxygenated blood through the aorta, the largest artery, to all parts of the body. Cells exchange oxygen for carbon dioxide, which enters the blood and returns through the vena cava back to the right side of the heart. Eventually, the right ventricle pumps carbon dioxide rich(deoxygenated)blood back to the lungs through the pulmonary arteries. Once in the lungs again, diffusion occurs and new oxygen is absorbed into the blood. These two systems are intimately connected and circulation between the heart and lungs is called pulmonary circulation.