The cardiorespiratory system generally refers to the interaction of the heart, blood vessels, and lungs as they work to take in oxygen for cellular use and remove waste products from the body. Arteries carry fresh oxygen to the body, while veins carry depleted blood back to the lungs to replenish the body's oxygen supply.
The cardiorespiratory system works to keep the human body both oxygenated and free of waste products. As oxygen enters the lungs from the air, it flows into the alveolar sacs, small air chambers within the lungs. From there, individual oxygen molecules continue to the bloodstream through small arteries within the alveolar sacs. This newly oxygenated blood moves into the left atrium and ventricle of the heart through the pulmonary veins. The blood circulates throughout the body via the aorta, eventually traveling through every organ and cell in the body.
Cells receive oxygen from the arterial blood, and the deoxygenated blood returns to the heart through the venous system, carrying carbon dioxide and other waste products. The deoxygenated blood enters the right atrium and ventricle of the heart via the vena cavae, where it returns to the alveoli in the lungs to exchange its carbon dioxide and waste products for fresh oxygen. This carbon dioxide then exits the body upon exhalation.