Cardiovascular System
The heart and blood vessels, which transport oxygen, nutrients, hormones, and wastes; blood flows right atrium, right ventricle, lungs, left atrium, left ventricle, body.
The pulmonary artery is the famous exception to ‘arteries carry oxygenated blood’ — it carries deoxygenated blood to the lungs, while the pulmonary veins return oxygenated blood to the left atrium. Valves between chambers prevent backflow, and exchange with tissues happens across capillary walls.