The circulatory system transports nutrients, water, oxygen, and waste throughout the body using blood, blood vessels, and the heart. Oxygen-rich blood is carried from the heart to body cells via arteries and returns to the heart via veins as oxygen-poor blood. The heart pumps blood in two separate circuits - the pulmonary circulation transports blood to the lungs to receive oxygen and the systemic circulation transports oxygenated blood to all body tissues before returning to the heart.