The circulatory, or cardiovascular, system is a complex body system that transports oxygen, nutrition, and waste throughout the body. The circulatory system networks the heart, blood vessels, arteries, and lungs in order to move blood and lymphatic fluid within the body.
The heart pumps oxygen-rich blood to the body through the arteries. Veins send blood back to the heart. The circulatory system develops very early in the human embryo, with the heartbeat starting at 4–5 weeks of gestation.