The circulatory system transports materials around the body using the heart and blood vessels. It has a double circulation where oxygenated blood from the lungs enters the left side of the heart and is pumped to the body, and deoxygenated blood returns to the right side of the heart to be pumped to the lungs. The heart is composed of four chambers - left and right atria and ventricles - with valves that prevent backflow of blood and ensure one-way circulation.