How does the circulatory system work?

This system is made up of two main components: the heart and the vasculature.  The heart is an organ with two sides, each of which is dedicated to carry out a different job.  The right side receives deoxygenated blood from the body and pumps it to the lungs; the left side receives oxygnated blood from the lungs and pumps it to the body.  Vessels which carry blood away from the heart to the tissues are called arteries.  Vessels which exist in the tissues as a place of exchange are called capillaries.  Vessels which transport blood back to the heart from the body are called veins.

Each vessel type is adapted to carry out its function optimally.  For instance, arteries are thick and muscular in order to deal with the high blood pressure; capillaries are one-cell thick which maximises the exchange of substances between the blood and the tissues; veins are thin-walled and contain valves in order to prevent blood from flowing backwards.