What does the heart do?
It is the pump that drives the blood to circulate.
What is the reason that blood circulates?
To bring nourishment (oxygen) to every cell of the body. The blood gets the oxygen from the lungs and delivers it to the cells all over the body.
Happily the internet is full of very fine expositions on the workings of the heart. Listed below are my favorites. They are marvellously informative.
In all of them the heart is shown as it physically appears in the body. The blood-carrying vessels are seen to intertwine commensurate with a cross over between opposite sides of the heart. But the heart cross over paths are not inherent to the design of the system. They are an artifact of packaging – of the plumbing. For its pumping function the packaging is advantageous. In the motion diagram image the package is unraveled to unveil the elemental simplicity of the circulation’s design. It is just a single loop. Click on GO to see it. Then on heart beat.
That this transportation system exists inside the body is not intuitive knowledge. It had to be discovered. The circulation of the blood was apparently understood in China by 300 BC. It was described by the Islamic physician, Ibn Al-Nafis in 1242. But it wasn’t appreciated in the Western World until 1628 when William Harvey rediscovered and publicized it in England.
http://www.pbs.org/wgbh/nova/heart/heartmap.html
http://www.heartpoint.com/theheart.html
http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_pumping.html