The heart - a giant pump, the most machine-like and yet the most human of organs. Every minute, it pumps about five litres of blood through a system of blood vessels nearly 100,000 kilometres long - that's about 7,500 litres of blood every day. But could a mechanical device do the job just as well? In this episode, Lyes Kadem tells us about what it means to engineer mechanical hearts, and about what might replace the pacemaker of the future.