Watch Inside the Human Body online: Episode 4 Hostile World

In the final episode of Inside The Human Body, Michael Mosley reveals the ingenious ways in which your body defends itself against a hostile world - where sunlight shatters your DNA and every breath contains microbes that can kill. We meet Cristian, a bull jumper whose muscles give him the strength to avoid a violent death, three-year-old Rowan whose internal army fights off the flu virus, and Johnny who gets injured almost every week to earn his living, relying upon his body's remarkable ability to heal And when injuries are too severe to be repaired, we see how surgeons use medical expertise to exploit the body's natural powers of healing. Richard Edwards is filmed having his damaged hands cut off and replaced with someone else's - the first time something this radical has ever been attempted.

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