As a kid of the 1970s, Mary Roach sat in front of the TV watching “The Six Million Dollar Man” promise a techno-rebirth. “We can rebuild him,” the narrator intoned. “Better than he was before. Better. Stronger. Faster.”

The idea of an astronaut stitched together with bionics was pure fantasy — at least at the time. But as Roach points out in her new book, “ Replaceable You: Adventures in Human Anatomy ” (W.W. Norton), the line between sci-fi and the operating room is starting to blur.

We are living in an era of rapid “discoveries that feel at once wondrous, improbable, and surreal,” ushering us toward a future of lab-grown parts, Roach writes. 4

The heart is where things get the most visceral. In one University of Michigan lab, surgeons use “heart-in-a-box” machines that keep a d

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