Human Body

Three systems that only make sense once you stop trusting the obvious answer. It opens on digestion, where the useful correction is that your gut is a tube running through you — so food is not inside your body until it crosses the wall, which happens further down than almost anyone guesses and not in the stomach at all. The second part follows what happens to that fuel once it is in the blood, and why blood passes through the heart twice per lap: lungs need a barrier too thin to survive the pressure a whole body requires, so the design refuses to compromise and re-pressurises in between. The third part asks what makes you breathe harder when you run, where the popular answer is wrong in a way that has killed strong swimmers. Each part hands its output to the next.