Human: The World Within

-Because our blood has to be conveyed to every cell in the body, the enormity of the blood vessels in our body is really mind-boggling. -A lot of simpler life forms don't need a circulatory system because they're just one-cell beings and they can rely on the diffusion of molecules within their one cell to carry everything around that they need to. But we as humans are made up of many cells, and we needed to evolve a system that would allow us to very efficiently move oxygen throughout our body. -Over the course of a day, blood will travel about 12,000 miles through arteries and veins. It's a system defined by constant motion, something that's also reflected in our evolution as a species. -Humans are absolutely built to move. I mean, if you look at our ancestry and the way we used to move around, our musculoskeletal system is designed for us to be agile, to go long periods of time by moving, and our hearts can perfectly accommodate this by making sure we get the right amount of blood when we need it. -But here's the tricky thing -- not only does our circulatory system allow us to move, it only works properly if we are moving. For most of us, modern life doesn't involve hunting or crossing continents on foot. And what we're finding out is that if we stay still for too long, the whole system breaks down.