When I graduated from architecture school, I knew almost nothing about science or the body. As an example, I though our digestive system simply separated food into solid or liquid and then pushed both down toward our no-no parts. I was amazed to learn about how food is broken down and either absorbed or excreted. Somewhere in this lesson, I picked up the tidbit that pee actually comes from your blood. Yeah… your blood. Grossly simplified, the nephrons in your kidneys filter blood, removing waste products and send them down to your bladder. In the microgravity of space, your bones don’t need to be as sturdy, so osteoclasts start acting on your bone matrix, leeching calcium and sending it into your bloodstream. The calcium is removed and excreted. So not only does pee come from your blood, but an astronaut can pee out his or her bones.
