Here's a fun science experiment you might have done in grade school. Get a bucket full of ice water that's big enough to fit your head. Now dunk your face in and see how long you can stay. Most people can't remain under for longer than a minute—and for the average human, it's pretty dangerous to try. But during that minute, your body goes through some pretty nifty changes. To conserve oxygen, your heart rate slows (what's called bradycardia), the veins in your extremities constrict, enabling you to distribute the oxygenated blood to your vital organs like your heart and lungs, and your spleen, which stores oxygenated red blood cells, contracts, releasing those cells into your bloodstream.