You may only think of blood as that yucky stuff you see when you cut yourself or have a nosebleed. But blood is really important for keeping us healthy, because it carries loads of things around our body, including the oxygen that we breathe.

Doctors really love blood, because doing tests on it can tell them so much about what's happening inside us. For example, testing a small sample of blood can tell them how much growth hormone you have in your body. Growth hormone is the stuff that makes our bones grow longer, so that we become taller.