You are an organism characterized by functions such as respiration and metabolism.
You can lose a limb and still be you, but if you stop breathing and digesting you are no longer the same living organism.
You are the tangle of neurons nestled in the grey matter found in your skull. You are your brain!
After all, you can lose a limb and still be you, but you cannot be the same person if your brain gets irreparably damaged or altogether destroyed.
You are your memories and your character traits; more generally, your psychology.
After all, if you completely lose your memories or your character traits change radically, people say that you have become a different person.