Heart is really a mindset, it is when nothing else matters but winning. When your brain shuts off that alarm that says "you just got hurt, stop this activity". It is when the noise of physical pain and discomfort is drowned by your brain screaming WIN WIN WIN! As a fighter, it is also taking an almost orgasmic pleasure in hurting an opponent, in watching him colapse under your attack. Being hit by your opponent does not scare you but makes you angry, makes you want to show him you can hit him even harder.
You have to have been born with that fire in your belly. it is there since birth! Bad parenting and influences as an infant can break your spirit as an adult... But to really have it as a grown up, it has to be in you since birth... You can always build on it and try to bring it out if it is not a dominant mindset.
In a tough fight, the kind where you take a lot of punishment, you are really having two battles. One against your opponet and the other against yourself. After being tired, hurt and fatigue sets in, you start hoping your opponent will land a good shot so that it can all be over. Your brain starts to tell you how nice it would be to stop fighting and go to the locker room to drink some Gatorade. The reward is not important anymore, the win means nothing, your brain just wants to spare your body from any more damage... Either you have the heart to tell you brain to "Shut The Fuck UP!" or you don't, you listen to it, and you give up the win.
-BRR