I believe the phrase you've got balls is related to testosterone. No one knows for sure but I read somewhere that the phrase started out as you've got big testicles, implying that you have a lot testosterone.
But anyway yeah, the phrase you've got balls is saying you have got a lot testosterone pumping inside of you. Think the phrase man up is something to do with armour, it is time to go to war so you need to protect your manhood by covering it up with some armour plate or something. So man up means cover up as it's time to go to war. That what I've read anyhow but no one knows for certain these things.
If you're interested on how the word pussy came to mean a vagina and cowardly, I wrote a post on this ages ago, the results are inconclusive but it's an eyeopening story:
