question about MLBers after hitting homers

[Sooga]

Now keep in mind I've never played organized baseball in any capacity; my only experience is the occasional softball game or college beerball game.

I'm curious about why when a MLB hitter comes back to the dugout after hitting a homer, he often looks exhausted. He's panting and heaving, chugging back that gatorade like he just finished a 100m Olympic sprint.

Is it that physically taxing to hit a homerun/trot the bases? Or is it mostly adrenaline? I'd be willing to guess it's not really adrenaline-related because I see this even in meaningless games/situations.