Physiologically, your brain weighs the situation and decides that you’d better be ready for anything, so it kicks your endocrine system in the pants for the adrenaline and cortisol and the like. There’s an issue in an old Science magazine – August 1999, I think – where a guy goes and measures the effects of stress on kids. Good read.
I hate public speaking, so naturally I got my degree in communications and a minor in marketing. XD
I coped by putting on the metaphorical game face. I would glance around the room, not really making eye contact but pretending that I was, and use my PowerPoints as a set of notecards – read the point, expound (rote memorization would be the death of me, so I’d wing the exposition), and move on. I suppose you could say that I would ignore the public. Imagining the audience in their underwear took too long.