I do a lot of climbing, am fairly lean, healthy, and have a low resting heart-rate. I've also always had the problem of nearing "blackout" occasionally if I stand up very quickly from zero physical activity - I've never actually passed out, but you have this weird sensation where your vision, balance, and in extreme cases even hearing, fades to various degrees until the pressure stabilises.

I wondered if it would be a similar effect to what happens when a pilot pulls a high-g manoeuvrer.