Blacking out while on roller coasters

The last couple of times I have been on a roller coaster at certain parts of the ride I black out. I can still hear everything going on the ride I just can't see for 3-5 seconds. It doesn't happen on every ride but it happens on loops and big drops. My friends did not have the same black outs that I did. Is this normal should I go for test? I am 19 years old and am in perfect health, so it seems. I eat alright and I exercise regularly. Thanks for any help!

[hammond]

It could be a sign of low blood pressue, as your going up, down left right and everywhere your body heart can't keep enough blood running to the head hence the blacking out. It could also be anxiety induced too.

[sparkeler]

It could just be the way your body reacts.  As if it goes in to some sort of shock as if your terrified of the drops.  You may not even be subconsciously aware of it, but its like the mind and body have a way of protecting you of trauma or potential disaster.