When i stand up ,my eyes black out and i get dizzy for a few seconds. why does that happen ?

Hypotension: Blacking out or vision getting dim when you stand up is a drop in blood pressure called orthostatic hypotension. It is really common in young, healthy people who have low blood pressure to start with. Increasing fluid intake and sometimes salt intake can help, especially in the summer months when you can get dehydrated more easily.