Explain POTS in one sentence, in layman's terms

Seeing a group of people later, first time since being diagnosed. Want to be prepared for the inevitable question, instead of spouting off confusing terminology. What would you say?

EDIT: So many awesome suggestions here, thank you! It seems if you mention "nervous system", you get confused looks, but the words "brain, blood and heart" are considered more serious. In the end, I went with, "my body doesn't have enough blood, and the blood I do have isn't pumped to my brain properly". This is accurate, as I do have low blood plasma and Hb. The fitness/health people in the group asked for the technical info later on.