Let me share something that happened to me last summer.
I'm 36 years old. 5'7" 150 pounds in good physical shape with no prior history of high blood pressure. Having grown up in Georgia, I have what I consider to be a fairly high tolerance to heat. Up untill last summer, I've never had any problem with heat whatsoever.
I had just returned to my job from my lunch break. All of a sudden, I got a little dizzy. I wrote it off as a product of the 70 hour weeks we had been working, and kept at it. The dizzyness got worse. I went and sat down in my air conditioned office, and called my boss on the phone.
In a matter of minutes, the dizzyness got worse, I was sweating bullets, it was really hard to breath. I told my now present boss, who was more scared than I was, to call an ambulance. From that point untill I woke up in the local ER with a saline bag dripping into my arm, I don't remember much.
After I came to, a doctor explained to me what happened. Because of long term dehydration, due to me not drinking enough water(I was a pepsi freak)over a long period of time, certain electrolytes that help control BP were depleted in my blood. This allowed my BP to spike real high, real quick, even though I had never had BP problems before.
In the year since, I've had three check ups and had tests run, with no further problems.