With the caveat this is my understanding and I am not a doctor, there are three reasons you might have low blood pressure:
- You’re extremely healthy and your system is very, very efficient
- You’re heart is weak/ill/deteriorating and simply can’t keep the pressure up
- We don’t know, you’re just that way.
Obviously, if you’re heart is having problems and can’t meet sudden changes in demand because of that illness (such as standing up quickly) that can be a serious problem and probably requires medical attention. The other two reasons are OK, so long as your system can handle changes in demand.
The “light-headed” thing is because your heart isn’t quite pumping hard enough to thoroughly oxygenate your brain. It them because a question of whether or not that is severe enough to pose a safety hazard or indicative of a serious problem.
In my case, when I took my first FAA physical they were a bit concerned because I was right at the bottom of “normal” by their standards (100/60) and, given that pilots are regularly subjected to greater than normal g-forces that require a heart healthy enough to meet the necessary power to oxygenate a brain at, say, 2g’s they wanted to be sure my pressure was low because of reason #1 rather that either of the other two. They did not want me to be light-headed at all. For someone not engaged in some activities a little light-headedness now and again might not be a problem or require any intervention so long as something serious isn’t going on.
If you have low blood pressure because of #1, when you run or exert yourself your blood pressure will rise a bit to meet the demands of the activity. If it’s for #2 it won’t, and that can be serious.