This is something that I have really struggled with because most people have never heard of the autonomic nervous system and don’t really know what a normal heart rate should be. Here is an example of what I usually tell people if they are curious:
The autonomic nervous system is the system in your body that controls all those things you don’t have to think about—heart rate, blood pressure, digestion, goose bumps, sweating, salivation, etc. When a person stands up, even if they don’t have POTS, their blood will pool in their feet because gravity pulls it down. The autonomic nervous system makes a lot of changes to blood vessels in order to push this blood out of the legs and back up to the heart. In patients with POTS, the ability to compensate for being upright doesn’t work well and the reasons for this are not completely understood by doctors. This means that when a patient with POTS stands, his/her heart has to beat really fast to try to keep blood flowing to their brain and other organs. If you look at the charts on a treadmill in the gym, my heart rate when I stand is in the “fat burning” or “aerobic” zone. This makes standing for any significant period of time challenging and exhausting. It also means that when I stand, there is less blood in my brain, making it harder to think and causing headaches and light-headedness. I also have other symptoms including digestive problems, body aches and difficulty with temperature regulation.
I will often end the conversation by adding that POTS is something that I have, but it is not what defines me as a person.
*NOTE: This is NOT a scientific explanation, but I find that if you include too much science, people will stop listening to you.