In POTS, your autonomic nervous system (the nervous system in charge of automatic body functions) doesn’t work efficiently when you sit or stand up.
Normally when you sit up or stand, gravity pulls some blood down to your tummy area, hands and feet. This makes your blood vessels narrow quickly and your heart rate increase slightly. This keeps blood flowing to your heart and brain and stops blood pressure dropping.
However, if you have POTS, your autonomic nervous system is slow to make this correction and you have a drop in blood supply to your heart and brain when you sit or stand up. This makes your heart race to make up for this drop in blood pressure.
You can develop POTS suddenly, or it can begin gradually.
There is a range of disorders similar to POTS (a few of which involve no rise in heart rate). These fall into the category of dysautonomia, which are conditions in which the autonomic nervous system isn’t working properly.
(Dysautonomia International, US, 2016)