Normally when a child stands up, the nervous system sends signals, and some hormones react. The heart beats a bit faster, and the blood vessels in the lower body get smaller. This helps make sure that enough blood can be pumped up to the brain. The heart rate goes back to normal after a minute or two.
But with POTS, this process doesn't work as it should. The body doesn't control the heart rate correctly after your child stands up. The heart rate stays too high for as long as your child remains standing. It usually improves when your child sits or lies back down.