If I remember my patho-physiology correctly: when blood volume drops the heart beats faster to get O2 infused blood to the major organs like the heart, lungs, brain etc. When a person is supine-lying down the blood is relatively even throughout the body, but when they sit or stand the blood drops rapidly to the legs and feet. The blood has been re-directed when you stand up. That's why you get dizziness or faint and why the heart beats so fast.It's one of them. The second will appear tomorrow.
I think it does. I think the increased heart rate is an overaction to reduced blood volume in the heart and brain (???)
Sometimes increasing sodium (Na+) in the body, like with an infusion of saline, increasing salt intake and drinking gatorade will help as the sodium in the table salt, saline infusion or the electrolytes (Sodium or Na+ are electrolytes) in the gatorade draws fluid into the blood vessels which increases blood volume.
It's a fine balance (homeostasis), usually, but people with POTS and ME don't have that balance of electrolytes which can lead to high BP, low BP, low blood volume and high heart rates. We are a mess inside.