Well, if you say "I just stood up" ,no doubt, u were previously sitting or even lying down. When u are sitting or lying down ur heart does not require much force or energy to send blood to the upper part of the body. But when u stand there is no chair to neutralize or reduce the effect of gravity on you, therefore the heart rate increases so that blood can be efficiently sent to the upper part of the body.
Please mark me as brainliest.
