When you stand up, gravity becomes the enemy, and is
blood towards the
lower parts of your body, and
your heart has to pump harder to move the blood through the vessels.. It sounds complicated but its a simple concept. This is also why people have increased pvc's after eating. Blood flows to the stomach and intestines to help with digestion, this shift in blood flow has an effect on the heart, making it work harder, which is why some people experience pvc's during this time..