Why does blood pressure and heart rate differ when measured in a reclining position and in a standing position?

It differs because your heart has to work harder when standing. the blood that reaches your legs has to travel back up through veins in your legs as gravity tries to pull it down. More blood pressure means there wont be as much flow back(veins don't have a lot of blood pressure to begin with). also to prevent back flow there are valves in your veins that close after each pass of blood. Another reason why is because while standing muscles are contracted and using ATP which leads to a need for more blood supply to ensure those muscles continue to contract. When you are in reclining position your muscles are relaxed and your cells aren't using as much ATP.