Final answer:
When changing body positions, the heart beats faster to maintain blood flow and pressure.
Step-by-step explanation:
When changing body positions from lying down to sitting and then to standing, the heart has to beat faster to compensate for the change in blood flow and maintain blood pressure.
When you stand up, blood tends to pool in the lower extremities due to gravity. To maintain a stable blood pressure, the heart rate increases to pump more blood to the brain and other vital organs.
For example, when you exercise, your heart rate increases to ensure that enough oxygen and nutrients are delivered to the working muscles.