If a person rises to a standing position too quickly blood d…

If a person rises to a standing position too quickly blood drains from the head.  A drop in blood pressure to the brain causes dizziness.  Pressure (baro) receptors in the arteries near the brain detect the drop in blood pressure and send signals to the cardiac center of the brainstem.  The cardiac centers of the brainstem signal the heart to accelerate the heartbeat, restoring the blood flow and pressure to the brain.  In this example of a feedback loop, the heart is the __________.