Final answer:
Specialized cardiac muscle cells called pacemaker cells control the heart rate by responding to signals from the autonomic nervous system and various hormones. These cells cannot be consciously controlled.
Step-by-step explanation:
Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.