Final answer:
Sinus rhythm refers to the normal rhythmic beating of the heart, established by pacemaker cells in the sinoatrial node. These cells create electrical signals that cause the atria and ventricles to contract. The heartbeat can also be influenced by the autonomic nervous system and hormones.
Step-by-step explanation:
The normal rhythmic beating of the heart is called sinus rhythm and is established by the pacemaker cells in the sinoatrial node, located in the right atrium of the heart. These pacemaker cells create electrical signals that cause the atria and ventricles to contract. The autonomic nervous system and hormones can also influence the heartbeat.