What system regulates involuntary body functions, such as heartbeat and digestion?

The autonomic nervous system is the correct answer, as it is responsible for regulating involuntary body functions that occur without conscious control. This includes vital processes like heartbeat, digestion, respiratory rate, and blood pressure. The autonomic nervous system has two main divisions: the sympathetic nervous system, which prepares the body for "fight or flight" responses, and the parasympathetic nervous system, which promotes "rest and digest" activities. This duality allows for a balanced regulation of internal bodily functions necessary for maintaining homeostasis.

While the endocrine system indeed regulates various bodily functions through hormones, it does not directly control involuntary processes but rather modifies them over time. The musculoskeletal system primarily manages voluntary movements and support of the body, with structures such as muscles and bones. The circulatory system focuses on transporting blood, nutrients, and waste throughout the body but does not regulate the functions of organs and systems. Thus, the autonomic nervous system is uniquely identified for its role in managing involuntary bodily functions.