Which system is responsible for regulating heart rate and contractility?

The autonomic nervous system is primarily responsible for regulating heart rate and contractility. This system has two branches: the sympathetic and parasympathetic systems, which work together to control the heart’s function. The sympathetic branch increases heart rate and the strength of contractions during stressful situations, known as the "fight or flight" response. Conversely, the parasympathetic branch decreases heart rate and promotes relaxation.

These regulatory mechanisms allow the heart to respond quickly to changing physiological demands, such as during exercise or stress, by adjusting the heart rate and the force of heart contractions. Therefore, the autonomous nature of this system directly influences cardiac output and overall heart function, making it the correct choice for this question.