Final answer:
An unexpected increase in heart rate due to a surprise phone call at 3:00 am is classified as an acute change, which is a short-term physiological reaction. This change is mediated by the sympathetic branch of the autonomic nervous system, indicating a 'fight or flight' response rather than a behavior controlled by the biological clock.
Step-by-step explanation:
The abrupt increase in heart rate due to a surprise phone call at 3:00 am is an example of an acute change. Acute changes are immediate and short-term physiological responses to sudden and often unexpected events or stimuli. This type of change differs from chronic changes, which are long-term and persistent, evolutionary changes, which occur over generations, and changes controlled by a biological clock, which are cyclical and predictable.
The heart's response in such a scenario is triggered by the autonomic nervous system (ANS), specifically the sympathetic branch, which prepares the body for 'fight or flight' reactions by increasing the heart rate to supply muscles with more oxygen. This response is not only crucial for immediate physical action, such as running away or defending oneself, but also for mental alertness. The pacemaker cells within the heart's cardiac muscle tissue receive signals from the ANS, causing the heart to beat faster.
Contrary to some circadian rhythms—which are regular biological processes that occur in a 24-hour cycle, like the sleep-wake cycle or daily fluctuations in body temperature—this increase in heart rate is an immediate reaction to an unexpected event, and it is not a behavior or change that is controlled by the biological clock.