Final answer:
An increase in the influence of the sympathetic nervous system leads to an increased heart rate and heightened alertness, which is part of the body's 'fight or flight' response to stress or danger.
Step-by-step explanation:
If the sympathetic nervous system increases its influence in the body, you should anticipate D) Increased heart rate and heightened alertness. The sympathetic nervous system is part of the autonomic nervous system and is activated during stressful or high-arousal situations. Its functions include dilating the pupil (pupil dilation), increasing the heart rate (tachycardia), dilating bronchi (which facilitates breathing), and inhibiting certain processes such as digestion and the contraction of the bladder.
During a sympathetic response, the body experiences a series of changes that prepare an individual to face a threat or stressful situation. Known colloquially as 'fight or flight,' this response includes releasing adrenaline into the bloodstream, which leads to various physiological outcomes such as increased heart rate, dilation of bronchial airways to allow more air exchange, and dilated pupils to increase visual acuity. These reactions are designed to optimize the body's performance under duress and are the opposite of what is seen in a parasympathetic response (relaxation state).