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all hungry before a stressful event, such as a sports game or an exam (when the sympathetic
division was primarily in action), but suddenly finding yourself starved afterward, as the
parasympathetic takes over. The two systems work together to maintain vital bodily functions,
resulting in homeostasis, the natural balance in the body’s systems.
The Body’s Chemicals Help Control Behavior: The Endocrine System
The nervous system is designed to protect us from danger through its interpretation of and
reactions to stimuli. But a primary function of the sympathetic and parasympathetic nervous
systems is to interact with the endocrine system to elicit chemicals that provide another system
for influencing our feelings and behaviors.
A gland in the endocrine system is made up of groups of cells that function to secrete hormones.
A hormone is a chemical that moves throughout the body to help regulate emotions and
behaviors. When the hormones released by one gland arrive at receptor tissues or other glands,
these receiving receptors may trigger the release of other hormones, resulting in a series of
complex chemical chain reactions. The endocrine system works together with the nervous
system to influence many aspects of human behavior, including growth, reproduction, and
metabolism. And the endocrine system plays a vital role in emotions. Because the glands in men
and women differ, hormones also help explain some of the observed behavioral differences
between men and women. The major glands in the endocrine system are shown in Figure 3.20
"The Major Glands of the Endocrine System".