Endocrine System: Glands, Hormones, and Fight-or-Flight

🧬 Endocrine System: Overview

The endocrine system works alongside the nervous system to control vital functions in the body.
It acts more slowly than the nervous system but has very widespread and powerful effects.
Hormones are secreted into the bloodstream and affect any cell in the body that has a receptor for that particular hormone.
Most hormones affect cells in more than one body organ, leading to many diverse and powerful responses.
Example: Thyroxine produced by the thyroid gland affects cells in the heart (increases heart rate) and also throughout the body to increase metabolic rates, which in turn affects growth rates.

🧪 Glands and Hormones

Endocrine system is one of the body's major information systems; it instructs glands to release hormones directly into the bloodstream.
Hormones are carried towards target organs in the body; communication is via chemicals.
Gland: An organ in the body that synthesises substances such as hormones.
Hormone: A biochemical substance that circulates in the blood but only affects target organs. They are produced in large quantities but disappear quickly. Their effects are very powerful.

🧠 The Pituitary Gland: Master Gland

The pituitary gland is the main endocrine gland located in the brain.
It is often called the 'master gland' because it controls the release of hormones from all the other endocrine glands in the body.
The main endocrine glands in the human body include: hypothalamus, pituitary, thyroid, parathyroid, adrenal glands, pancreas, ovaries (female), and testes (male).
The diagram (referred to as on the right) lists these key glands.

🏃‍♀💨 Endocrine System and Autonomic Nervous System (ANS): Fight or Flight

Often the endocrine system and the autonomic nervous system work in parallel during stressful events.
When a stressor is perceived (e.g., a friend jumps out to frighten you or you think about upcoming exams), the hypothalamus activates the pituitary gland, triggering activity in the sympathetic branch of the ANS.
The ANS shifts from its normal resting parasympathetic state to the physiologically aroused sympathetic state.
Adrenaline (the stress hormone) is released from the adrenal medulla (part of the adrenal gland near the kidneys) into the bloodstream.
Adrenaline triggers physiological changes in the body (e.g., increased heart rate) that create the physiological arousal necessary for the fight or flight response.

🚀 Immediate and Automatic Response

All of this happens in an instant as soon as the threat is detected (e.g., the heart starts beating faster almost immediately when frightened).
This is an acute response and an automatic reaction in the body.
The physiological changes associated with the sympathetic response explain why stress, panic, or even excitement are often experienced as a 'sick' feeling or butterflies in the stomach.

🧘‍♀ Parasympathetic Action: Rest and Digest

After the threat has passed, the parasympathetic nervous system returns the body to its resting state.
The parasympathetic branch works in opposition to the sympathetic branch and is antagonistic to its actions.
It acts as a 'brake' and reduces the activities of the body that were increased by the sympathetic branch, producing the rest and digest state.

🔑 Key Terms

Endocrine system: One of the body's major information systems that instructs glands to release hormones directly into the bloodstream. These hormones are carried toward target organs in the body. Communicates via chemicals.
Gland: An organ in the body that synthesises substances such as hormones.
Hormone: A biochemical substance that circulates in the blood but only affects target organs. They are produced in large quantities but disappear quickly. Their effects are very powerful.
Fight or flight response: The way an animal responds when stressed. The body becomes physiologically aroused in readiness to fight an aggressor or, in some cases, flee.
Adrenaline: A hormone produced by the adrenal glands which is part of the human body's immediate stress response system. Adrenaline has a strong effect on the cells of the cardiovascular system
—stimulating heart rate, contracting blood vessels and dilating air passages.

🔗 Connections and Real-World Relevance

The endocrine system complements the nervous system, providing a dual mechanism for rapid and widespread physiological control.
Hormone action requires receptor presence on target cells, ensuring specificity amid widespread circulatory signals.
The fight or flight response explains common experiences of stress, panic, or excitement (e.g., rapid heartbeat, butterflies in the stomach).
Understanding these processes helps in recognizing how acute stress differs from resting states and why recovery (parasympathetic rebound) is essential after a threat.

🗺 Quick Reference: Summary of Pathways

Stressor perceived → hypothalamus activated → pituitary engaged → sympathetic branch of ANS activated → adrenal medulla secretes adrenaline → physiological arousal (increased heart rate, etc.) → threat passes → parasympathetic system restores rest and digest state.

💡 Practical Implications

Acute stress triggers rapid, automatic bodily changes designed for quick action.
The experience of 'butterflies' or a sick feeling reflects the body's arousal and redistribution of resources during sympathetic activation.
Knowing that parasympathetic activity helps return to baseline can inform strategies for calming after stress (breathing, relaxation) to promote the rest and digest state.