Adrenal Glands: Cortex, Medulla and Adrenaline Explained

The adrenal glands sit on top of each kidney. Each has an outer cortex making steroid hormones and an inner medulla making adrenaline and noradrenaline for rapid stress responses.

They matter because they handle both long-term balance (salt, blood pressure, metabolism, stress adaptation) and instant emergencies. Adrenaline's “fight or flight” effects are felt within seconds of fright or excitement.

Key Concepts

• Adrenal cortex zones: glomerulosa (aldosterone), fasciculata (cortisol), reticularis (androgens).
• Aldosterone: mineralocorticoid regulating Na+, K+ and blood pressure.
• Cortisol: glucocorticoid regulating glucose, stress and inflammation.
• Adrenal medulla: modified sympathetic neurons secreting adrenaline (~80%) and noradrenaline (~20%).
• ACTH: pituitary hormone stimulating cortisol production.

How It Works

1. Stress signal reaches hypothalamus → CRH → pituitary ACTH → cortex releases cortisol (minutes to hours effects).
2. Low blood pressure/volume triggers aldosterone via renin-angiotensin system, increasing kidney Na+ and water retention.
3. Sympathetic nerves directly stimulate medulla, releasing adrenaline into blood within seconds.
4. Adrenaline increases heart rate, dilates airways and pupils, mobilises glucose and fatty acids.
5. Feedback of cortisol to brain/pituitary limits the response once stress passes.

Worked Example

Problem: Compare timing of medulla vs cortex in sudden fright.

Step 1 (medulla, seconds): Adrenaline surge raises pulse and breathing before conscious analysis completes.

Step 2 (cortex, minutes): ACTH rises, cortisol follows, sustaining glucose supply and blood pressure.

Step 3: Aldosterone support maintains volume if stress/blood loss continues.

Result: Fast system buys immediate survival; slower steroid system sustains it. Both originate from the same small gland.

Common Misconceptions

• Myth: Adrenaline comes from the cortex. Correction: Adrenaline/noradrenaline come from the medulla; cortex makes steroids like cortisol and aldosterone.
• Myth: Cortisol is only a harmful “stress hormone.” Correction: Normal cortisol is essential for blood pressure, glucose and waking rhythm; only chronic excess is harmful.
• Myth: Adrenal glands are part of kidney function. Correction: They sit on kidneys but are endocrine glands with independent blood/hormone roles.

Real-World Applications

An EpiPen delivers adrenaline to reverse anaphylaxis (severe allergy). Corticosteroid medicines mimic cortisol to reduce inflammation in asthma and autoimmune disease. Addison's disease (adrenal insufficiency) requires lifelong cortisol/aldosterone replacement.

One gland, two systems: medulla for seconds, cortex for sustained control of stress, salt and energy. The cortex also responds to severe illness, injury and surgery by increasing cortisol output to maintain blood pressure and glucose availability, which is why critical-care doctors monitor adrenal function in prolonged shock, and why patients on long-term steroid medicines must taper doses gradually to allow their own adrenal cortex to resume normal ACTH responsiveness.