## Anatomy and Function of the Adrenal Gland **Key Point:** The adrenal gland has two distinct functional zones: the cortex (produces steroid hormones) and the medulla (produces catecholamines). Cortisol is NOT a product of the adrenal medulla. ### Adrenal Cortex vs. Adrenal Medulla | Feature | Adrenal Cortex | Adrenal Medulla | | --- | --- | --- | | **Embryologic origin** | Mesodermal (coelomic epithelium) | Ectodermal (neural crest) | | **Primary hormones** | Cortisol, aldosterone, androgens | Epinephrine, norepinephrine | | **Control** | ACTH from anterior pituitary | Sympathetic nervous system | | **Feedback regulation** | Cortisol → negative feedback on HPA axis | No feedback regulation | **High-Yield:** The adrenal medulla is innervated by preganglionic sympathetic fibres and secretes catecholamines in response to stress. It does NOT produce cortisol. ## HPA Axis Feedback Mechanism 1. Hypothalamus releases CRH (corticotropin-releasing hormone) 2. Anterior pituitary releases ACTH (adrenocorticotropic hormone) 3. Adrenal cortex (zona fasciculata) produces cortisol 4. High cortisol → negative feedback suppression of CRH and ACTH **Clinical Pearl:** Cortisol feedback is biphasic — low cortisol stimulates the HPA axis; high cortisol suppresses it. This is the basis of the dexamethasone suppression test (DST) used in diagnosing Cushing syndrome. ## Circadian Rhythm of Cortisol **Key Point:** Cortisol follows a robust circadian pattern: - Peak: 06:00–08:00 hours (upon waking) - Nadir: 23:00–01:00 hours (midnight) - Driven by CRH pulsatility from the hypothalamus, which is entrained to the sleep-wake cycle **Mnemonic:** **ACTH-Cortisol Rhythm** = **A**wake and **C**ortisol **R**ise (morning peak).
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