Anatomy and Physiology 2e · The Endocrine System
The Adrenal Glands
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In 30 seconds
The adrenal glands are paired, pyramid-shaped endocrine glands perched on top of each kidney, like small hats. Each gland is really two endocrine organs in one: an outer Adrenal cortex Outer steroid-making region (glomerulosa, fasciculata, reticularis). Full entry →, which makes steroid hormones, and an inner Adrenal medulla Inner catecholamine-making region (chromaffin cells). Full entry →, which makes catecholamine hormones. The cortex produces three classes of steroids from cholesterol — mineralocorticoids (mainly Aldosterone Mineralocorticoid increasing renal Na⁺ reabsorption and K⁺ secretion. Full entry →), glucocorticoids (mainly Cortisol Glucocorticoid raising blood glucose, mobilizing fuel, dampening inflammation. Full entry →), and androgens (mainly DHEA Weak adrenal androgen from the reticularis. Full entry →) — with distinct jobs: salt and water balance, stress metabolism, and sex-hormone precursor supply. The medulla, controlled directly by sympathetic nerves, releases Epinephrine Medullary catecholamine (~80%) acting on alpha and beta receptors. Full entry → and Norepinephrine Medullary catecholamine (~20%) acting mainly on alpha receptors. Full entry → for the classic "fight-or-flight" response.
The two regions have different embryonic origins (cortex from mesoderm, medulla from neural crest), different control systems, and different hormones — a pair of glands that happen to share a capsule. Understanding the cortex–medulla split is the most important organizing idea in this topic.
Why this matters
The adrenal glands connect almost everything: they are the effectors of the body's stress systems (both the fast nervous and slower hormonal responses), they defend blood pressure and volume through aldosterone, and they supply the androgens behind many aspects of secondary sexual development. Adrenal disorders are high-yield exam material because their symptoms map directly onto the hormones: too little cortisol and aldosterone (adrenal insufficiency), too much cortisol (Cushing syndrome), too much aldosterone (primary aldosteronism), or a medullary tumor (pheochromocytoma) each produce distinct pictures. The CRH–ACTH–cortisol axis also gives a second full example of the pituitary template — and explains real-world phenomena like the stress response and the daily rhythm of alertness.
The college version
Core Concepts
Two organs, one capsule
Embryologically, the cortex arises from mesoderm and the medulla from neural crest cells (the same origin as sympathetic neurons). That split explains the wiring: the cortex is controlled by hormones (ACTH from the anterior pituitary; angiotensin II and potassium for aldosterone), while the medulla is controlled by direct sympathetic preganglionic nerve fibers. In fact, the medulla behaves like a modified sympathetic ganglion, releasing hormones into the blood instead of onto a synapse.
The cortex: three zones, three hormone families
The cortex has three concentric layers, each making a different steroid class:
- Zona glomerulosa (outermost): mineralocorticoids, chiefly aldosterone.
- Zona fasciculata (middle): glucocorticoids, chiefly cortisol.
- Zona reticularis (innermost): androgens such as DHEA (dehydroepiandrosterone).
A common memory hook is "salt, sugar, sex" from outside in (GFR).
Aldosterone: salt, water, and blood pressure
Aldosterone acts on the distal tubules and collecting ducts of the kidney, increasing sodium reabsorption and potassium (and hydrogen) secretion. Water follows sodium, so aldosterone expands blood volume and raises blood pressure. Its secretion is driven mainly by the renin–angiotensin–aldosterone system (triggered by low blood pressure, low volume, or low sodium) and directly by high blood potassium. Hence aldosterone excess classically produces high blood pressure with low potassium, while deficiency produces low blood pressure with high potassium.
Cortisol: the stress and metabolism hormone
Cortisol is controlled by the familiar axis: CRH from the hypothalamus → ACTH from the anterior pituitary → cortisol, with cortisol feeding back to suppress CRH and ACTH. Its effects include raising blood glucose through Gluconeogenesis Making new glucose from non-carbohydrate sources. Full entry → in the liver; mobilizing amino acids from muscle and fatty acids from fat stores; helping maintain blood pressure and vascular responsiveness to catecholamines; and broadly dampening inflammation and immune activity. Cortisol follows a daily rhythm, peaking in early morning, and surges during stress.
Adrenal androgens
The reticularis produces weak androgens (mainly DHEA) that are converted to testosterone and estrogens in peripheral tissues. They contribute to traits such as pubic and axillary hair growth and are largely minor in adults, but they become clinically important in enzyme defects such as congenital adrenal hyperplasia, where precursor steroids are diverted toward androgen production.
The medulla: fight-or-flight catecholamines
The medulla's Chromaffin cells Medullary cells making catecholamines. Full entry → release epinephrine (about 80%) and norepinephrine (about 20%) when stimulated by sympathetic preganglionic fibers. These catecholamines prepare the body for action: faster heart rate and stronger contractions, bronchodilation, glycogen breakdown to raise blood glucose, redirection of blood flow to skeletal muscle, and pupil dilation. Epinephrine acts on both alpha and beta receptors, giving it broader "hormonal" reach than norepinephrine, which acts mainly on alpha receptors (and is also the sympathetic postganglionic neurotransmitter).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Cortex (steroids, hormone-controlled) | Medulla (catecholamines, nerve-controlled) | Different tissue, control, and hormones. |
| Cortisol (sugar/stress/immune) | Aldosterone (salt/volume/potassium) | Separate zones, triggers, and jobs. |
| Aldosterone excess (high BP, low K⁺) | Aldosterone deficiency (low BP, high K⁺) | Mirror images: one hoards sodium, the other wastes it. |
| Epinephrine (alpha + beta, broad) | Norepinephrine (mainly alpha, narrower) | Different receptor profiles, different reach. |
| Pheochromocytoma (catecholamine excess) | Cushing syndrome (cortisol excess) | Medullary tumor with episodic surges vs chronic steroid excess. |
| Adrenal androgen excess (virilization) | Cortisol excess (Cushing features) | Different zones and different physical pictures. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The adrenal glands are like a two-story emergency station sitting on top of each kidney. The outside office (cortex) handles long-term jobs: it sends messages to keep salt, water, sugar, and stress in balance. The inside office (medulla) handles sudden emergencies — when you are scared or surprised, it pours out "go fast" chemicals that make your heart pound and your body ready to run or fight.
Worked example
Stage a two-part stress response and watch the glands divide the work. At the sound of sudden danger, sympathetic nerves fire: the medulla receives the signal instantly and pours epinephrine and norepinephrine into the blood. Within seconds, heart rate climbs, airways widen, the liver releases glucose, and blood is redirected to muscles — the fast, nervous-system response. Minutes later, the hormonal response builds: the hypothalamus releases CRH, the pituitary releases ACTH, and the cortex secretes cortisol, sustaining blood glucose and dampening inflammation so the body can keep performing. When the crisis ends, cortisol falls back under negative feedback.
Now imagine the cortex fails (as in classic Addison disease): without aldosterone, sodium is lost and potassium climbs, so blood volume and pressure drop; without cortisol, blood glucose can fall and stress responses falter. The textbook picture includes salt wasting, low blood pressure, and hyperpigmentation from unsuppressed ACTH. These are educational descriptions of classic presentations, not diagnostic criteria.
Key takeaways
- Cortex = steroids: zona glomerulosa → aldosterone (salt/BP), zona fasciculata → cortisol (sugar/stress), zona reticularis → androgens. Hook: "salt, sugar, sex."
- Medulla = catecholamines (epinephrine ~80%, norepinephrine ~20%), controlled by sympathetic preganglionic nerves; a modified sympathetic ganglion.
- Aldosterone: Na⁺ reabsorbed, K⁺/H⁺ excreted, water follows → raises volume and pressure. Driven by RAAS and blood K⁺.
- Cortisol axis: CRH → ACTH → cortisol, with negative feedback; diurnal morning peak.
- Cortisol raises blood glucose, mobilizes fuel, supports blood pressure, suppresses inflammation.
- Catecholamines: tachycardia, bronchodilation, glycogenolysis, blood-flow redistribution, pupil dilation.
- Classic textbook pictures: adrenal insufficiency (low BP, high K⁺, low Na⁺); cortisol excess (Cushing: central weight gain, high glucose); aldosterone excess (Conn: hypertension, low K⁺); medullary tumor (pheochromocytoma: catecholamine surges). Educational, not diagnostic.
- Two stress speeds: fast (sympathetic/catecholamines, seconds) and slow (CRH–ACTH–cortisol, minutes to hours).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name the three cortical zones from outside in and the hormone family each produces.
Show answer
Zona glomerulosa → mineralocorticoids (aldosterone); zona fasciculata → glucocorticoids (cortisol); zona reticularis → androgens (DHEA). Hook: "salt, sugar, sex."
What two signals drive aldosterone secretion, and what does it do in the kidney?
Show answer
The renin–angiotensin–aldosterone system (low blood pressure/volume or low sodium) and high blood potassium. In the kidney, sodium reabsorption and potassium (and hydrogen) secretion increase; water follows sodium.
Write the cortisol axis and identify where negative feedback acts.
Show answer
CRH (hypothalamus) → ACTH (anterior pituitary) → cortisol (cortex). Cortisol feeds back negatively on both CRH and ACTH.
Why is the medulla described as a modified sympathetic ganglion?
Show answer
It derives from neural crest, receives direct sympathetic preganglionic innervation, and releases catecholamines into the blood rather than onto a synapse — a sympathetic ganglion that secretes hormones.
List four classic effects of epinephrine during fight-or-flight.
Show answer
Increased heart rate and contractility, bronchodilation, glycogen breakdown raising blood glucose, and blood-flow redistribution to skeletal muscle (plus pupil dilation).
In adrenal insufficiency, why do potassium and blood pressure behave as they do?
Show answer
Aldosterone is lost, so sodium (and water) are wasted — lowering blood volume and pressure — while potassium cannot be excreted normally and rises.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Adrenal cortex
- Outer steroid-making region (glomerulosa, fasciculata, reticularis).
- Adrenal medulla
- Inner catecholamine-making region (chromaffin cells).
- Aldosterone
- Mineralocorticoid increasing renal Na⁺ reabsorption and K⁺ secretion.
- Cortisol
- Glucocorticoid raising blood glucose, mobilizing fuel, dampening inflammation.
- DHEA
- Weak adrenal androgen from the reticularis.
- Renin–angiotensin–aldosterone system (RAAS)
- Cascade activated by low blood pressure/volume that raises aldosterone.
- Chromaffin cells
- Medullary cells making catecholamines.
- Epinephrine
- Medullary catecholamine (~80%) acting on alpha and beta receptors.
- Norepinephrine
- Medullary catecholamine (~20%) acting mainly on alpha receptors.
- Gluconeogenesis
- Making new glucose from non-carbohydrate sources.
Sources & references
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