8.9: Adrenal Glands

The adrenal glands sit atop the kidneys and function as powerful endocrine organs whose outer cortex produces steroid hormones regulating metabolism, fluid balance, and stress, while the inner medulla releases epinephrine to trigger the body’s fight-or-flight response.

Master this section and you'll be able to

• Describe the location and structure of the adrenal glands
• Identify the hormones produced by the adrenal cortex and adrenal medulla, and summarize their target cells and effects
• Explain the role of the adrenal medulla in the fight-or-fight response.
• Recognize two disorders caused by cortisol hyper- and hyposecretion. 

Anatomy of the Adrenal (Suprarenal) Glands

Perched like little caps on top of each kidney, the adrenal glands — also called suprarenal glands — are small but mighty powerhouses of the endocrine system (See figure below). Each gland is wrapped in a tough fibrous capsule that anchors it to the kidney and protects its delicate inner tissues. Despite their size, the adrenal glands have an exceptionally rich blood supply, receiving one of the highest rates of blood flow in the entire body. This makes sense — when these glands release their hormones, they need them to reach target tissues fast.

Structurally, each adrenal gland has two distinct regions with very different jobs:

• The outer adrenal cortex, which produces vital steroid hormones that help regulate metabolism, salt balance, and stress responses.

• The inner adrenal medulla, which functions more like a neuroendocrine organ, releasing adrenaline and noradrenaline to kick the body into high gear during “fight-or-flight” situations.

Figure \(\PageIndex{1}\): The Adrenal Glands. Both adrenal glands sit atop the kidneys and are composed of an outer cortex and an inner medulla, all surrounded by a connective tissue capsule. The cortex can be subdivided into additional zones. (Image credit: "The Adrenal Glands" by Jennifer Lange is licensed under CC BY-NC-SA 4.0; includes section of "Slagter - Drawing Anatomy of the kidney, adrenal glands and bladder - no labels" by Ron Slagter, license: CC BY-NC-SA.)

 

Figure \(\PageIndex{2}\): Adrenal Gland Histology.  The adrenal gland is located on top of each kidney.  The histology seen here shows the different parts of the gland that is responsible for the production of different types of hormones.  A - Zona Glomerulosa, B - Zona Fasciculata, C - Zona Reticularis, and D - Medulla. (Image credit: “Histology of  Adrenal Gland" by Yancy Aquino is licensed under CC BY-NC-SA 4.0, slide provided by the Regents of the University of Michigan Medical School © 2022 under CC BY-NC-SA 3.0.) 

 

Stress!

Adrenal Glands and the Stress Response

One of the adrenal glands’ most important jobs is to help the body respond to stress — whether that stress is physical, psychological, or both.

• Physical stress might come from an injury, exposure to cold weather without proper clothing, or malnutrition.
• Psychological stress can stem from arguments, looming deadlines, or simply having a rough day.

The body’s overall reaction to stress follows a pattern called the General Adaptation Syndrome (GAS), which occurs in three stages. 

1. Alarm Reaction – The “Fight-or-Flight” Response

This is the body’s immediate reaction to short-term stress. It’s powered by the adrenal medulla, which releases the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline).
These hormones rapidly prepare the body for intense physical action by:

• Stimulating the liver and muscles to convert glycogen to glucose, raising blood sugar levels.
• Increasing heart rate, pulse, and blood pressure.
• Dilating the airways to boost oxygen intake.
• Directing blood flow toward vital organs like the heart, lungs, brain, and skeletal muscles, while reducing flow to the digestive system, kidneys, and skin.

Other typical signs include dry mouth, dilated pupils, decreased appetite, and tunnel vision — all part of a body ready for action.
Once the threat passes, the body quickly returns to normal conditions.
 

2. Resistance Stage – Adapting to Ongoing Stress

If the stressor persists, the body enters a phase of adjustment and adaptation. For example, someone who is starving might experience increased signals to the digestive system to maximize nutrient absorption. The goal is to maintain stability (homeostasis) while the stress continues.
 

3. Exhaustion Stage – The Consequences of Chronic Stress

Prolonged or repeated stress can eventually overwhelm the body’s adaptive systems. This final stage of GAS is mediated by the adrenal cortex, particularly through the hormone cortisol.
Long-term elevation of cortisol can lead to:

• Fatigue and depression
• Suppressed immunity
• Muscle wasting and weight changes
• Cardiovascular problems, including increased risk of heart attack

 

The Adrenal Cortex

The adrenal cortex is the outer part of the adrenal gland, made up of layers of cells packed with lipids (fats) that give the tissue a yellowish color. It has three distinct regions, each responsible for producing different steroid hormones. These hormones help the body manage long-term stress, blood pressure and volume, nutrient balance, fluid and electrolyte levels, and inflammation.

The three regions are:

1. Zona glomerulosa – controls minerals and water balance

2. Zona fasciculata – manages metabolism and long-term stress

3. Zona reticularis – produces small amounts of sex hormones

 

1. Zona Glomerulosa – Regulating Minerals and Blood Pressure

The outermost layer, the zona glomerulosa, produces mineralocorticoids, hormones that regulate sodium (Na⁺) and potassium (K⁺) levels — key to maintaining fluid balance and healthy blood pressure.

The most important mineralocorticoid is aldosterone. It acts mainly on the kidneys, helping the body to:

• Retain sodium (Na⁺) and excrete potassium (K⁺)
• Hold on to water, which increases blood volume and blood pressure

Aldosterone secretion is triggered when blood pressure or sodium levels are low — or when potassium levels are high. It’s also part of a hormone cascade called the renin–angiotensin–aldosterone system (RAAS):

1. The kidneys release renin when blood pressure drops.

2. Renin activates a molecule that becomes angiotensin II.

3. Angiotensin II constricts blood vessels, signals the kidneys to retain water, and tells the adrenal cortex to release aldosterone.

For people with high blood pressure, medications called ACE inhibitors block the enzyme that produces angiotensin II, lowering blood pressure by interrupting this system.

 

2. Zona Fasciculata – Managing Long-Term Stress

The middle layer, the zona fasciculata, produces glucocorticoids, which help the body handle long-term stress and regulate glucose metabolism. The main hormone here is cortisol (some of which the liver converts to cortisone).

Cortisol is controlled by the HPA axis (Hypothalamus–Pituitary–Adrenal):

1. The hypothalamus releases CRH.

2. CRH triggers the anterior pituitary to release ACTH.

3. ACTH stimulates the adrenal cortex to produce cortisol.

When cortisol levels rise, the body shifts energy priorities:

• Glycogen is broken down to glucose for energy.
• Fat stores are converted into fatty acids.
• Muscle proteins are broken down into amino acids.

Cortisol ensures cells have enough fuel during ongoing stress, but too much for too long can harm the body—especially the hippocampus, the brain region critical for learning and memory, which is very sensitive to stress hormones.

You’ve probably seen cortisol’s effects in medicine: hydrocortisone creams for rashes, prednisone tablets for inflammation, and steroid inhalers for asthma. These drugs work by suppressing the immune and inflammatory responses, mimicking cortisol’s natural effects. 

 

3. Zona Reticularis – Supplementary Sex Hormones

The innermost layer, the zona reticularis, produces small amounts of androgens, a class of sex hormones. Androgens are often called “male” sex hormones because they promote traits like facial hair, a deeper voice, and greater muscle mass, but both males and females produce them — in the testes for males and in the ovaries and adrenal cortex for both sexes. All steroid hormones share a common origin from cholesterol and can be converted into one another through enzymes. For example, androgens such as testosterone can be converted into estrogens (like estradiol) by the enzyme aromatase in tissues such as the ovaries, fat, and even the brain. After menopause, when ovarian hormone production declines, the adrenal cortex becomes an important source of estrogens for individuals with ovaries.

The adrenal androgens are often referred to as "weak androgens" and serve as precursors that the body converts into more potent androgens like testosterone and estrogen. So, while the gonads (ovaries and testes) are the main producers, these adrenal androgens provide a backup supply.

 

 Table \(\PageInd ex{1}\): Hormones of the Adrenal Glands 

Adrenal gland region	Associated hormones	Effect
Adrenal cortex (Zona Glomerulosa)	Aldosterone	Increases blood Na^+ levels
Adrenal cortex (Zona Fasciculata)	Cortisol, corticosterone, cortisone	Increase blood glucose levels
Adrenal cortex (Zona Reticularis)	Androgens	Supplement gonadal androgens
Adrenal medulla	Epinephrine, norepinephrine	Stimulate fight-or-flight response

 

The Adrenal Medulla

The adrenal medulla is the inner core of the adrenal gland and is made up of chromaffin cells — specialized cells that are actually modified postganglionic sympathetic neurons. Like the rest of the sympathetic nervous system, these cells develop from neural crest tissue, the same embryonic origin as the sympathetic ganglia.

The adrenal medulla serves as a bridge between the nervous and endocrine systems. It functions like a neural structure because it is controlled by direct sympathetic nerve stimulation, yet it behaves like an endocrine gland because it releases its secretions — epinephrine (adrenaline) and norepinephrine (noradrenaline) — directly into the bloodstream. These hormones rapidly prepare the body for “fight-or-flight” situations by increasing heart rate, blood pressure, and energy availability.

Unlike most hormones, the effects of the adrenal medulla are immediate but short-lived, giving the body a quick burst of energy and alertness during sudden stress.

 

DISORDERS OF THE...

Endocrine System: Disorders Involving the Adrenal Glands

Hormones are secretive messengers that quietly work behind the scenes to keep the body in balance. Most of the time, their actions go unnoticed — until something disrupts that balance. The best way to understand what a hormone really does is to study what happens when there is either hypersecretion (too much hormone) or hyposecretion (too little hormone). These conditions reveal the hormone’s true influence on the body and help explain how each contributes to maintaining homeostasis.

For example, hypersecretion of thyroid hormones can make a person feel restless and overheated, while hyposecretion slows metabolism and causes fatigue. Too much insulin can drop blood glucose dangerously low, whereas too little leads to diabetes. By examining these hormonal imbalances, we can see how hormones regulate energy, mood, growth, and nearly every physiological system in our body.

Several disorders result from dysregulation — either hypersecretion or hyposecretion  — of adrenal hormones. 

One well-known example is Cushing’s disease, a condition caused by hypersecretion of cortisol. Cushing’s disease is often triggered by a pituitary tumor that secretes excessive amounts of ACTH, which in turn overstimulates cortisol production. The excess cortisol leads to elevated blood glucose levels and the accumulation of fat deposits on the face and neck, resulting in the classic “moon face” and “buffalo hump” appearance. Other common symptoms include rapid weight gain, hair loss, and muscle weakness.

Because cortisol also affects metabolism and the immune system, chronically high levels can increase the risk of type 2 diabetes, impair immune defenses, and reduce resistance to infection. Long-term cortisol hypersecretion can also interfere with memory.

Figure \(\PageIndex{3}\): Cushings Syndrome. A. The different symptoms of a person with Cushing's syndrome.  B. A picture of a person with Cushing's syndrome showing the prominent stretch marks on the abdomen.  (Image credit: "A. Cushing Syndrome Symptoms" and "B. Cushign Syndrome" by Yancy Aquino is licensed under CC BY-NC-SA 4.0, modification of original by OpenStax.)

 

In contrast, the hyposecretion of corticosteroids can result in Addison’s disease, a rare disorder that causes low blood glucose levels and low blood sodium levels. The signs and symptoms of Addison’s disease are vague and are typical of other disorders as well, making diagnosis difficult. They may include general weakness, abdominal pain, weight loss, nausea, vomiting, sweating, and cravings for salty food.

Figure \(\PageIndex{4}\): Addison's Disease.  A 69-year-old female with tiredness and a persistent tan. Facial appearance at presentation (left), showing pigmentation, and after treatment (right). (Image credit: "Addison's Disease" by Yancy Aquino is licensed under CC BY-NC-SA 4.0, modification of original by OpenStax.)