15.5: Adrenal Glands

By the end of this section, you will 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

The adrenal glands, also known as the suprarenal glands, are wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule of dense connective tissue (Figure \(\PageIndex{1}\)). The adrenal glands have a rich blood supply and experience one of the highest rates of blood flow in the body. They are served by several arteries branching off the aorta, including the suprarenal and renal arteries. Blood flows to each adrenal gland at the adrenal cortex and then drains into the adrenal medulla. Adrenal hormones are released into the circulation via the left and right suprarenal veins.  The adrenal gland consists of an outer cortex and an inner medulla.

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.)

The adrenal cortex, as a component of the hypothalamic-pituitary-adrenal (HPA) axis, secretes steroid hormones important for the regulation of the long-term stress response, blood pressure and blood volume, nutrient uptake and storage, fluid and electrolyte balance, and inflammation. The HPA axis involves the stimulation of hormone release of adrenocorticotropic hormone (ACTH) from the pituitary by the hypothalamus. ACTH then stimulates the adrenal cortex to produce the hormone cortisol.

The adrenal medulla is the central portion of the adrenal gland composed of chromaffin cells, which are modified postganglionic sympathetic neurons. These cells secrete epinephrine and norepinephrine into the bloodstream, and have the general effect of preparing the body for fight or flight.  The adrenal medulla has features of both the neural and endocrine systems: it is made of modified neural tissue and its activity is regulated by a direct neural input, while the outputs are released into blood and act at distant sites by activating specific receptors.  The actions of the medullary hormones are far more rapid and short-lived than those of most "classic" hormones. 

One of the major functions of the adrenal gland is to respond to stress. Stress can be either physical or psychological or both. Physical stresses include exposing the body to injury, walking outside in cold and wet conditions without a coat on, or malnutrition. Psychological stresses include the perception of a physical threat, a fight with a loved one, or just a bad day at school.

The body responds in different ways to short-term stress and long-term stress following a pattern known as the general adaptation syndrome (GAS). Stage one of GAS is called the alarm reaction. This is short-term stress, the fight-or-flight response, mediated by the hormones epinephrine and norepinephrine from the adrenal medulla. Their function is to prepare the body for extreme physical exertion. Both epinephrine and norepinephrine signal the liver and skeletal muscle cells to convert glycogen into glucose, resulting in increased blood glucose levels. These hormones increase the heart rate, pulse, and blood pressure to prepare the body to fight the perceived threat or flee from it. In addition, the pathway dilates the airways, raising blood oxygen levels. It also prompts vasodilation, further increasing the oxygenation of important organs such as the lungs, brain, heart, and skeletal muscle. At the same time, it triggers vasoconstriction to blood vessels serving less essential organs such as the gastrointestinal tract, kidneys, and skin, and down regulates some components of the immune system. Other effects include a dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision.Once this stress is relieved, the body quickly returns to normal. 

If the stress is not relieved quickly, the body adapts to the stress in the second stage called the stage of resistance. If a person is starving for example, the body may send signals to the gastrointestinal tract to maximize the absorption of nutrients from food.

If the stress continues for a longer term however, the body responds with symptoms quite different than the fight-or-flight response. During the stage of exhaustion, individuals may begin to suffer depression, the suppression of their immune response, severe fatigue, or even a fatal heart attack. These symptoms are mediated by the hormones of the adrenal cortex, especially cortisol, released as a result of signals from the HPA axis.

Adrenal hormones also have several non–stress-related functions, including the increase of blood sodium and glucose levels, which will be described in detail below.

Hormones of the Adrenal Cortex

The adrenal cortex consists of multiple layers of lipid-storing cells that occur in three structurally distinct regions: the zona glomerulosa, the zona fasciculata, and the zona reticularis. Each region secretes its own set of hormones.

Hormones of the Zona Glomerulosa

The most superficial region of the adrenal cortex is the zona glomerulosa, which produces a group of hormones collectively referred to as mineralocorticoids because of their effect on body minerals, especially sodium and potassium. These hormones are essential for fluid and electrolyte balance.

Aldosterone is the major mineralocorticoid. It is important in the regulation of the concentration of sodium (Na⁺) and potassium (K⁺) ions in urine, sweat, and saliva. For example, it is released in response to elevated blood K⁺, low blood Na⁺, low blood pressure, or low blood volume. In response, aldosterone increases the excretion of K⁺ and the retention of Na⁺, which in turn increases blood volume and blood pressure. Its secretion is prompted when CRH from the hypothalamus triggers ACTH release from the anterior pituitary.

Aldosterone is also a key component of the renin-angiotensin-aldosterone system (RAAS) in which specialized cells of the kidneys secrete the enzyme renin in response to low blood volume or low blood pressure. Renin the triggers and enzymatic cascade that results in the production of angiotensin II, which has three major functions:

1. Initiating vasoconstriction of the arterioles, decreasing blood flow;
2. Stimulating kidney tubules to reabsorb NaCl and water, increasing blood volume;
3. Signaling the adrenal cortex to secrete aldosterone, the effects of which further contribute to fluid retention, restoring blood pressure and blood volume.

For individuals with hypertension, or high blood pressure, drugs are available that block the production of angiotensin II. These drugs, known as ACE inhibitors, block the enzyme that produces angiotensin II, thus mitigating the latter’s ability to increase blood pressure.

Hormones of the Zona Fasciculata

The intermediate region of the adrenal cortex is the zona fasciculata, named as such because the cells form small fascicles (bundles) separated by tiny blood vessels. The cells of the zona fasciculata produce hormones called glucocorticoids because of their role in glucose metabolism. The most important of these is cortisol, some of which the liver converts to cortisone.  In response to long-term stressors, the hypothalamus secretes CRH, which in turn triggers the release of ACTH by the anterior pituitary. ACTH triggers the release of the glucocorticoids. Their overall effect is to inhibit tissue building while stimulating the breakdown of stored nutrients to maintain adequate fuel supplies. In conditions of long-term stress, for example, cortisol promotes the catabolism of glycogen to glucose, the catabolism of stored triglycerides into fatty acids and glycerol, and the catabolism of muscle proteins into amino acids. The hippocampus, which is part of the temporal lobe of the cerebral cortices and important in memory formation, is highly sensitive to stress levels because of its many glucocorticoid receptors.

You are probably familiar with prescription and over-the-counter medications containing glucocorticoids, such as cortisone injections into inflamed joints, prednisone tablets and steroid-based inhalers used to manage severe asthma, and hydrocortisone creams applied to relieve itchy skin rashes. These drugs reflect another role of cortisol—the down regulation of the immune system, which inhibits the inflammatory response.

Hormones of the Zona Reticularis

The deepest region of the adrenal cortex is the zona reticularis, which produces small amounts of a class of steroid sex hormones called androgens. During puberty and most of adulthood, androgens are produced in the gonads. The androgens produced in the zona reticularis supplement the gonadal androgens. They are produced in response to ACTH from the anterior pituitary and are converted in the tissues to testosterone or estrogens. After menopause in those with ovaries, as the functions of the ovaries decline, the main source of estrogens becomes the androgens produced by the zona reticularis.

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

Adrenal gland region	Associated hormones	Effect
Adrenal cortex	Aldosterone	Increases blood Na^+ levels
Adrenal cortex	Cortisol, corticosterone, cortisone	Increase blood glucose levels
Adrenal medulla	Epinephrine, norepinephrine	Stimulate fight-or-flight response

DISORDERS OF THE...

Endocrine System: Disorders Involving the Adrenal Glands

Several disorders are caused by the dysregulation of the hormones produced by the adrenal glands. For example, Cushing’s disease is a disorder characterized by high blood glucose levels and the accumulation of lipid deposits on the face and neck. It is caused by hypersecretion of cortisol. The most common source of Cushing’s disease is a pituitary tumor that secretes cortisol or ACTH in abnormally high amounts. Other common signs of Cushing’s disease include the development of a moon-shaped face, a buffalo hump on the back of the neck, rapid weight gain, and hair loss. Chronically elevated glucose levels are also associated with an elevated risk of developing type 2 diabetes. In addition to hyperglycemia, chronically elevated glucocorticoids compromise immunity, resistance to infection, and memory, and can result in rapid weight gain and hair loss.

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.

Concept Review

The adrenal glands, located superior to each kidney, consist of two regions: the adrenal cortex and adrenal medulla. The adrenal cortex—the outer layer of the gland—produces mineralocorticoids, glucocorticoids, and androgens. The adrenal medulla, at the core of the gland, produces epinephrine and norepinephrine.

The adrenal glands mediate a short-term stress response and a long-term stress response. A perceived threat results in the secretion of epinephrine and norepinephrine from the adrenal medulla, which mediate the fight-or-flight response. The long-term stress response is mediated by the secretion of CRH from the hypothalamus, which triggers ACTH, which in turn stimulates the secretion of corticosteroids from the adrenal cortex. The mineralocorticoids, chiefly aldosterone, cause sodium and fluid retention, which increases blood volume and blood pressure.

Review Questions

Q. The adrenal glands are attached superiorly to which organ?

A. thyroid

B. liver

C. kidneys

D. hypothalamus

Answer

C

Q. What secretory cell type is found in the adrenal medulla?

A. chromaffin cells

B. neuroglial cells

C. follicle cells

D. oxyphil cells

Answer

A

Q. Cushing’s disease is a disorder caused by ________.

A. abnormally low levels of cortisol

B. abnormally high levels of cortisol

C. abnormally low levels of aldosterone

D. abnormally high levels of aldosterone

Answer

B

Q. Which of the following responses is not part of the fight-or-flight response?

A. pupil dilation

B. increased oxygen supply to the lungs

C. suppressed digestion

D. reduced heart rate

Answer

D

Critical Thinking Questions

Q. What are the three regions of the adrenal cortex and what hormones do they produce?

Answer

A. The outer region is the zona glomerulosa, which produces mineralocorticoids such as aldosterone; the next region is the zona fasciculata, which produces glucocorticoids such as cortisol; the inner region is the zona reticularis, which produces androgens.

Q. Compare and contrast the short-term and long-term stress response.

Answer

A. The short-term stress response involves the hormones epinephrine and norepinephrine, which work to increase the oxygen supply to organs important for extreme muscular action such as the brain, lungs, and muscles. In the long-term stress response, the hormone cortisol is involved in catabolism of glycogen stores, proteins, and triglycerides, glucose and ketone synthesis, and downregulation of the immune system.

Glossary

adrenal cortex

outer region of the adrenal glands consisting of multiple layers of epithelial cells and capillary networks that produces mineralocorticoids and glucocorticoids

adrenal glands

endocrine glands located at the top of each kidney that are important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels; also known as suprarenal glands

adrenal medulla

inner layer of the adrenal glands that plays an important role in the stress response by producing epinephrine and norepinephrine

angiotensin-converting enzyme

the enzyme that converts angiotensin I to angiotensin II

alarm reaction

the short-term stress, or the fight-or-flight response, of stage one of the general adaptation syndrome mediated by the hormones epinephrine and norepinephrine

aldosterone

hormone produced and secreted by the adrenal cortex that stimulates sodium and fluid retention and increases blood volume and blood pressure

chromaffin

neuroendocrine cells of the adrenal medulla

cortisol

glucocorticoid important in gluconeogenesis, the catabolism of glycogen, and downregulation of the immune system

epinephrine

primary and most potent catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called adrenaline

general adaptation syndrome (GAS)

the human body’s three-stage response pattern to short- and long-term stress

glucocorticoids

hormones produced by the zona fasciculata of the adrenal cortex that influence glucose metabolism

mineralocorticoids

hormones produced by the zona glomerulosa cells of the adrenal cortex that influence fluid and electrolyte balance

norepinephrine

secondary catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called noradrenaline

stage of exhaustion

stage three of the general adaptation syndrome; the body’s long-term response to stress mediated by the hormones of the adrenal cortex

stage of resistance

stage two of the general adaptation syndrome; the body’s continued response to stress after stage one diminishes

zona fasciculata

intermediate region of the adrenal cortex that produce hormones called glucocorticoids

zona glomerulosa

most superficial region of the adrenal cortex, which produces the hormones collectively referred to as mineralocorticoids

zona reticularis

deepest region of the adrenal cortex, which produces the steroid sex hormones called androgens

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology