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.  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.)

Figure \(\PageIndex{2}\):The Adrenal Gland slide.  The adrenal gland is locacted on top of each kidney.  The histolgy 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.) Copy and Paste Caption here. (Copyright; author via source)

 

STRESS!

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.

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 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.

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. These cords of cells are oriented perpendicular to the surface of the organ.  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 hypothalamic-pituitary-adrenal (HPA) axis activates: 1) the hypothalamus secretes CRH, which 2) triggers the release of ACTH by the anterior pituitary, and 3) ACTH triggers the release of the glucocorticoids. The 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 (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 central portion of the adrenal gland composed of chromaffin cells, which are modified postganglionic sympathetic neurons. The adrenal medulla develops from neural crest cells, which are also the origin of sympathetic ganglia. 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. 

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.

Figure \(\PageIndex{3}\): 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.)

 

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

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

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