Adrenal Gland Definition
The adrenal gland has several metabolic roles in the human body. Our two adrenal glands can be found lying on top of each of our kidneys. Together, they release the hormones that help us metabolize, undergo sexual maturation as we grow, as well as respond to stress. The latter may be known to us as the primal “fight or flight” response. It is the knee-jerk reaction we get upon seeing a life-endangering stimulus. Cortisol is the key to coordinating the processes that allow us to fight or run from this perceived danger. There is an obvious evolutionary value in being able to do this. Anything that shortens the time in which we can engage our muscles and run will likely increase our chance of survival. But even more low key scenarios trigger the release of cortisol. A feeling we may be familiar with is the intense focus we feel when studying for an exam the night before we have it. Cortisol is largely responsible for this newfound focus. In spite of all of the negative connotations associated with stress, in normal amounts the stress hormones allow us to focus better to perform the tasks at hand.
Thus, the adrenal glands play a functional role in our alertness, growth, and more. The main products released by the adrenal glands are cortisol, epinephrine, aldosterone, and adrenal androgens, or sex hormones. We will discuss each in more detail.
The image depicts a simple illustration of the adrenal glands and surrounding structures.
Adrenal Gland Structure
Our adrenal glands are two organs shaped in the form of a triangle that spans three inches in length. When a biopsy is taken of the adrenal gland, two sections are immediately visualized. The outermost layer of the adrenal gland is called the adrenal cortex, while the inner layer is named the adrenal medulla. Besides having physical differences that give them a distinct appearance, they release hormones independently of each other. In fact, the cortex contains zones of different cell types starting with the outermost “shell” or capsule, followed by “zones” named the zona glomerulosa, zona fasciculate, and the zona reticularis. This division facilitates some versatility. The medulla will secrete epinephrine in response to emotional or physical stress, while the outer adrenal cortex will make steroids and metabolic hormones like aldosterone and cortisol. However, it is safe to say that there is a lot of overlap between their functions.
Adrenal Cortex Zones:
- Zona Glomerulosa: Secretes Mineralcorticoids (i.e. cortisol)
- Zona Fasiculata: Secretes Glucocorticoids
- Zona Reticularis: Secretes Androgens
The adrenal gland is supplied by three arteries: the superior suprarenal artery, a branch of the inferior phrenic artery, and the middle suprarenal artery which directly branches off of the abdominal aorta. As for the nerve supply, the adrenal glands are innervated by the sympathetic thoracic spinal cord fibers.
Adrenal Gland Function
The most unique function of the adrenal glands is their ability to orchestrate responses in light of stress. This has the most primal purpose of helping us evade danger and, therefore, of prolonging our lifespan. However, the adrenal gland is also involved in more mundane, but equally important, aspects of our everyday life.
Everyday roles of the adrenal glands:
- Via aldosterone, which we will discuss in more detail below, the adrenal gland allows our kidneys to regulate blood pressure via salt and water exchange between the kidneys and our surrounding blood vessels. In the absence of aldosterone, the kidney will lose lots of salt to urine which will draw water from our vessels and remove it from our system. This will most certainly lead to dehydration.
- Via cortisol, the body is not only able to respond to stress in potentially life-saving circumstances, but it will also help us regulate our body’s metabolism by initiating glucose production and by circulating fatty acids and amino acids to our cells.
- Via adrenal androgens, the adrenal gland helps create differences between the sexes by initiating the development of our sex organs and secondary traits.
Adrenal Gland Hormones
Like any endocrine gland, the beauty of the adrenal glands lies in its secretions. In general, the endocrine gland will produce several types of hormone: androgens, cortisol, aldosterone, and norepinephrine. Let us begin with a discussion of androgens. The zona reticularis of the adrenal cortex is responsible for releasing androgen hormones that help give rise to secondary sex traits in human males. “Secondary” traits can be thought of as the changes that occur once puberty starts, including bodily changes like pubic hair growth, the Adam’s apple, and muscle and hair growth. Females also use androgens but they are instead secreted by the ovaries and repurposed into estrogen hormone.
Aldosterone, which is released by the zona glomerulosa of the cortex, plays a huge role in our kidneys. Our kidneys can be thought of as big filters that will help us excrete waste and excess fluid from our cells and blood vessels, while allowing us to reabsorb the ions that we need to maintain an ionic balance and a good blood pressure. One of these important components is salt, or Na+Cl–. Salt is able to modulate the levels of fluid in our vessels, otherwise known as our blood pressure! Based on simple rules of diffusion, having more salt reabsorbed will lead water to be reabsorbed into our vessels in greater amounts, as well, since “water follows salt.” Aldosterone is thus able to directly modulate the excretion of salt by either increasing or decreasing the number of salt (Na/Cl) channels in the walls of our nephrons (kidneys). The presence of aldosterone will encourage our blood vessels to retain more salt via numerous salt channels, which in turn encourages the reabsorption of water. This results in concentrated urine, and a healthy blood pressure. When this process is compromised, as with Addison’s disease, insufficient amounts of cortisol and aldosterone are made when the body is under stress – such as when fighting off an infection. Its symptoms vary from fatigue, to dizziness and chronic nausea secondary to low blood pressure and low salt levels.
The zona fasciculata, in turn, creates the stress hormones we have discussed before: cortisol and its derivatives. Cortisol is inherently a steroid hormone that responds to stressful situations and when our blood sugar drops too low. In effect, it will stimulate gluconeogenesis, or glucose creation, to counter the low blood sugar and will aid in the metabolic breakdown of food. It also suppresses the immune system and decreases bone formation. Healthy amounts of cortisol increase our focus; however, chronic overexpression of cortisol has many scientists worried for fear of memory interference, bone density loss, and heart disease in affected patients.
We have discussed plenty about the adrenal cortex, but it is important to note that the adrenal medulla uniquely makes epinephrine and norepinephrine. These water-soluble compounds are the ones responsible for giving us a “rush” whenever we are confronted with a stressful situation. Its effects are characterized by a fastened breathing and heart rate, and a constriction of blood vessels that redirects blood flow to our muscles. This allows our muscles to immediately engage for quick movement.
Adrenal Gland Tumor
Since the adrenal gland is constantly producing hormones that are vital and well-circulated in the body, there is a chance of risking an imbalance. For example, an overactive gland or even a benign tumor on the adrenal gland will cause it to make too much cortisol. This will disrupt our blood pressure, our heart health, and even our body’s response to stress. Adrenal gland tumors can be benign or malignant.
There are various kinds of malignant, or “cancerous,” gland tumors.
- Adrenocortical cancer will originate in the adrenal cortex. There are two types. The functioning tumor is the most common and will continue to produce cortisol, aldosterone, and androgens. Non-functioning tumors will, on the other hand, not produce hormones which will result in other deficiencies.
- Adrenal Pheochromocytomas originate in the adrenal medulla and are very rare.
- Adrenal Paragangliomas begin within or outside the adrenal gland.
A well-studied benign adrenal gland tumor is commonly known as Cushing’s Syndrome. This syndrome will overproduce cortisol, and thus disrupt heart function and the processes that the body engages in when responding to a stressful stimulus. Cushing’s syndrome is quite rare, with only two-to-four new cases per one million Americans each year. Other causes for an overproduction of cortisol may be an adenoma, or benign tumor anywhere on the adrenal gland, or long-term use of corticosteroid medications like prednisone.
1. In which of the following way does Aldosterone perform its roles in the human body?
A. It acts directly on water channels to stimulate water retention
B. It works via calcium ions that increase water absorption
C. It acts on salt by increasing the number of sodium channels in our nephrons to facilitate water retention
D. It acts on salt by decreasing the number of sodium channels in our nephrons to facilitate water retention
2. A lack of Aldosterone will result in which of the following conditions?
A. Too much water retention
B. A dehydrated state
C. Heightened blood pressure
D. Stimulates overproduction of glucose
3. Correctly identify the zone of the adrenal cortex that releases cortisol:
A. Zona reticularis
B. Zona fasciculata
C. Zona glomerulosa
D. Does not apply, zones are found in adrenal medulla
- Medline Plus (2017). “Adrenal Gland Disorders.” US National Library of Medicine NIH. Retrieved on 2017-07-23 from https://medlineplus.gov/adrenalglanddisorders.html
- Sargis, Rober MD, PhD. “An Overview of the Adrenal Glands.” Endocrine Web. Retrieved on 2017-07-22 from https://www.endocrineweb.com/endocrinology/overview-adrenal-glands
- MD Anderson Cancer Center. “Adrenal Tumors.” The University of Texas. Retrieved on 2017-07-22 from https://www.mdanderson.org/cancer-types/adrenal-tumors.html
- Cancer.net Editorial Board (2016). “Adrenal Gland Tumor: Introduction.” Cancer.Net. Retrieved on 2017-07-23 from http://www.cancer.net/cancer-types/adrenal-gland-tumor/introduction
- Aviva (2017). “Structure and Function: Adrenal Glands.” Aviva. Retrieved on 2017-07-23 from https://www.aviva.co.uk/health-insurance/home-of-health/medical-centre/medical-encyclopedia/entry/structure-and-function-adrenal-glands/
- You and your hormones (2017). “Adrenal Glands.” Your Hormones Info.. Retrieved on 2017-07-24 from http://www.yourhormones.info/glands/adrenal-glands/