Definition
The adrenal gland has several metabolic roles in the human body, which it completes by releasing hormones. The two adrenal glands within the body release hormones that help control the metabolism, undergo sexual maturation as we grow, and respond to stress.
Overview
The stress response is 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 hormone coordinating the processes that allow us to fight or run from this perceived danger. There is 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 from the adrenal gland. 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, precursors of sex hormones. We will discuss each in more detail.
Adrenal Gland Function
The most unique function of the adrenal glands is their ability to orchestrate responses to stressors. 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 Location
Our two adrenal glands can be found lying on top of each of our kidneys. Here, they can release hormones into the bloodstream which stimulate various aspects of the stress response.
The image above 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 Mineralocorticoids (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 Hormones
Like any endocrine gland, the beauty of the adrenal glands lies in its secretions. In general, the adrenal gland will produce several types of hormone: androgens, cortisol, aldosterone, and norepinephrine.
Androgens
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 like the changes that occur once puberty starts, including bodily changes like pubic hair growth, Adam’s apple formation, and muscle and hair growth. Females also use androgens but they are instead secreted by the ovaries and repurposed into estrogen hormone.
Aldosterone
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, which breaks down into small ionized atoms within a solution. Salt is able to modulate the levels of fluid in our vessels, directly affecting the 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 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.
Cortisol
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 fully discussed 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 increased 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 Disorders
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. Tumor symptoms can include a variety of symptoms, typically related to the release or blockage of hormones.
There are various kinds of malignant 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 of 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 overproduction of cortisol may be an adenoma, or benign tumor anywhere on the adrenal gland, or long-term use of corticosteroid medications like prednisone.
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