Function of the Adrenal Glands
Each adrenal gland has an outer region, called the cortex, and an inner region, called the medulla. Each of these regions contains highly specialized cells that secrete distinct hormones to carry out different physiologic functions (PubMed Health 2011a).
- The adrenal cortex secretes 3 types of hormones: glucocorticoids, mineralocorticoids, and androgens.
- Glucocorticoids (eg, cortisol) control inflammation and regulate the body’s response to infections and stress. They also play a role in maintaining blood pressure, blood sugar, and cardiovascular function (Charmandari 2010).
- Mineralocorticoids (eg, aldosterone) regulate sodium and potassium levels in the body and thereby help maintain blood pressure and water balance, mainly via the kidneys (Farman 2001).
- Adrenal androgens (eg, dehydroepiandrosterone [DHEA]) are precursors of the sex hormones testosterone and estrogen (Gurnell 2008). In addition, the adrenal glands also produce a small amount of testosterone (PubMed Health 2011a).
The adrenal medulla produces the catecholamine hormones, which compriseepinephrine (also known as adrenaline), norepinephrine (noradrenaline), and dopamine (PubMed Health 2011a). Norepinephrine and epinephrine are primarily responsible for the “fight or flight” response to stress or fear (Arun 2004).
The “fight or flight” response manifests as increased heart rate and blood pressure, rapid breathing, and greater blood flow to muscles (Head 2009; Molina 2005). These physiologic responses arise via activation of the sympathetic nervous system. The sympathetic nervous system is a part of the involuntary nervous system, which controls processes such as breathing, heart rate, and metabolism (PubMed Health 2012; Molina 2005).
A precisely regulated relationship exists between the adrenal hormones and hormones secreted by the hypothalamus (a small region located at the center of the brain) and the pituitary gland (a pea-shaped structure located at the base of the brain). These 3 structures influence one another and collectively comprise the hypothalamic-pituitary-adrenal (HPA) axis (Bonfiglio 2011; Head 2009). The HPA axis is crucial to the regulation of a variety of physiologic functions including the body’s response to stress. For example, one of the actions of the hypothalamus is to direct the pituitary gland to release adrenocorticotropic hormone (ACTH), which regulates the production and secretion of hormones from the adrenal cortex.
Under normal healthy conditions, the secretion of hypothalamic, pituitary, and adrenal cortex hormones is finely controlled by each of the other glands (Head 2009). For instance, increasing cortisol levels signal the pituitary to reduce ACTH secretion, which in turn decreases cortisol secretion (Head 2009; Bonfiglio 2011). Under chronic stress or disease conditions, however, this feedback system can become imbalanced (Anagnostis 2009; Foley 2010).
Impaired adrenal function
Impaired function of the adrenal glands may lead to either increased or decreased production of adrenal hormones. Cushing’s syndrome and Addison’s disease are conditions characterized by abnormal adrenal function.
In Cushing’s syndrome, blood levels of cortisol remain high over an extended period of time and cause characteristic changes in the body (Pivonello 2008; Bertagna 2009). People with Cushing’s typically have a rounded “moon” face, gain weight around the trunk, and have slender arms and legs. Their skin is often thin and can have a bruised appearance with stretch marks. Other features include muscle weakness, susceptibility to infection, elevated blood sugar levels (hyperglycemia), and weak bones (osteoporosis). These changes are often accompanied by mood disorders such as anxiety and depression. In children, excess cortisol can lead to stunted growth. Furthermore, men can exhibit reduced fertility and libido, while women can exhibit hirsutism (abnormal hair growth on face, neck, thighs, and chest) and menstrual disorders (Pozza 2012; Stratakis 2008). Excess secretion of adrenal androgens may also lead to virilization (presence of external male characteristics in females or in boys before puberty) (Stratakis 2008).
Addison’s disease is an uncommon, debilitating disease that is rarely identified in its early stages. In Addison’s disease, the function of the adrenal cortex progressively declines over time, resulting in glucocorticoid and mineralocorticoid deficiency, as well as reduced levels of DHEA and androgens (Nieman 2006; Ten 2001; Gebre-Medhin 2000; Luken 1999). The typical early symptoms of Addison’s disease are weakness, low blood pressure upon standing, and fatigue. People with Addison’s disease gradually develop an often heavy pigmentation of the skin (especially around bony prominences, skin folds, and on the back of arms and legs) and a bluish discoloration of the mucous membrane lining the mouth (Ten 2001). Cortisol and aldosterone deficiency together cause changes in blood levels of sodium and potassium and a decrease in plasma volume, which can lead to extreme dehydration and shock (Padidela 2010). Trauma, surgery, and infections in people with reduced adrenal function may result in adrenal crisis, a life-threatening condition that can lead to extreme weakness, severe body pain, low blood pressure, and fever (Mattke 2002; Omori 2003).