With this post, we introduce you to the adrenal glands. We will review the anatomy and function of these glands as well as the functions of the hormones they secrete. Although these glands are small, they are essential for life, and the hormones they secrete regulate complex metabolic processes in every cell of our bodies. Indeed, our health and well-being depend on these little glands.
We have two adrenal glands, located high in the abdomen, just above each kidney, and just below the diaphragm. These glands are named for their location near the kidneys (“ad” is Latin for “near” and renes is Latin for “kidney”) but they function completely separately from the kidneys. The adrenal glands have a conical shape and sit atop the kidneys much like a child’s conical birthday party hat. These glands make several hormones that are essential for sustaining life and good health, and a variety of medical problems arise when these glands malfunction.
If one views the adrenal gland in cross-section, 3 layers become apparent. The outer layer is a fibrous shell called the capsule. Beneath the capsule is a middle layer called the cortex. Below the cortex, in the core of the gland, is the medulla. The middle layer, the cortex, contains 3 sub-layers, or zones. Starting from the outer sub-layer and moving towards the core, one finds the zona glomerulosa, the zona fasciculata, and the zona reticularis. Each zone of the cortex, and the inner core, the medulla medulla, synthesize and secrete specific hormones. The zona glomerulosa secretes a steroid hormone called aldosterone, necessary for maintaining proper sodium and potassium balance in the blood and maintaining blood volume. The zona fasciculata secretes a steroid hormone called cortisol, necessary for synchronizing metabolic processes throughout the whole body to the 24-hour circadian cycle and for surviving stress of various kinds. The zona reticularis secretes a steroid hormone called dehydroepiandrosterone (commonly known by the acronym DHEA), a hormone with weak testosterone-like properties that mediates the growth of secondary body hair (such as pubic and axillary hair) when children pass through puberty. DHEA also serves as a precursor hormone for the synthesis of both testosterone and estrogen. And finally, the inner core of the gland, the medulla, secretes the catecholamine hormones epinephrine (about 80%) and norepinephrine (about 20%). The catecholamine hormones are also important mediators of the stress response.
Perhaps the most and important adrenal hormone is cortisol. Cortisol probably made its first evolutionary appearance more than 500 million years ago, before the dawn of the most primitive vertebrates, like the lamprey. The first steroid hormone to evolve was probably estrogen, and cortisol evolved from estrogen. Like estrogen, cortisol is now found in all vertebrates, all fish, amphibians, reptiles, birds, and mammals. Cortisol has been conserved through 500 million years of evolution because it performs multiple essential functions. First, it synchronizes metabolism throughout the whole organism to the 24-hour circadian cycle. Cortisol levels rise and fall every 24 hours and keep all of life’s processes in sync. Second, cortisol is essential for surviving the fundamental stresses of life in the wilderness, like starvation, low blood sugar, hypothermia, low blood pressure, infection, and the effects of extreme exertion. The other steroid hormones made by the adrenal gland, including aldosterone and dehydroepiandrosterone came much later during evolution.
The adrenal medulla is really an extension of the sympathetic nervous system. The tips of sympathetic nerve fibers release epinephrine and norepinephrine to cause immediate effects under the control of the brain. These same signaling chemicals, epinephrine and norepinephrine, are released directly into the bloodstream by the adrenal medulla so that these signals can circulate widely and reach tissues that do not receive nerve signals directly. The epinephrine and norepinephrine signaling is also essential for stress survival. These mediators increase the heart rate and the force of heart contraction, they constrict blood vessels and elevate blood pressure, they dilate the airways to improve lung ventilation, they release fats and sugars from storage depots to provide metabolic fuel, and they slow down “nonessential” functions like the gastrointestinal system.
The effects of epinephrine and norepinephrine, released by the adrenal medulla, are immediate. However, the effects of cortisol, released by the adrenal cortex, can play out over weeks and months. Acting together, these hormones keep our bodies synchronized to the circadian cycle and allow us to survive short-term and long-term stress.
Several disorders of the adrenal glands can lead to overproduction of certain hormones, underproduction of certain hormones, or production of hormones at the wrong time of day. Diagnoses encountered in an adrenal endocrine practice include: Adrenal Insufficiency, Adrenal Fatigue, Cushing Syndrome, Addison’s Disease, Conn Syndrome, Idiopathic Hyperaldosteronism, Congenital Adrenal Hyperplasia, Adrenal Adenoma, Adrenal Carcinoma, and many others. We will discuss the diagnosis and treatment of these and other disorders in future posts. If you have questions or concerns about your adrenal gland function, consider consulting an experienced endocrinologist with expertise in adrenal disease. Diagnostic and therapeutic regimens can sometimes be quite complex, but an expert in adrenal disease will lead you to a correct diagnosis and an optimal treatment plan.
Ó David R. Brown, M.D., Ph.D.
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