The adrenal glands, also known as the suprarenal glands, are small, triangular-shaped organs situated directly on top of each kidney. These glands are composed of two distinct regions: the outer adrenal cortex and the inner adrenal medulla. The adrenal medulla is the central core of the gland, specializing in the rapid release of hormones integral to the body’s stress response. The medulla is essentially a neuroendocrine organ, bridging the gap between the nervous system and the endocrine system. This allows it to secrete powerful chemical messengers directly into the bloodstream, enabling fast, widespread physiological change.
The Hormones Produced by the Adrenal Medulla
The adrenal medulla synthesizes and secretes a class of compounds known as catecholamines. The two primary hormones produced are epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones are created and stored within specialized secretory cells called chromaffin cells, which make up the bulk of the medulla tissue.
The synthesis begins with the amino acid tyrosine, which is converted through multiple enzymatic steps into dopamine, then norepinephrine, and finally, epinephrine. Epinephrine constitutes the majority of the total catecholamine output, typically accounting for about 80 percent of the hormones released.
Sympathetic Control and Release Mechanism
The adrenal medulla is unique among endocrine glands because it receives direct nervous system input, bypassing the typical two-neuron chain of the sympathetic nervous system. Chromaffin cells are considered modified postganglionic sympathetic neurons that have lost their axons and instead secrete hormones directly into the blood vessels.
The signal for release originates from preganglionic sympathetic nerve fibers that travel via the splanchnic nerve to the medulla. Upon activation, these nerve fibers release the neurotransmitter acetylcholine directly onto the chromaffin cells. Acetylcholine binds to receptors on the chromaffin cell membrane, triggering a cascade that leads to the rapid exocytosis of stored catecholamines into the bloodstream.
Systemic Impact of the “Fight or Flight” Response
The primary function of the hormones released by the adrenal medulla is to mobilize the body’s resources for immediate, high-intensity action, known as the “fight or flight” response. Epinephrine and norepinephrine circulate as hormones, binding to adrenergic receptors throughout the body to coordinate a widespread physiological shift.
Cardiovascular Effects
In the cardiovascular system, these hormones maximize blood flow to the muscles and brain. Epinephrine increases both the heart rate (tachycardia) and the force of contraction, significantly boosting cardiac output. Norepinephrine is a powerful vasoconstrictor, causing blood vessels in less essential areas like the skin, kidneys, and digestive tract to narrow.
Metabolic Effects
The metabolic effects are important for fueling the sudden increase in physical activity. Epinephrine stimulates the liver to break down stored glycogen into glucose (glycogenolysis) and promotes the creation of new glucose from non-carbohydrate sources (gluconeogenesis). The hormones also stimulate lipolysis in adipose tissue, releasing fatty acids into the bloodstream to serve as an additional energy substrate.
Respiratory and Other Effects
In the respiratory system, circulating catecholamines cause the smooth muscles surrounding the bronchioles to relax, a process called bronchodilation. This widening of the airways maximizes oxygen uptake to meet the increased metabolic demand. Simultaneously, the response causes the pupils to dilate, enhancing visual acuity. To conserve energy, the adrenal hormones suppress bodily functions not immediately required for survival, such as slowing the activity of the digestive system and temporarily inhibiting urine production.
Conditions Related to Medulla Dysfunction
Dysfunction of the adrenal medulla typically involves the overproduction of catecholamines, as isolated underproduction is rare and often compensated for by the nervous system. The most clinically significant condition of hyperfunction is a pheochromocytoma, a tumor of the chromaffin cells.
The constant or episodic high levels of catecholamines lead to severe symptoms reflecting a chronic “fight or flight” state. Patients often experience sustained or fluctuating high blood pressure (hypertension), which can be difficult to control. Other common manifestations include severe headaches, excessive sweating (diaphoresis), and episodes of a rapid, forceful heartbeat (palpitations). If left untreated, the resulting severe hypertension can lead to serious complications, including stroke or heart attack.