The adrenal cortex is the outer part of the adrenal glands, small, triangular-shaped glands located on top of each kidney. It produces and releases hormones involved in regulating metabolism, immune responses, blood pressure, and stress reactions. The cortex is organized into distinct layers, each with its own specific hormonal output, which is fundamental to how the body maintains its internal balance.
Anatomy and Layers of the Adrenal Cortex
The adrenal gland is composed of two distinct parts: the inner medulla and the outer cortex, which is the larger section. The cortex is structurally organized into three concentric zones, each with a unique cellular arrangement and function.
The outermost layer, situated just beneath the gland’s capsule, is the zona glomerulosa, with cells arranged in dense, oval-shaped clusters. Deep to this is the zona fasciculata, the thickest of the three layers. The cells in this middle zone are arranged in long, straight columns or bundles.
The innermost layer, which lies adjacent to the adrenal medulla, is the zona reticularis, characterized by a network of branching cell cords. The appearance of each zone reflects its specialized role in steroid hormone production, a process known as steroidogenesis, which begins with cholesterol.
Hormone Production and Function
The zona glomerulosa produces mineralocorticoids, with aldosterone being the principal hormone in this group. Aldosterone’s primary function is to regulate the balance of sodium and potassium in the blood. It acts on the kidneys to increase sodium reabsorption and potassium excretion, which helps manage blood volume and maintain stable blood pressure.
The middle layer, the zona fasciculata, produces glucocorticoids, the most prominent of which is cortisol. Cortisol is often called the “stress hormone” because its levels increase in response to physical or psychological stress, triggering a rise in blood sugar to provide the body with immediate energy. Beyond the stress response, cortisol plays a part in regulating the metabolism of proteins, fats, and carbohydrates, suppressing inflammation, and modulating the sleep-wake cycle.
The innermost layer, the zona reticularis, is the site of production for adrenal androgens. These are primarily weak male sex hormones, such as dehydroepiandrosterone (DHEA) and androstenedione. In both sexes, these hormones serve as precursors that can be converted into more potent sex hormones, like testosterone and estrogen, in other tissues throughout the body, such as the gonads.
Regulation of Adrenal Hormones
The release of hormones from the adrenal cortex is controlled by feedback systems. The regulation of cortisol is managed by the Hypothalamic-Pituitary-Adrenal (HPA) axis. This system begins when the hypothalamus releases corticotropin-releasing hormone (CRH) in response to stress or low cortisol levels. CRH then travels to the pituitary gland, stimulating it to secrete adrenocorticotropic hormone (ACTH).
ACTH travels through the bloodstream to the adrenal cortex, where it signals the zona fasciculata to produce and release cortisol. As cortisol levels rise in the blood, they exert a negative feedback effect on both the hypothalamus and the pituitary gland, inhibiting the release of CRH and ACTH. This self-regulating loop ensures that cortisol levels are kept within a narrow, healthy range.
The regulation of aldosterone involves the Renin-Angiotensin-Aldosterone System (RAAS). This system is activated when the kidneys detect a drop in blood pressure or a decrease in sodium levels. In response, the kidneys release an enzyme called renin. Renin initiates a cascade of events, leading to the production of angiotensin II, which stimulates the zona glomerulosa to secrete aldosterone. Aldosterone then acts on the kidneys to conserve sodium and water, helping to restore blood volume and pressure.
Disorders of the Adrenal Cortex
Imbalances in adrenal cortex hormone production can lead to health problems. When the adrenal cortex produces an excess of cortisol, it results in Cushing’s Syndrome. Common signs of this disorder include:
- Significant weight gain, particularly in the face and abdomen
- High blood pressure
- Muscle weakness
- Easy bruising
An overproduction of aldosterone leads to Conn’s Syndrome, or primary hyperaldosteronism. This condition causes the body to retain excess sodium and lose potassium, leading to high blood pressure. It can also cause symptoms like muscle cramps and weakness.
When the adrenal cortex fails to produce sufficient hormones, the condition is called adrenal insufficiency. The most comprehensive form is Addison’s Disease, a deficiency in both cortisol and aldosterone. Symptoms can include:
- Chronic fatigue
- Weight loss
- Low blood pressure (hypotension)
- Patches of darkened skin
This condition requires lifelong hormone replacement therapy.