The suprarenal glands, also known as the adrenal glands, are small, triangular-shaped organs that sit atop each kidney. These glands are part of the endocrine system, a network of glands that produce and release hormones directly into the bloodstream. Each suprarenal gland is comprised of two distinct parts: an inner region called the medulla and an outer region known as the cortex. The suprarenal cortex constitutes the larger, outer portion of the gland and is responsible for synthesizing a variety of steroid hormones. These hormones play a role in regulating numerous bodily processes, including metabolism, immune responses, and the balance of fluids and salts.
The Three Zones of the Cortex
The suprarenal cortex organizes into three distinct layers or zones, each with a unique cellular arrangement. These zones are arranged concentrically, starting from the outermost layer and moving inward towards the medulla. The outermost layer is the zona glomerulosa, which features small, polyhedral cells arranged in rounded clusters.
Beneath the zona glomerulosa lies the zona fasciculata, which is the thickest of the three cortical layers. This middle zone is characterized by cells arranged in long, straight cords or columns, separated by sinusoidal capillaries. These cells appear vacuolated due to lipid droplets, involved in hormone synthesis.
The innermost layer, adjacent to the suprarenal medulla, is the zona reticularis. This zone consists of smaller cells arranged in irregular, branching cords.
Cortical Hormones and Their Roles
Each zone of the suprarenal cortex produces specific classes of steroid hormones, each with distinct roles. The outermost zona glomerulosa is the primary site for the production of mineralocorticoids, with aldosterone being the main hormone in this class. Aldosterone plays a direct role in regulating blood pressure by managing the levels of sodium and potassium in the body. It acts on the kidneys to promote the reabsorption of sodium back into the bloodstream while increasing the excretion of potassium, thereby influencing water retention and overall blood volume.
The zona fasciculata is responsible for synthesizing glucocorticoids, predominantly cortisol. Cortisol is involved in the body’s stress response. It influences metabolism by increasing blood glucose levels through gluconeogenesis, and it also impacts the metabolism of fats and proteins. Cortisol also possesses anti-inflammatory properties and helps modulate the immune system.
The innermost zona reticularis produces gonadocorticoids, which are weak androgens like dehydroepiandrosterone (DHEA) and androstenedione. These serve as precursors for more potent sex hormones, including testosterone. In both males and females, these hormones contribute to the development of secondary sexual characteristics, particularly during puberty, and serve as a secondary source of sex hormones throughout life.
How Hormone Release is Controlled
Hormone production and release from the suprarenal cortex are precisely regulated by complex feedback systems. Cortisol release from the zona fasciculata is primarily controlled by the Hypothalamic-Pituitary-Adrenal (HPA) axis. When the body experiences stress, the hypothalamus releases corticotropin-releasing hormone (CRH), signaling the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH then travels to the suprarenal cortex, stimulating the zona fasciculata to produce and release cortisol.
Aldosterone release from the zona glomerulosa is largely managed by the Renin-Angiotensin-Aldosterone System (RAAS). This system activates when blood pressure, blood volume, or sodium levels decrease. The kidneys respond by releasing renin, which initiates a cascade leading to angiotensin II formation. Angiotensin II then directly stimulates the zona glomerulosa to synthesize and secrete aldosterone, helping restore blood pressure and electrolyte balance.
Consequences of Imbalance
When the suprarenal cortex produces too much or too little of its hormones, it can lead to various health conditions. Overproduction of cortisol can result in Cushing’s syndrome. Symptoms include weight gain, particularly around the midsection and face, easy bruising, and muscle weakness.
An excess of aldosterone, a condition known as Conn’s syndrome or primary hyperaldosteronism, can lead to high blood pressure that is resistant to typical treatments. This imbalance causes low potassium levels, resulting in muscle cramps, fatigue, and heart rhythm abnormalities.
Conversely, underproduction of suprarenal cortex hormones can lead to Addison’s disease, also known as adrenal insufficiency. This condition involves insufficient production of both cortisol and aldosterone. Symptoms of Addison’s disease can include chronic fatigue, muscle weakness, weight loss, low blood pressure, and characteristic darkening of the skin.