Adrenal GlandEdit

The adrenal glands are small, triangular endocrine organs perched atop the kidneys. They play a central role in managing stress, metabolism, electrolyte balance, and cardiovascular tone. Each gland weighs roughly a few grams and consists of two distinct regions that operate with different sets of hormones: the outer cortex and the inner medulla. The cortex manufactures steroids such as aldosterone, cortisol, and adrenal androgens, while the medulla releases catecholamines like epinephrine and norepinephrine in response to sympathetic stimulation. Hormone production is governed by signals from the brain and the kidneys, and it follows circadian rhythms and the body’s response to stressors.

Anatomy and histology - Location and structure: The adrenal glands sit on top of the kidneys within the retroperitoneal space. They are encapsulated and connected to the vascular system that carries blood to and from the kidneys. The two glands together form a key part of the endocrine axis that coordinates metabolism, fluid balance, and blood pressure. See Adrenal Gland. - Cortex and its zones: The cortex is derived from mesoderm and is divided into three zones with distinct hormone products: - zona glomerulosa: produces mineralocorticoids, chiefly aldosterone, which regulate sodium and potassium balance and influence blood pressure. See Zona Glomerulosa. - zona fasciculata: produces glucocorticoids, chiefly cortisol, which affect glucose metabolism, immune responses, and stress adaptation. See Cortisol. - zona reticularis: produces adrenal androgens such as dehydroepiandrosterone (DHEA) and androstenedione, contributing to androgenic effects, especially after puberty. See DHEA and Androstenedione. - Medulla: The inner core, derived from neural crest cells, houses chromaffin cells that secrete catecholamines—epinephrine (adrenaline) and norepinephrine—in response to acute stress. These hormones prepare the body for a rapid “fight-or-flight” response. See Adrenal Medulla and Epinephrine. - Vascularization and innervation: Blood supply comes from arteries that deliver arterial blood to the cortex and medulla, with drainage through specific adrenal veins. Sympathetic innervation via the splanchnic nerves modulates medullary hormone release. See Suprarenal arteries and Splanchnic nerves.

Hormones and functions - Mineralocorticoids: Aldosterone acts on the distal nephron to promote sodium reabsorption and potassium excretion, helping regulate extracellular fluid volume and blood pressure. This system is tightly connected to the renin–angiotensin–aldosterone axis. See Aldosterone and Renin–Angiotensin System. - Glucocorticoids: Cortisol exerts widespread effects on energy metabolism, immune function, and stress responsiveness. It also participates in maintaining vascular tone and helping the body adapt to long-term challenges. See Cortisol. - Adrenal androgens: DHEA and androstenedione contribute to androgen pools, particularly after puberty, and can influence secondary sexual characteristics and libido in some contexts. See DHEA and Androstenedione. - Catecholamines: Epinephrine and norepinephrine amplify heart rate, cardiac output, blood glucose mobilization, and blood flow to muscles during stress. See Epinephrine and Norepinephrine. - Regulation: Hormone release is integrated by the hypothalamic-pituitary-adrenal axis, with CRH from the hypothalamus stimulating ACTH release from the pituitary, which in turn drives cortisol synthesis in the cortex. Cortisol exerts negative feedback on both the pituitary and the hypothalamus. See Hypothalamus, Pituitary gland, and Hypothalamic-pituitary-adrenal axis.

Physiology and regulation - The hypothalamic-pituitary-adrenal axis: The cortex responds to ACTH, which is released in response to CRH. Cortisol then modulates energy metabolism and immune activity, while feeding back to limit further ACTH release. The axis is influenced by stress, circadian rhythms, and metabolic state. See ACTH and CRH. - Electrolyte and fluid balance: Aldosterone’s effect on renal salt handling interacts with the kidneys’ control of volume status and blood pressure, aided by the renin–angiotensin system. See Aldosterone and Renin–Angiotensin System. - Stress response: The medulla’s catecholamines are released rapidly during acute stress, working in concert with cortisol to preserve energy, maintain hemodynamic stability, and prepare the body for action. See Epinephrine and Norepinephrine.

Clinical significance - Adrenal insufficiency: Primary adrenal insufficiency (Addison’s disease) arises from autoimmune destruction or other damage to the cortex, resulting in insufficient cortisol and often aldosterone. Symptoms include fatigue, weight loss, and low blood pressure; treatment relies on hormone replacement. See Addison's disease. - Secondary adrenal insufficiency: Pituitary or hypothalamic disease can reduce ACTH production, leading to cortisol deficiency with relatively preserved mineralocorticoid function. See Secondary adrenal insufficiency. - Cushing’s syndrome and disease: Excess cortisol activity can stem from endogenous overproduction or exogenous steroid use. Clinical features include weight gain, glucose intolerance, and high blood pressure. See Cushing's syndrome. - Hyperaldosteronism: Primary aldosteronism (Conn’s syndrome) results in excess aldosterone, with hypertension and hypokalemia. See Hyperaldosteronism. - Pheochromocytoma: A tumor of the adrenal medulla can cause episodic or sustained hypertension, headaches, sweating, and tachycardia due to excess catecholamines. See Pheochromocytoma. - Congenital adrenal hyperplasia: Genetic defects in steroidogenic enzymes, especially 21-hydroxylase deficiency, disrupt cortisol and aldosterone production and shift precursors toward androgens. See Congenital adrenal hyperplasia. - Adrenal incidentalomas and tumors: Benign or malignant growths in the gland may require evaluation for hormone excess or malignancy. See Adrenal incidentaloma. - Therapeutic approaches: Management ranges from hormone replacement therapy for insufficiency to surgical removal of adrenal tumors, depending on the condition. See Adrenalectomy and Hormone replacement therapy.

Development and evolution - Embryology: The adrenal cortex develops from mesoderm, while the adrenal medulla originates from neural crest cells. This dual origin underpins the distinct hormone profiles of the cortex and medulla. See Embryology. - Evolutionary notes: The adrenal system is a vertebrate adaptation that supports energy mobilization and rapid responses to environmental challenges, contributing to survival across diverse ecological niches. See Evolutionary biology.

See also - Adrenal Gland - Adrenal Cortex - Adrenal Medulla - Zona Glomerulosa - Zona Fasciculata - Zona Reticularis - Aldosterone - Cortisol - DHEA - Androstenedione - Epinephrine - Norepinephrine - Pheochromocytoma - Addison's disease - Cushing's syndrome - Congenital adrenal hyperplasia - Renin–Angiotensin System - Hypothalamus - Pituitary gland - Hypothalamic-pituitary-adrenal axis