The Adrenal Glands are paired endocrine organs situated on the superior poles of the kidneys. Each gland has an outer cortex and an inner medulla that differ substantially in embryological origin, structure and hormonal function. The adrenal cortex produces steroid hormones including cortisol, aldosterone and adrenal androgens, while the adrenal medulla produces catecholamines, principally adrenaline and noradrenaline.
The cortex is divided into three major zones known as the zona glomerulosa, zona fasciculata and zona reticularis, each specialised for different steroidogenic pathways. Aldosterone is produced mainly by the zona glomerulosa, cortisol by the zona fasciculata and adrenal androgens by the zona reticularis. The medulla contains chromaffin cells that are functionally related to sympathetic nervous system cells. This organisation allows the adrenal glands to coordinate long term endocrine regulation with rapid responses to acute stress.
Cortisol is a glucocorticoid hormone that affects carbohydrate, protein and lipid metabolism, immune activity, vascular responsiveness and adaptation to stress. Its secretion is controlled primarily by the hypothalamic pituitary adrenal axis. Corticotropin releasing hormone from the hypothalamus stimulates adrenocorticotropic hormone secretion from the pituitary, which stimulates the adrenal cortex to produce cortisol. Rising cortisol concentrations exert negative feedback on the hypothalamus and pituitary.
Cortisol secretion follows a circadian rhythm, usually reaching higher concentrations around the beginning of the active period and declining later, although individual timing varies. Stress, illness and other physiological demands can alter this rhythm. Cortisol helps maintain blood glucose and cardiovascular responsiveness during physiological stress, but chronic excessive exposure can produce widespread metabolic and immune effects.
Aldosterone is a mineralocorticoid that regulates sodium retention and potassium excretion, thereby contributing to extracellular fluid volume and blood pressure. Its secretion is controlled primarily by the renin angiotensin aldosterone system and by extracellular potassium concentration rather than by adrenocorticotropic hormone. Renin initiates a cascade producing angiotensin II, which stimulates aldosterone secretion from the zona glomerulosa. Aldosterone acts mainly on the distal nephron and collecting duct, increasing sodium reabsorption and promoting potassium secretion.
Water follows retained sodium, influencing extracellular fluid volume. Excess aldosterone can contribute to hypertension and low potassium concentrations, whereas inadequate aldosterone can contribute to salt wasting, low blood pressure and hyperkalaemia. Because aldosterone and cortisol have different regulatory systems despite both being adrenal cortical steroids, endocrine evaluation must distinguish their physiological pathways.
The adrenal medulla forms part of the sympathetic stress response. Sympathetic preganglionic neurons stimulate chromaffin cells, causing rapid release of adrenaline and noradrenaline into the circulation. These catecholamines increase cardiac activity, redistribute blood flow, influence airway calibre and mobilise energy substrates during acute stress. Adrenaline has particularly strong effects on cardiac and metabolic responses, while noradrenaline has important vascular effects. Catecholamine release is rapid compared with cortisol secretion and can occur within seconds of acute stimulation. This provides a useful physiological division in which the nervous system and adrenal medulla coordinate immediate responses, while the hypothalamic pituitary adrenal axis contributes to more sustained endocrine adaptation.
Adrenal cortical disorders include adrenal insufficiency and hormone excess syndromes. Primary adrenal insufficiency results from inadequate adrenal cortical hormone production and can affect cortisol and aldosterone production. Autoimmune destruction is an important cause in many populations, while infection, haemorrhage, genetic disorders and other processes can also damage the cortex. Secondary adrenal insufficiency results from inadequate pituitary stimulation and may preserve aldosterone production because aldosterone is regulated mainly by the renin angiotensin system.
Excess cortisol production can produce Cushing syndrome, which may arise from adrenal, pituitary or other causes. Primary hyperaldosteronism results from excessive autonomous aldosterone production and can contribute to hypertension and potassium abnormalities. Diagnosis often requires dynamic hormonal testing because single measurements can be difficult to interpret.
The adrenal glands also produce adrenal androgens such as dehydroepiandrosterone and related steroids. Their physiological importance varies with age and sex, but they contribute to the overall pool of androgenic steroid precursors. During puberty, adrenal androgen production increases as part of adrenarche, a process that is distinct from gonadarche even though both occur during development. Adrenal steroid synthesis begins with cholesterol and proceeds through multiple enzyme mediated steps.
Genetic defects affecting steroidogenic enzymes can alter the balance among cortisol, aldosterone and androgen production. Congenital adrenal hyperplasia is a group of inherited disorders involving such enzyme deficiencies, with consequences depending on the affected pathway. These disorders demonstrate how closely interconnected adrenal steroid pathways are and how redirecting precursor molecules can produce characteristic hormonal patterns.
Adrenal tumours can be benign or malignant and may produce hormones or remain hormonally inactive. Pheochromocytomas arise from chromaffin cells and can produce excessive catecholamines, leading to episodic or persistent cardiovascular and metabolic effects. Adrenal cortical adenomas can sometimes produce cortisol or aldosterone, while adrenal cortical carcinomas are rare malignant tumours that can produce steroids or present as large masses. Imaging and biochemical testing are used together when evaluating adrenal abnormalities. Because some adrenal tumours release hormones intermittently, clinical suspicion and appropriate testing are important. The adrenal glands are also involved in physiological responses to infection, trauma, fasting and other stressors, demonstrating why their disorders can have effects across cardiovascular, metabolic, immune and electrolyte systems.
The Adrenal Glands demonstrate the integration of endocrine and nervous regulation. Their cortex produces hormones with relatively prolonged systemic effects, while the medulla converts sympathetic neural activation into a rapid circulating chemical response. Cortisol, aldosterone, adrenal androgens, adrenaline and noradrenaline have different receptors, target tissues and regulatory mechanisms. The kidneys are particularly important targets of adrenal hormones, while the pituitary and hypothalamus regulate part of cortical function. Adrenal physiology therefore cannot be understood by studying the glands alone. Modern endocrinology uses hormone measurements, suppression and stimulation tests, imaging, genetic analysis and clinical assessment to distinguish disorders of production, regulation and target tissue response.
