Adrenal Medulla and Cortex: Stress, Metabolism and Salt Balance

One gland contains two different endocrine tissues

A human typically has one adrenal gland above each kidney. An adrenal gland has an outer cortex and a central medulla; these are physically adjacent but differ in developmental origin, hormones and methods of control. The medulla is involved in rapid responses to perceived threats or acute demands. The cortex produces steroid hormones important in longer-term metabolism, immune responses and maintenance of the extracellular fluid environment. A cross-section helps prevent confusion because the central medulla is not merely the innermost 'layer' of cortical steroid-producing tissue.

Adrenal medulla releases catecholamines

Adrenal medullary chromaffin cells respond to sympathetic neural activity. When the brain detects a threat, strong physical exertion or another acute demand, nerve signals can rapidly stimulate secretion of adrenaline and noradrenaline. The body becomes more prepared for action: heart rate and force of contraction may increase, breathing rate can rise, pupils dilate and sweating or piloerection may occur. These changes are sometimes called a fight-or-flight response, although real behavioural reactions are more varied than just two options.

Catecholamines also mobilise available fuels. They promote breakdown of glycogen stores, increasing glucose release into blood, and can enhance the breakdown of stored lipids. These actions supply fuels for active tissues during acute demand. The source also notes effects on protein metabolism. Catecholamines are not the same as cortisol, although both are associated with stress; their chemical nature and time course differ.

The cortex has three recognised zones

From outside to inside, the adrenal cortical layers are zona glomerulosa, zona fasciculata and zona reticularis. Zona glomerulosa predominantly makes mineralocorticoids, especially aldosterone. Zona fasciculata chiefly produces glucocorticoids, especially cortisol. Zona reticularis produces relatively weak adrenal androgen precursors and contributes to certain developmental changes. The zones are not sealed containers, but their specialised enzyme expression explains why hormone products differ.

Cortisol supports metabolic and stress adaptation

Cortisol raises the availability of glucose during fasting or physiological stress partly by promoting gluconeogenesis, the manufacture of glucose from non-carbohydrate precursors. It encourages mobilisation of amino acids and fatty acids and can change the way tissues use fuels. Its systemic actions help maintain blood pressure and cardiovascular responsiveness in appropriate contexts. Physiological cortisol is necessary; too much or too little can produce serious problems.

Glucocorticoids can suppress inflammatory reactions and reduce several immune responses. This is why related synthetic medicines are used to control inflammatory or autoimmune conditions under medical supervision. It would be misleading to say that cortisol simply destroys immunity: natural cortisol contributes to normal regulation, and the effects depend on amount and duration. The source also notes roles in erythropoiesis and renal function, which are additional examples of broad tissue influence.

Aldosterone regulates electrolytes and water

Aldosterone acts mainly in the distal nephron, including principal cells of the late distal tubule and collecting duct. It promotes sodium reabsorption and potassium secretion, increasing the kidney's capacity to conserve sodium. Water conservation often follows sodium retention depending on water permeability and ADH action. Aldosterone thereby supports extracellular fluid volume and blood pressure. It is incorrect to say that aldosterone and ADH are identical because one mainly adjusts sodium transport while the other greatly increases water permeability.

The source also notes altered phosphate ion excretion under aldosterone-related regulation. In modern physiology, the main high-value distinction remains sodium conservation and potassium excretion. Aldosterone secretion responds substantially to the renin–angiotensin system and elevated potassium, while ACTH has a smaller short-term effect. This distinguishes aldosterone control from the ACTH–cortisol axis that controls glucocorticoid secretion.

Cortical underproduction and Addison disease

When adrenal cortical output is inadequate, normal adaptation to fasting, stress and fluid balance becomes difficult. Weakness, fatigue, low blood pressure and disturbances of salt–water balance can occur, with additional manifestations depending on cause and severity. A severe acute deficiency is a medical emergency. The source associates underproduction of adrenal corticoids with Addison disease; this should be understood as a failure of adrenal cortical hormones, not a catecholamine excess from the medulla.

How the two adrenal compartments cooperate

During an acute threat, the medulla can rapidly release catecholamines. In parallel, hypothalamic–pituitary signalling increases ACTH, driving cortisol production from the cortex over a different time course. Cortisol helps maintain the metabolic and cardiovascular environment in which rapid catecholamine actions operate. Chronic steroid balance and rapid neural-adrenal reactions are linked but cannot be collapsed into one hormone pathway.

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