Each individual normally has two adrenal glands, both situated atop each kidney. These glands are divided into two portions: the central adrenal medulla and the outer adrenal cortex. Each secrete different hormones and are used for different functions.
Adrenal Medulla
The adrenal medulla serves as a part of the autonomic nervous system. ==Stimulation of preganglionic sympathetic nerve fibers==, which directly travel to the cells of the adrenal medulla, causes the release of catecholamine hormones epinephrine and norepinephrine. About 90% of the secretions of the adrenal medulla is epinephrine, also known as adrenaline.
- These catecholamines are used to regulate metabolic pathways for catabolism of stored fuels to meet caloric needs with endogenous sources. This is often utilized to prepare for a challenge (fight-or-flight response).
- They also reduce blood flow to tissues not necessary for the emergency response e.g. decreased blood flow to the GIT in order to shunt blood towards more “necessary” tissues like cardiac and skeletal muscles.
- Induction of the release of free fatty acids, increase the basal metabolic rate, and blood glucose level.
Adrenal Cortex
A functioning adrenal cortex is necessary for life. Adrenocortical secretions allow for adaptation to all kinds of stress.
Glucocorticoids
The prototypal glucocorticoid is hydrocortisone; the hormone is cortisol. These have an important influence on glucose metabolism (as evident by its name).
- Increased hydrocortisone secretion results in elevated blood glucose levels.
- Glucocorticoids are secreted in response to ACTH secretion from the anterior pituitary gland which utilizes a negative feedback system: glucocorticoids in the blood inhibit corticotropin-releasing hormone (CRH) from the hypothalamus, which then inhibits ACTH secretion from the anterior pituitary gland.
- Glucocorticoids are used in medicine in the form of corticosteroids to inhibit inflammatory responses to tissue injury and to suppress allergic manifestations (immunosuppressant). However, they have various side effects:
- Diabetes Mellitus, osteoporosis, Peptic Ulcers, muscle wasting (from protein breakdown), poor wound healing, and redistribution of body fat.
- Large amounts of exogenous glucocorticoids inhibit ACTH and endogenous glucocorticoids, which may result in adrenal cortex atrophy. Sudden discontinuation of corticosteroid administration may result in insufficiency due to the inability of an atrophied adrenal cortex to respond adequately.
Mineralocorticoids
Mainly in the form of aldosterone, the salt-saving hormone. As its name suggests, its major effect is on electrolyte metabolism.
- These act principally on the renal tubules and GI epithelium to increase sodium (cation) absorption in exchange for potassium or hydrogen (anions) excretion. It may be released in response to decreased blood pressure and dehydration; it may also be released in response to hyperkalemia due to its potassium-wasting characteristic.
- Compared to glucocorticoids, ACTH only minimally affects aldosterone secretion. Instead, Angiotensin II (an arteriolar vasoconstrictor) presence in the bloodstream is what primarily stimulates its secretion. Angiotensin II, in turn, is resulting from the RAAS system.
Adrenal Sex Hormones
Androgens are the third major type of steroid hormones produced by the adrenal cortex, whose effects are similar to those of male sex hormones.
- The adrenal cortex may also secrete small amounts of female sex hormones (estrogens).
- ACTH also controls the production of adrenal sex hormones.
- Adrenal androgens have minimal effects when at normal levels, but may cause masculinization; androgenital syndrome if produced in excess such as in inborn enzyme deficiencies.