The Adrenals - Introduction

This is a multi part piece about: The Adrenal Gland Function, Assessment and Resolution.

GENERAL

The two adrenal glands are located immediately anterior to the kidneys, encased in a connective tissue capsule and usually partially buried in an island of fat. Like the kidneys, the adrenal glands lie beneath the peritoneum.

CIRCULATION

• tied with thyroid for greatest blood supply/gram of tissue
• up to 60 arterioles enter each adrenal

Inspection of an adrenal gland that has been sectioned reveals two distinct regions. The medulla and the Cortex.

• An inner medulla, which is a source of the catecholamines epinephrine and norepinephrine. The chromaffin cell is the principle cell type. The medulla is richly innervated by preganglionic sympathetic fibers and is, in essence, an extension of the sympathetic nervous system.

• An outer cortex, which secretes several classes of steroid hormones (glucocorticoids and mineralocorticoids, plus a few others). Histologic examination of the cortex reveals three concentric zones of cells that differ in the major steroid hormones they secrete.

Despite their organization into a single gland, the medulla and cortex are functionally different endocrine organs, and have different embryological origins. The medulla derives from ectoderm (neural crest), while the cortex develops from mesoderm.

They sit on top of kidneys, make corticosteroids (cortisol), mineralocorticoids (aldosterone) and catecholamines (dopamine, epinephrine and norepinephrine), also sex steroids (DHEA, progesterone, estrogen, testosterone).

• Surrounded adipose capsule and the renal fascia
• Located at about T11-12
• right side lower due to liver taking up space.

Removal of the adrenal glands leads to death within just a few days. Observation of such an unfortunate subject would reveal several key derangements:

•The concentration of potassium in extracelluar fluid becomes dramatically elevated.
•Urinary excretion of sodium is high and the concentration of sodium in extracellular fluid decreases significantly.
•Volume of extracellular fluid and blood decrease.
•The heart begins to function poorly, cardiac output declines and shock ensues.

These phenomena are a direct result of loss of mineralocorticoid activity, and can largely be prevented by replacement of salts and mineralocorticoids. Clearly mineralocorticoids are acutely critical for maintenance of life!

In contrast to loss of mineralocorticoids, failure to produce glucocorticoids is not acutely life-threatening. Nevertheless, loss or profound diminishment of glucocorticoid secretion leads to a state of deranged metabolism and an inability to deal with stressors which, if untreated, is fatal.

In addition to their physiologic importance, glucocorticoids are also among the most frequently used drugs, and often prescribed for their anti-inflammatory and immunosuppressive properties.

Adrenal Gland

The zona fasciculata and zona reticularis, which are both normally activated by ACTH from the anterior pituitary, would atrophy, so that you'd have less adrenal production of cortisol and sex steroids. The glomerulosa (produces aldosterone) and the medulla (produces dopamine, epinephrine and nor-epinephrine) would be unaffected.

Medulla
The adrenal medulla is really an extension of the central nervous system and secretes the catecholamines norepinephrine (noradrenalin) and epinephrine (adrenalin) in response to stimulation of the sympathetic nervous system. These substances aid in the "fight or flee" reaction in response to immediate stress. When these catecholamines are released into the general circulation, their effects last from 30 seconds to two minutes.

Chromaffin cells (derived from neural crest) are main source of catecholamines.

• Main source of Dopamine, precursor of Epinephrine & Norepinephrine.
• Catecholamines are water soluble, derived from amino acid tyrosine.
• Adrenal medulla is from ectoderm tissue in embryo.
• Considered specialized ganglia of the sympathetic nervous system.
• Releases hormones into the blood.

Cortex Three (3) Layers of Cortex: Zonas
Makes corticosteroid hormones from:

HPA axis controls cortisol/corticosterone synthesis

• Normal production: 35–40 mg of cortisone acetate per day
• Also makes androgens: testosterone
• Also regulates water and electrolyte concentrations via aldosterone
• Cortex regulated by neuroendocrine hormones from pituitary gland and by renin-angiotensin system (Aldosterone)

Cortex Zonas

• Zona Glomerulosa makes mineralocorticoids (aldosterone)
• Zona Fasciculata makes glucocorticoids (cortisol)
• Zone Reticularis makes sex steroids (DHEA)

ZONA GLOMERULOSA

• Mineralocorticoids including aldosterone
• Aldostone release provoked by RAA system, triggered by low blood volume
• Aldosterone; distal convoluted tubules excrete K+, reabsorbtion Na+ and H2O--> increase volume and blood pressure.
• Licorice works to increase blood pressure by increasing mineralocorticoid production

ZONA FASCICULATA

• glucocorticoids including cortisol and corticosterone
• stimulated by ACTH from anterior pituitary-->increased cAMP
• some cortisol secretion persists even when there's not ACTH

ZONA RETICULARIS

Makes DHEA (dehydroepiandrosterone) in response to ACTH signal from anterior pituitary which is controlled by hypothalamic signaling and other sex steroids too: estrogen, testosterone.

next Mineralcorticosteriod & Glucocorticosteroid..............................