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Adrenal gland
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Adrenal gland

In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. They are chiefly responsible for regulating the stress response through the synthesis of corticosteroids and catecholamines, including cortisol and adrenalin.

Table of contents
1 Overview
2 Adrenal medulla
3 Adrenal cortex
4 Pathology
5 Related topics
6 References

Overview

Anatomically, the adrenal gland is situated on the anteriosuperior aspect of the kidney and receives its blood supply from the adrenal arteries. It is separated into two distinct structures, the adrenal medulla and the adrenal cortex, both of which receive regulatory input from the nervous system. As its name suggests, the adrenal medulla is situated within the adrenal gland, while the adrenal cortex covers its surface.

The adrenal medulla is the body's main source of the catecholamine hormones epinephrine and norepinephrine. By contrast, some cells of the adrenal cortex belong to the hypothalamic-pituitary-adrenal axis and are the source of cortisol synthesis. Other cortical cells produce androgens such as testosterone, while some regulate water and electrolyte concentrations by secreting aldosterone.

Adrenal medulla

Composed mainly of hormone-producing chromaffin cells, the adrenal medulla is the principal site of the conversion of the amino acid tyrosine into the catecholamines epinephrine and norepinephrine (also called adrenaline and noradrenaline, respectively). Medullary cells are derived from the embryonic neural crest and, as such, are simply modified neurons. In particular, they are postganglionic cells of the sympathetic nervous system, receiving innervation from corresponding preganglionic fibers. Moreover, as the synapses between pre- and postganglionic fibers are called ganglia, the adrenal medulla is actually a ganglion of the sympathetic nervous system.

In response to stressors such as exercise or imminent danger, medullary cells release catecholamines into the blood in a 70:30 ratio of epinephrine to norepinephrine. Notable effects of epinephrine and norepinephrine include increased heart rate, blood vessel constriction, bronchiole dilation, and increased metabolism, all of which are characteristic of the fight-or-flight response.

Adrenal cortex

Situated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of mineralocorticoids and glucocorticoids, including aldosterone and cortisol. It is also a secondary site of androgen synthesis.

The cortex can be divided into three distinct layers of tissue based on cell type and function:

Zona glomerulosa

In response to increased potassium levels or decreased blood flow to the kidneys, cells of the zona glomerulosa secrete the mineralocorticoid aldosterone into the blood as part of the renin-angiotensin system. Aldosterone regulates the body's concentration of electrolytes, primarily sodium and potassium, by acting on the distal convoluted tubule of kidney nephrons to:

Zona fasciculata

Cortical cells responsible for the production of glucocorticoids are the primary effectors of adrenocorticotropic hormone (ACTH). Produced by the anterior pituitary gland in response to hypothalamic corticotropin-releasing hormone, ACTH stimulates cortical cells to secrete glucocorticoids. This three-organ endocrine system is commonly called the hypothalamic-pituitary-adrenal axis.

The primary glucocorticoid released by the adrenal gland is cortisol. Upon binding to its target, cortisol enhances metabolism in several ways:

Zona reticularis

Cells of the zona reticularis provide a secondary source of androgens such as testosterone, dihydrotestosterone (DHT), androstenedione, and dehydroepiandrosterone (DHEA). These enhance muscle mass, stimulate cell growth, and aid in the development of the secondary sexual characteristics.

Pathology

Related topics

References


Endocrine system - Adrenal glands Edit
Medulla - Chromaffin cells - Cortex - Zona glomerulosa - Zona fasciculata - Zona reticularis

Endocrine system
Adrenal gland; - Corpus luteum; - Hypothalamus - Ovaries - Pancreas - Parathyroid gland; - Pineal gland; - Pituitary gland; - Testes - Thyroid gland;