Adrenal gland

Adrenal gland
Endocrine system
Adrenal gland
Latin	glandula suprarenalis
Gray's	subject #277 1278
System	endocrine
Artery	superior suprarenal artery, middle suprarenal artery, Inferior suprarenal artery
Vein	suprarenal veins
Nerve	celiac plexus, renal plexus
Lymph	lumbar glands
Precursor	mesoderm, neural crest
MeSH	Adrenal+Glands
Dorlands/Elsevier	Adrenal gland

In mammals, the adrenal glands (also known as suprarenal glands) are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped. They are chiefly responsible for releasing hormones in response to stress through the synthesis of corticosteroids such as cortisol and catecholamines such as epinephrine. The adrenal glands affect kidney function through the secretion of aldosterone, a hormone involved in regulating the osmolarity of blood plasma.

Anatomy and Physiology

Anatomically, the adrenal glands are located in the retroperitoneum superior to the kidneys, bilaterally. They are surrounded by an adipose capsule and renal fascia. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra. Each adrenal gland has two distinct structures, the outer adrenal cortex and the inner medulla, both of which produce hormones. The cortex mainly produces cortisol, aldosterone and androgens, while the medulla chiefly produces epinephrine and norepinephrine. The combined weight of the adrenal glands in an adult human ranges from 7 to 10 grams.^[1]

A CT scan in which the Adrenals are shown as the triangular-shaped organs on top of the kidneys

Cortex

The adrenal cortex is devoted to the synthesis of corticosteroid and androgen hormones. Specific cortical cells produce particular hormones including aldosterone, cortisol, and androgens such as androstenedione. Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35–40 mg of cortisone acetate per day.^[2] In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted from the pituitary gland which are under the control of the hypothalamus, as well as by the renin-angiotensin system.

The adrenal cortex comprises three zones, or layers. This anatomic zonation can be appreciated at the microscopic level, where each zone can be recognized and distinguished from one another based on structural and anatomic characteristics.^[3] The adrenal cortex exhibits functional zonation as well: by virtue of the characteristic enzymes present in each zone, the zones produce and secrete distinct hormones.^[3]

Zona glomerulosa (outer)

The outermost layer, the zona glomerulosa is the main site for production of mineralocorticoids, mainly aldosterone, which is largely responsible for the long-term regulation of blood pressure. Aldosterone's effects are on the distal convoluted tubule and collecting duct of the kidney where is causes increased reabsorption of sodium and increased excretion of both potassium (by principal cells) and hydrogen ions (by intercalated cells of the collecting duct). Sodium retention is also a response of the salivary ducts, distal colon, and sweat glands to aldosterone receptor stimulation. The major stimulus to produce aldosterone is angiotensin II while ACTH from the pituitary only produces a transient effect. Angiotensin is stimulated by the juxtaglomerular cells when renal blood pressure drops below 90 mmHg.^[4]

Zona fasciculata

Situated between the glomerulosa and reticularis, the zona fasciculata is responsible for producing glucocorticoids, such as 11-deoxycorticosterone, corticosterone, and cortisol in humans. Cortisol is the main glucocorticoid under normal conditions and its actions include mobilization of fats, proteins, and carbohydrates, but it does not increase under starvation conditions.^[4] Additionally, cortisol enhances the activity of other hormones including glucagon and catecholamines. The zona fasciculata secretes a basal level of cortisol but can also produce bursts of the hormone in response to adrenocorticotropic hormone (ACTH) from the anterior pituitary.

Zona reticularis

The inner most cortical layer, the zona reticularis produces androgens, mainly dehydroepiandrosterone (DHEA) DHEA sulfate (DHEA-S), and androstenedione (the precursor to testosterone) in humans.^[4]

Medulla

The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. It secretes approximately 20% norepinephrine and 80% epinephrine.^[4] The chromaffin cells of the medulla, named for their characteristic brown staining with chromic acid salts, are the body's main source of the circulating catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). Catecholamines are derived from the amino acid tyrosine and these water-soluble hormones are the major hormones underlying the fight-or-flight response.

To carry out its part of this response, the adrenal medulla receives input from the sympathetic nervous system through preganglionic fibers originating in the thoracic spinal cord from T5–T11.^[5] Because it is innervated by preganglionic nerve fibers, the adrenal medulla can be considered as a specialized sympathetic ganglion.^[5] Unlike other sympathetic ganglia, however, the adrenal medulla lacks distinct synapses and releases its secretions directly into the blood.

Cortisol also promotes epinephrine synthesis in the medulla. Produced in the cortex, cortisol reaches the adrenal medulla and at high levels, the hormone can promote the upregulation of phenylethanolamine N-methyltransferase (PNMT), thereby increasing epinephrine synthesis and secretion.^[3]

Blood supply

Although variations of the blood supply to the adrenal glands (and indeed the kidneys themselves) are common, there are usually three arteries that supply each adrenal gland:

• The superior suprarenal artery is provided by the inferior phrenic artery
• The middle suprarenal artery is provided by the abdominal aorta
• The inferior suprarenal artery is provided by the renal artery

Venous drainage of the adrenal glands is achieved via the suprarenal veins:

• The right suprarenal vein drains into the inferior vena cava
• The left suprarenal vein drains into the left renal vein or the left inferior phrenic vein.

The suprarenal veins may form anastomoses with the inferior phrenic veins. Since the right supra-renal vein is short and drains directly into the inferior vena cava it is likely to injure the latter during removal of right adrenal for various reasons.

The adrenal glands and the thyroid gland are the organs that have the greatest blood supply per gram of tissue. Up to 60 arterioles may enter each adrenal gland.^[6] This may be one of the reasons lung cancer commonly metastasizes to the adrenals.

Terminology

The adrenal glands are named for their location relative to the kidneys. The term "adrenal" comes from ad- (Latin, "near") and renes (Latin, "kidney"). Similarly, "suprarenal" is derived from supra- (Latin, "above") and renes.

References

1. ^ Page 18 in: Boué A, Nicolas A, Montagnon B (June 1971). "Reinfection with rubella in pregnant women". Lancet 297 (7712): 1251–3. doi:10.1016/S0140-6736(71)91775-2. PMID 4104713. 
2. ^ Jefferies, William McK (2004). Safe uses of cortisol. Springfield, Ill: Charles C. Thomas. ISBN 0-398-07500-X. 
3. ^ ^{a b c} Whitehead, Saffron A.; Nussey, Stephen (2001). Endocrinology: an integrated approach. Oxford: BIOS. pp. 122. ISBN 1-85996-252-1. 
4. ^ ^{a b c d} Dunn R. B.; Kudrath W., Passo S.S., Wilson L.B. (2011). "10" (in English). Kaplan USMLE Step 1 Physiology Lecture Notes. p. 263-289. 
5. ^ ^{a b} Sapru, Hreday N.; Siegel, Allan (2007). Essential Neuroscience. Hagerstown, MD: Lippincott Williams & Wilkins. ISBN 0-7817-9121-9. 
6. ^ Mirilas P, Skandalakis JE, Colborn GL, Weidman TA, Foster RS, Kingsnorth A, Skandalakis LJ, Skandalakis PN (2004). Surgical Anatomy: The Embryologic And Anatomic Basis Of Modern Surgery. McGraw-Hill Professional Publishing. ISBN 960-399-074-4. 

External links

• MedlinePlus Encyclopedia 002219
• Virtual Slidebox at Univ. Iowa Slide 272
• Anatomy Atlases - Microscopic Anatomy, plate 15.292 - "Adrenal Gland"
• Histology at BU 14501loa
• SUNY Labs 40:03-0105 - "Posterior Abdominal Wall: The Retroperitoneal Fat and Suprarenal Glands"
• Adrenal Gland, from Colorado State University
• Cross section at UV pembody/body8a