Difference between revisions of "Adrenal Glands - Anatomy & Physiology"

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==Adrenal Glands==
 
==Adrenal Glands==
  
−
The adrenal glands are paired bodies lying cranial to the kidneys within the retroperitoneal space. The glands consist of two layers; the '''cortex''' and '''medulla'''.
 
−
 
−
The '''adrenal cortex''' is red to light brown in colour and is composed of three zones. From the outer to inner, the layers are;
 
  
−
1. zona glomerulosa
+
The adrenal glands are paired bodies lying cranial to the kidneys within the retroperitoneal space. The glands consist of two layers; the cortex and medulla.  
  
−
2. zona fasciculata  
+
The adrenal cortex is red - light brown in colour and is comprised of three zones. These zones all produce hormones derived from cholesterol which is abundant in the cells. From outer to inner the layers are zona glomerulosa, zona fasciculata and zona reticularis. The adrenal cortex represents 80-90% of the adrenal gland.
  
−
3. zona reticularis
+
The adrenal medulla is primarily involved in the production of catecholamines; epinephrine and norepinephrine. In fetal life the adrenal medulla plays a role in the autonomic nervous system. The medulla acts as a sympathetic ganglion with the postganglionic cells lacking axons. Through sympathetic preganglionic fiber stimulation the medullary cells secrete catecholamines. The adrenal medulla represents only 10-20% of the adrenal gland.
−
 
 
−
All these zones produce hormones derived from cholesterol, which is abundant in the cells. The '''adrenal cortex''' represents 80-90% of the adrenal gland.
 
−
 
 
−
The '''adrenal medulla''' is primarily involved in the production of catecholamines; epinephrine and norepinephrine. In fetal life, the adrenal medulla plays a role in the autonomic nervous system. The medulla acts as a sympathetic ganglion with the postganglionic cells lacking axons. Through sympathetic preganglionic fiber stimulation, the medullary cells secrete catecholamines. The adrenal medulla represents only 10-20% of the adrenal gland.
 
  
 
[[Image:Adrenal Gland Schematic..jpg|700px|small>'''Schematic Diagram of the Adrenal Gland</small>|frameless]]
 
[[Image:Adrenal Gland Schematic..jpg|700px|small>'''Schematic Diagram of the Adrenal Gland</small>|frameless]]
Line 20: Line 20:
  
 
==Embryological Origin==
 
==Embryological Origin==
 +
The adrenal glands develop from two separate embryological tissues; the neural crest ectoderm and the intermediate mesoderm. The adrenal cortex develops from the intermediate mesoderm. The medulla originates from neural crest cells migrating from sympathetic ganglion. Mesodermal cells then surround the medulla.
  
−
The adrenal glands develop from two separate embryological tissues; the '''neural crest ectoderm''' and the '''intermediate mesoderm'''. The medulla originates from neural crest cells migrating from sympathetic ganglion. Mesodermal cells then surround the medulla. The adrenal cortex develops from the intermediate mesoderm. The fetal cortex develops in the centre, with the permanent cortex surrounding it. By 4 months of age the adrenal gland is fully developed.
+
The fetal cortex develops in the centre with the permanent cortex surrounding it. By 4 months of age the adrenal gland is fully developed.
  
 
==Anatomy==
 
==Anatomy==
  
−
Even though the adrenal glands gain their name through their relationship with the kidney, they are in fact more closely related to the major vessels, including the aorta and the caudal vena cava.  
+
Even though the adrenal glands gain their name through their relationship with the kidney they are in fact more closely related to the major vessels. They are closely connected to the aorta and caudal vena cava.
  
−
The adrenal glands are elongated and are often asymmetrical, being moulded around the neighbouring vessels. Their size varies greatly and generally those of juveniles are larger than adults, and those of lactating or pregnant animals are larger than reproductively inactive animals. A medium-sized dog's adrenals will on average measure 2.5 x 1 x 0.5cm. The adrenal glands are firm, and their capsule is easily fractured on flexion. The cortex on appearance is yellow and radially striated, whilst the medulla is darker with a more uniform appearance. The '''zona glomerulosa''' is narrow and the cells are in a whorled pattern. The '''zona fasiculata''' is wide and the cells lie in columns and the '''zona reticularis''' is more randomly organised.
+
They are elongated and are often asymmetrical being moulded around the neighbouring vessels. Their size varies greatly and generally those of juveniles are larger than adults, and those of lactating or pregnant animals are larger than reproductively inactive animals. A medium-sized dog's adrenals will on average measure 2.5x1x0.5cm.
 +
 
 +
They are firm and their capsule is easily fractured on flexion. The cortex on appearance is yellow and radially striated whilst the medulla is darker with a more uniform appearance.
 +
 
 +
The zona glomerulosa is narrow and the cells are in a whorled pattern. The zona fasiculata is wide and the cells lie in columns. The zona reticularis is more randomly organised.
  
 
==Vascular Supply==
 
==Vascular Supply==
  
−
The oxygenated supply is from various branches of the following neighbouring trunks; aorta, renal artery, lumbar artery, phrenicoabdominal artery and the cranial mesenteric arteries. After perfusion of the gland, the blood pools in a central vein and then exits the gland through the hilus. This then joins up with the caudal vena cava or one of its tributaries.
+
The oxygenated supply is from various branches of the following neighbouring trunks; aorta, renal artery, lumbar artery, phrenicoabdominal artery and the cranial mesenteric arteries.
 +
 
 +
After perfusion of the gland the blood pools in a central vein and then exit the gland through the hilus. This then joins up with the caudal vena cava or one of it's tributaries.
  
 
==Function==
 
==Function==
  
 
===Adrenal Cortex===
 
===Adrenal Cortex===
−
The function of the cortex is to produce '''adrenocorticoid hormones'''. The three zones each make a specific type of hormone:
+
The function of the cortex is to produce ''adrenocorticoid hormones''. The three zones each make a specific type of hormone:
−
 
+
* The Zona Glomerulosa secretes '''mineralocorticoids'''
−
The Zona Glomerulosa secretes '''mineralocorticoids''',
+
* The Zona Fasciculata secretes '''glucocorticoids'''
−
 
+
* The Zona Reticularis secretes sex steroids or '''androgens'''
−
The Zona Fasciculata secretes '''glucocorticoids''',
+
They are formed from the esterified cholesterol that is stored in intracellular lipid droplets. Below is the steps of how the adrenocorticoid hormones are made from cholesterol.
−
 
 
−
The Zona Reticularis secretes sex steroids or '''androgens'''.
 
−
 
 
−
The hormones are formed from the esterified cholesterol that is stored in intracellular lipid droplets. Below is the steps of how the adrenocorticoid hormones are made from cholesterol.
 
−
 
 
 
1) The P450scc enzyme converts cholesterol to pregnenolone by the removal of a side chain. This is regulated by ACTH and is the rate determining step of adrenocorticoid hormone production. This occurs in the mitochondria.
 
1) The P450scc enzyme converts cholesterol to pregnenolone by the removal of a side chain. This is regulated by ACTH and is the rate determining step of adrenocorticoid hormone production. This occurs in the mitochondria.
−
 
 
2) Hydroxylation reactions occur in the mitochondria and endoplasmic reticulum converting pregnenolone into specific hormones. The location within the cortex determines the hormones produced.
 
2) Hydroxylation reactions occur in the mitochondria and endoplasmic reticulum converting pregnenolone into specific hormones. The location within the cortex determines the hormones produced.
  
−
====Mineralocorticoids====
+
===='''Mineralocorticoids'''====
−
These hormones regulate the metabolism of inorganic ions, such as sodium, potassium and chloride. Several hormones are produced but the most important is '''aldosterone'''. [[Aldosterone]] is formed in the zona glomerulosa by the conversion of pregnenolone to cortisol and then to corticosterone, before finally becoming aldosterone. It circulates by binding to albumin and cortisol-binding globulin. It's plasma half-life is 20 minutes. It is eventually inactivated in the liver, where it binds to glucuronic acid and is excreted in bile and urine.
+
These hormones regulate the metabolism of inorganic ions, such sodium, potassium and chloride. Several hormones are produced but the most important is '''aldosterone'''.
−
 
+
It is formed in the zona glomerulosa by the conversion of pregnenolone to cortisol and then to corticosterone before finally becoming aldosterone. It circulates by binding to albumin and cortisol-binding globulin. It's plasma half-life is 20 minutes. It is eventually inactivated in the liver, where it binds to glucuronic acid and is excreted in bile and urine.
−
'''Regulation and action'''
 
  
−
It's main regulators of synthesis and secretion are; '''Renin''' and extracellular concentrations of potassium ions. Its main action is upon renal regulation and electrolyte and fluid balance. These are fully explained within the '''[[Renin Angiotensin Aldosterone System|renin-angiotensin]] and [[Aldosterone|aldosterone system]] pages.
+
'''Regulation and action''' - It's main regulators of synthesis and secretion are; '''Renin''' and extracellular concentrations of potassium ions. Its main action is upon renal regulation and electrolyte and fluid balance. These are fully explained within the '''[[Important Hormonal Regulators of the Kidney - Anatomy & Physiology|renin-angiotensin and aldosterone system]] page.
  
−
====Glucocorticoids====
+
===='''Glucocorticoids'''====
−
These hormones are important in regulating glucose metabolism; the most important of them is '''cortisol'''. This is formed within the zona fasciculata by the conversion of pregnenolone directly to cortisol. It is transported by cortisol-binding globulin and has a half-life of 90 minutes.
+
These hormones are important in regulating glucose metabolism; the most important of them is '''cortisol'''. This is formed within the zona fasiculata by the conversion of pregnenolone directly to cortisol. It is transported by cortisol-binding globulin and has a half-life of 90 minutes.
  
 
'''Regulation of Synthesis and Secretion'''
 
'''Regulation of Synthesis and Secretion'''
−
 
 
This is regulated by the hypothalamus-pituitary adrenal axis as follows:
 
This is regulated by the hypothalamus-pituitary adrenal axis as follows:
 
[[Image:ACTH axis.jpg|right|thumb|205px|<small><center>'''Schematic Diagram of the Hypothalamic Pituitary Adrenal Axis, Copyright John Tulloch</center></small>]]
 
[[Image:ACTH axis.jpg|right|thumb|205px|<small><center>'''Schematic Diagram of the Hypothalamic Pituitary Adrenal Axis, Copyright John Tulloch</center></small>]]
  
−
An environmental, or internal stimuli (such as stress or light regulating the circadian rhythm) causes the hypothalamus to release ACTH-releasing hormone ('''ACTH-RH'''). ACTH-RH flows to the pituitary gland, stimulating the production of adrenocorticotrophic hormone ('''ACTH'''). ACTH binds to receptors on the cells of the adrenal cortex, thus activating adenyl cyclase. This leads to increased cAMP levels, and the mobilization of more cholesterol and so the production of more cortisol. This is the rate limiting step and is determined by the P-450-enzyme. Cortisol then has a negative feedback effect on ACTH-RH and ACTH secretion.
+
* An environmental or internal stimuli (such as stress or light regulating the circadian rhythm) causes the hypothalamus to release ACTH-releasing hormone ('''ACTH-RH''').
 +
* ACTH-RH flows to the pituitary gland stimulating the production of adrenocorticotrophic hormone ('''ACTH''').
 +
* ACTH binds to receptors on cells of the adrenal cortex. Thus activating adenyl cyclase.
 +
* This leads to increased cAMP levels and the mobilization of more cholesterol and so the production of more cortisol. This is the rate limiting step and is determined by the P-450-enzyme.
 +
* Cortisol then has a negative feedback effect on ACTH-RH and ACTH secretion.
  
 
'''Action'''
 
'''Action'''
 +
These are essential hormones and have multiple effects around the body.
 +
* Cortisol has a '''permissive action'''. I.e. without cortisol certain chemical reactions within cells are unable to occur, for example the production of epinephrine and glucagon.
 +
* Cortisol is a '''stress hormone'''. In times of stress it is released and enhances the effect of norepinephrine on blood pressure. It is also essential for the day to day maintenance of normal blood pressure. It will also stimulate gluconeogenesis and inhibit certain tissues utilization of glucose.
 +
* It stimulates degradation of fats and proteins.
 +
* It inhibits DNA synthesis in some tissues. This along with degradation of protein results in a '''growth-inhibiting effect''' in times of stress.
 +
* It can have '''anti-inflammatory''' effects. It inhibits the formation of prostaglandins and cytokines and reduces the ability of white blood cells being transported to an injured area. It helps keep in check inflammatory processes that could potentially cause tissue damage.
 +
* It is also '''immunosuppressive''' by reducing the lymphocyte numbers in circulation and reducing antibody production.
  
−
These are essential hormones and have multiple effects around the body. Cortisol has a '''permissive action''', i.e. without cortisol certain chemical reactions within cells are unable to occur, for example the production of epinephrine and glucagon. Cortisol is a '''stress hormone''', in times of stress it is released and enhances the effect of norepinephrine on blood pressure. It is also essential for the day to day maintenance of normal blood pressure and will also stimulate gluconeogenesis and inhibit certain tissues utilization of glucose. Cortisol stimulates the degradation of fats and proteins and inhibits DNA synthesis in some tissues. This along with degradation of protein, results in a '''growth-inhibiting effect''' in times of stress. It can also  have '''anti-inflammatory''' effects. It inhibits the formation of prostaglandins and cytokines and reduces the ability of white blood cells being transported to an injured area. It helps keep in check inflammatory processes that could potentially cause tissue damage. Cortisol is also '''immunosuppressive''', by reducing the lymphocyte numbers in circulation and reducing antibody production.
+
===='''Androgens'''====
 +
These hormones function as male sex hormones. The most important of these is '''testosterone'''. This is produced by the conversion of pregnenolone to dehydroepiandrosterone, then to androstenedione and finally to testosterone. This occurs in the zona reticularis. They bind in the blood to sex hormone-binding globulin (SHBG) and albumin. Most adrenal androgens have little physiological action untill they are coverted to testosterone.
 +
They have little importance within the male. In women adrenal androgens, which are converted to oestrogen in adipose tissue, is the most important source of oestrogen after the menopause.
  
−
====Androgens====
+
==='''Adrenal Medulla'''===
−
These hormones function as male sex hormones. The most important of these is '''testosterone'''. This is produced by the conversion of pregnenolone to dehydroepiandrosterone, then to androstenedione and finally to testosterone. This occurs in the zona reticularis. They bind in the blood to sex hormone-binding globulin (SHBG) and albumin. Most adrenal androgens have little physiological action until they are converted to testosterone. They have little importance within the male, however, in women adrenal androgens, which are converted to oestrogen in adipose tissue, is the most important source of oestrogen after the menopause.
+
The adrenal medulla converts the amino acid to '''catecholamines'''. This group of hormones contain '''epinephrine''' and '''norepinephrine'''. All of the bodie's circulating epinephrine is produced by the adrenal medulla, where as norepinephrine comes from both the medulla and postganglionic sympathetic neurons.
−
 
 
−
===Adrenal Medulla===
 
−
 
 
−
The adrenal medulla converts the amino acid to '''catecholamines'''. This group of hormones contains '''epinephrine''' and '''norepinephrine'''. All of the bodies circulating epinephrine is produced by the adrenal medulla, where as norepinephrine comes from both the medulla and postganglionic sympathetic neurons.
 
  
 
'''Synthesis, secretion and metabolism'''
 
'''Synthesis, secretion and metabolism'''
  
−
Tyrosine is firstly converted to dihydroxyphenylalanine by tyrosine hydroxylase. This is the rate-limiting step. Dihydroxyphenylalanine is then converted into dopamine, then into norepinephrine and finally into epinephrine. They are stored in secretory vesicles and are released by exocytosis. They are metabolised in the liver and kidney and their half-lives are only 1-3 minutes. Some unmetabolised catecholamines are excreted in the urine.
+
Tyrosine is firstly converted to dihydroxyphenylalanine by tyrosine hydroxylase. This is the rate-limiting step. Dihydroxyphenylalanine is then coverted into dopamine, then into norepinephrine and finally into epinephrine. They are stored in secretory vesicles and are released by exocytosis. They are metabolised in the liver and kidney and their half-lives are only 1-3 minutes. Some unmetablised catecholamines are excreted in the urine.
  
 
'''Regulation of synthesis and secretion'''
 
'''Regulation of synthesis and secretion'''
  
−
They are released in response to stress by the firing of preganglionic sympathetic nerve fibres in the adrenal medulla. This causes release of acetylcholine, thus depolarizing the cell membrane. An action potential is generated and an influx of calcium ions results in the exocytosis of the catecholamine vesicles.
+
They are released in response to stress by the firing of preganglionic sypathetic nerve fibres in the adrenal medulla. This causes release of acetylcholine, thus depolarizing the cell membrane. An action potential is generated and an influx of calcium ions results in the exocytosis of the catecholamine vesicles.
  
 
'''Adrenergic Receptors'''
 
'''Adrenergic Receptors'''
−
 
 
The catecholamines bind to two types of receptor proteins in cell membranes to elicit a response; these are alpha and beta receptors. The following is a table of where the receptors are located in the body and their effects when stimulated.  
 
The catecholamines bind to two types of receptor proteins in cell membranes to elicit a response; these are alpha and beta receptors. The following is a table of where the receptors are located in the body and their effects when stimulated.  
  
Line 193: Line 200:
 
</TR></TABLE>
 
</TR></TABLE>
  
−
==Histology==
+
==Histology of the Adrenal Glands==
−
 
 
 
<gallery>
 
<gallery>
−
Image:Histology of the Adrenal Glands..jpg|<small>'''Histological section of the Adrenal Gland, from the [[Endocrine Histology resource|Histology of the Endocrine system tutorial]]</small>
+
Image:Histology of the Adrenal Glands..jpg|<small>'''Histological section of the Adrenal Gland</small>
−
Image:Histology of the Adrenal Glands showing zones..jpg|<small>'''Histological section of the Adrenal Gland Cortical zones, from the [[Endocrine Histology resource|Histology of the Endocrine system tutorial]]</small>
+
Image:Histology of the Adrenal Glands showing zones..jpg|<small>'''Histological section of the Adrenal Gland Cortical zones</small>
−
Image:Histology of the Adrenal Glands Medulla..jpg|<small>'''Histological section of the Adrenal Gland showing Medulla and Zona Reticularis, from the [[Endocrine Histology resource|Histology of the Endocrine system tutorial]]</small>
+
Image:Histology of the Adrenal Glands Medulla..jpg|<small>'''Histological section of the Adrenal Gland showing Medulla and Zona Reticularis</small>
 
</gallery>
 
</gallery>
  
−
==Functional anatomy==
 
−
[[Image:Normal adrenal cortex.jpg|right|thumb|125px|<small><center>'''Normal Adrenal Cortex'''. Courtesy of A. Jefferies</center></small>]]
 
−
The adrenal glands are essential to life.
 
−
They are closely applied to the cranial poles of each kidney.  Consists of a cortex and medulla, each having different embryological origin.  In some lower animals the two components actually exist as separate endcrine glands.
 
−
*'''Cortex''': Secretes a variety of steroid hormones derived from ''cholesterol''. 
 
−
**''Zona glomerulosa'': Secretes '''Aldosterone'''.  Concerned with electrolyte and fluid homeostasis.
 
−
**''Zona Fasciculata'': Secretes '''Glucocorticoids'''.  Controls metabolism of lipid, protein and carbohydrate.
 
−
**''Zona Reticularis'': Produces '''Sex hormones'''.  Supplements gonadal sex hormone secretion.
 
−
*'''Medulla''': Embryologically similar origin to the sympathetic nervous system.  Secretes ''catecholamine hormones'' E.g. Adrenaline, noradrenaline.<br>
 
−
Secretion from the medulla is directly controlled by the sympathetic nervous
 
−
system, allowing a rapid response.  In contrast, the secretions from the cortex are controlled by the hormones ACTH and other circulating hormones.
 
−
 
−
As with any endocrine gland there are two main types of pathology:
 
−
*Insufficiency leading to [[Hypoadrenocorticism|'''Addisons disease''']].
 
−
*Excess leading to [[Hyperadrenocorticism|'''Cushings disease''']].
 
−
 
−
 
−
 
−
{{Template:Learning
 
−
|powerpoints = [[Endocrine Histology resource|Histology of the Endocrine system]]
 
−
|OVAM = [http://www.onlineveterinaryanatomy.net/content/ovine-adrenal-gland-histology Ovine Adrenal Gland Histology 1]<br>[http://www.onlineveterinaryanatomy.net/content/ovine-adrenal-gland-histology-zona-fasiculate-and-zona-glomerulosa Ovine Adrenal Gland Histology 2]
 
−
|Vetstream = [https://www.vetstream.com/canis/search?s=adrenals Adrenal diseases]
 
−
}}
 
−
 
−
==Webinars==
 
−
<rss max="10" highlight="none">https://www.thewebinarvet.com/endocrinology/webinars/feed</rss>
 
  
−
[[Category:Endocrine System - Anatomy & Physiology]]
+
[[Category:Endocrine System]]
−
[[Category:A&P Done]]
 

Revision as of 12:24, 20 August 2010

()Map ENDOCRINE SYSTEM (Map)



Adrenal Glands

The adrenal glands are paired bodies lying cranial to the kidneys within the retroperitoneal space. The glands consist of two layers; the cortex and medulla.

The adrenal cortex is red - light brown in colour and is comprised of three zones. These zones all produce hormones derived from cholesterol which is abundant in the cells. From outer to inner the layers are zona glomerulosa, zona fasciculata and zona reticularis. The adrenal cortex represents 80-90% of the adrenal gland.

The adrenal medulla is primarily involved in the production of catecholamines; epinephrine and norepinephrine. In fetal life the adrenal medulla plays a role in the autonomic nervous system. The medulla acts as a sympathetic ganglion with the postganglionic cells lacking axons. Through sympathetic preganglionic fiber stimulation the medullary cells secrete catecholamines. The adrenal medulla represents only 10-20% of the adrenal gland.

frameless


Embryological Origin

The adrenal glands develop from two separate embryological tissues; the neural crest ectoderm and the intermediate mesoderm. The adrenal cortex develops from the intermediate mesoderm. The medulla originates from neural crest cells migrating from sympathetic ganglion. Mesodermal cells then surround the medulla.

The fetal cortex develops in the centre with the permanent cortex surrounding it. By 4 months of age the adrenal gland is fully developed.

Anatomy

Even though the adrenal glands gain their name through their relationship with the kidney they are in fact more closely related to the major vessels. They are closely connected to the aorta and caudal vena cava.

They are elongated and are often asymmetrical being moulded around the neighbouring vessels. Their size varies greatly and generally those of juveniles are larger than adults, and those of lactating or pregnant animals are larger than reproductively inactive animals. A medium-sized dog's adrenals will on average measure 2.5x1x0.5cm.

They are firm and their capsule is easily fractured on flexion. The cortex on appearance is yellow and radially striated whilst the medulla is darker with a more uniform appearance.

The zona glomerulosa is narrow and the cells are in a whorled pattern. The zona fasiculata is wide and the cells lie in columns. The zona reticularis is more randomly organised.

Vascular Supply

The oxygenated supply is from various branches of the following neighbouring trunks; aorta, renal artery, lumbar artery, phrenicoabdominal artery and the cranial mesenteric arteries.

After perfusion of the gland the blood pools in a central vein and then exit the gland through the hilus. This then joins up with the caudal vena cava or one of it's tributaries.

Function

Adrenal Cortex

The function of the cortex is to produce adrenocorticoid hormones. The three zones each make a specific type of hormone:

  • The Zona Glomerulosa secretes mineralocorticoids
  • The Zona Fasciculata secretes glucocorticoids
  • The Zona Reticularis secretes sex steroids or androgens

They are formed from the esterified cholesterol that is stored in intracellular lipid droplets. Below is the steps of how the adrenocorticoid hormones are made from cholesterol. 1) The P450scc enzyme converts cholesterol to pregnenolone by the removal of a side chain. This is regulated by ACTH and is the rate determining step of adrenocorticoid hormone production. This occurs in the mitochondria. 2) Hydroxylation reactions occur in the mitochondria and endoplasmic reticulum converting pregnenolone into specific hormones. The location within the cortex determines the hormones produced.

Mineralocorticoids

These hormones regulate the metabolism of inorganic ions, such sodium, potassium and chloride. Several hormones are produced but the most important is aldosterone. It is formed in the zona glomerulosa by the conversion of pregnenolone to cortisol and then to corticosterone before finally becoming aldosterone. It circulates by binding to albumin and cortisol-binding globulin. It's plasma half-life is 20 minutes. It is eventually inactivated in the liver, where it binds to glucuronic acid and is excreted in bile and urine.

Regulation and action - It's main regulators of synthesis and secretion are; Renin and extracellular concentrations of potassium ions. Its main action is upon renal regulation and electrolyte and fluid balance. These are fully explained within the renin-angiotensin and aldosterone system page.

Glucocorticoids

These hormones are important in regulating glucose metabolism; the most important of them is cortisol. This is formed within the zona fasiculata by the conversion of pregnenolone directly to cortisol. It is transported by cortisol-binding globulin and has a half-life of 90 minutes.

Regulation of Synthesis and Secretion This is regulated by the hypothalamus-pituitary adrenal axis as follows:

Schematic Diagram of the Hypothalamic Pituitary Adrenal Axis, Copyright John Tulloch
  • An environmental or internal stimuli (such as stress or light regulating the circadian rhythm) causes the hypothalamus to release ACTH-releasing hormone (ACTH-RH).
  • ACTH-RH flows to the pituitary gland stimulating the production of adrenocorticotrophic hormone (ACTH).
  • ACTH binds to receptors on cells of the adrenal cortex. Thus activating adenyl cyclase.
  • This leads to increased cAMP levels and the mobilization of more cholesterol and so the production of more cortisol. This is the rate limiting step and is determined by the P-450-enzyme.
  • Cortisol then has a negative feedback effect on ACTH-RH and ACTH secretion.

Action These are essential hormones and have multiple effects around the body.

  • Cortisol has a permissive action. I.e. without cortisol certain chemical reactions within cells are unable to occur, for example the production of epinephrine and glucagon.
  • Cortisol is a stress hormone. In times of stress it is released and enhances the effect of norepinephrine on blood pressure. It is also essential for the day to day maintenance of normal blood pressure. It will also stimulate gluconeogenesis and inhibit certain tissues utilization of glucose.
  • It stimulates degradation of fats and proteins.
  • It inhibits DNA synthesis in some tissues. This along with degradation of protein results in a growth-inhibiting effect in times of stress.
  • It can have anti-inflammatory effects. It inhibits the formation of prostaglandins and cytokines and reduces the ability of white blood cells being transported to an injured area. It helps keep in check inflammatory processes that could potentially cause tissue damage.
  • It is also immunosuppressive by reducing the lymphocyte numbers in circulation and reducing antibody production.

Androgens

These hormones function as male sex hormones. The most important of these is testosterone. This is produced by the conversion of pregnenolone to dehydroepiandrosterone, then to androstenedione and finally to testosterone. This occurs in the zona reticularis. They bind in the blood to sex hormone-binding globulin (SHBG) and albumin. Most adrenal androgens have little physiological action untill they are coverted to testosterone. They have little importance within the male. In women adrenal androgens, which are converted to oestrogen in adipose tissue, is the most important source of oestrogen after the menopause.

Adrenal Medulla

The adrenal medulla converts the amino acid to catecholamines. This group of hormones contain epinephrine and norepinephrine. All of the bodie's circulating epinephrine is produced by the adrenal medulla, where as norepinephrine comes from both the medulla and postganglionic sympathetic neurons.

Synthesis, secretion and metabolism

Tyrosine is firstly converted to dihydroxyphenylalanine by tyrosine hydroxylase. This is the rate-limiting step. Dihydroxyphenylalanine is then coverted into dopamine, then into norepinephrine and finally into epinephrine. They are stored in secretory vesicles and are released by exocytosis. They are metabolised in the liver and kidney and their half-lives are only 1-3 minutes. Some unmetablised catecholamines are excreted in the urine.

Regulation of synthesis and secretion

They are released in response to stress by the firing of preganglionic sypathetic nerve fibres in the adrenal medulla. This causes release of acetylcholine, thus depolarizing the cell membrane. An action potential is generated and an influx of calcium ions results in the exocytosis of the catecholamine vesicles.

Adrenergic Receptors The catecholamines bind to two types of receptor proteins in cell membranes to elicit a response; these are alpha and beta receptors. The following is a table of where the receptors are located in the body and their effects when stimulated.

Target Tissue
Receptor Type
Responses
Eye
alpha-1
radial muscle contraction
Pancreas
alpha-2
decreased insulin secretion
Cardiovascular system
alpha-2
vasoconstriction
urinary bladder
alpha-2
sphincter contraction
uterus
alpha-2
contraction
male sex organs
alpha-2
ejaculation, detumescence
central nervous system
alpha-2
stimulation
skin
alpha-2
piloerection, sweat production
cardiovascular system
beta-1
increased heart rate, increased contractility, increased conduction velocity
renin secretion
beta-1
stimulation
liver
beta-2
glycogenolysis, lipolysis, gluconeogenesis
adipose tissue
beta-2
lipolysis
skeletal muscle
beta-2
glycogenolysis
pancreas
beta-2
increased insulin secretion
cardiovascular system
beta-2
vasodilation in skeletal muscle arterioles, coronary arteries and all veins
bronchial muscles
beta-2
relaxation
gastrointestinal tract
beta-2
relaxation
urinary bladder
beta-2
detrusor relaxation
uterus
beta-2
relaxation
male sex organs
beta-2
erection?
eye
beta-2
ciliary muscle relaxation

Histology of the Adrenal Glands

  • Histological section of the Adrenal Gland

  • Histological section of the Adrenal Gland Cortical zones

  • Histological section of the Adrenal Gland showing Medulla and Zona Reticularis

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