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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 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 comprised of three zones. From the outer to inner, the layers are;  

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

1. zona glomerulosa

3. zona reticularis

3. zona reticularis

These zones all 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.

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]]

==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 it's 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 exits the gland through the hilus. This then joins up with the caudal vena cava or one of its tributaries.

==Function==

==Function==

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 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 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.

'''Regulation and action'''  

'''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.

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.

===='''Glucocorticoids'''====

====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.

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.

'''Regulation of Synthesis and Secretion'''

'''Regulation of Synthesis and Secretion'''

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.

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'''====

====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 until they are coverted 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.

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.

==='''Adrenal Medulla'''===

===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.

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 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.

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.

'''Regulation of synthesis and secretion'''

'''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.

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.

'''Adrenergic Receptors'''

'''Adrenergic Receptors'''

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Image:Histology of the Adrenal Glands..jpg|<small>'''Histological section of the Adrenal Gland. RVC 2008</small>

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 showing zones..jpg|<small>'''Histological section of the Adrenal Gland Cortical zones. RVC 2008</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 Medulla..jpg|<small>'''Histological section of the Adrenal Gland showing Medulla and Zona Reticularis RVC 2008</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>

</gallery>

</gallery>

[[Category:Endocrine System - Anatomy & Physiology]]

[[Category:Endocrine System - Anatomy & Physiology]]

[[Category:To Do - A&P]]

[[Category:A&P Done]]