Adrenal Glands, Pancreas and Local Hormones

step 2: Structure of The Adrenal Gland (Suprarenal) (30 minutes)

·         Anatomically, the adrenal glands (suprarenal) are located in the thoracic abdomen situated 'on' top of the kidneys one on each side, specifically on their anterosuperior aspect.

·         They are also surrounded by the adipose capsule and the renal fascia. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra and receive their blood supply from the adrenal arteries.

·         The adrenal gland is separated into two distinct structures, both of which receive regulatory input from the nervous system

Figure 2:  Gross Section Appearance, Microstructure Vasculature and Ultrastructure of The Adrenal Glands

Adrenal Cortex

·         The adrenal cortex is devoted to the synthesis of corticosteroid hormones from cholesterol. Some cells belong to the hypothalamic-pituitary-adrenal axis and are the source of cortisol and corticosterone synthesis.

·         Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35–40 mg of cortisone acetate per day    Other cortical cells produce androgens such as testosterone, while some regulate water and electrolyte concentrations by secreting aldosterone.

·         In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted by the pituitary gland and hypothalamus, as well as by the renin-angiotensin system.

·         The cortex is divided into three zones, or layers. This division is sometimes referred to as "functional zonation".  Moving from the outermost layer in,

o   Zona glomerulosa, the zona glomerulosa is the main site for production of mineralocorticoids, namely aldosterone, which plays an important role in the body's sodium homeostasis.

o   Zona fasciculata, the zona fasciculata is responsible for producing glucocorticoids, chiefly cortisol in humans. Cortisol secretion is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary, by binding to a cell surface receptor and in turn increasing intracellular cAMP. In the absence of ACTH, the zona fasciculata secretes a basal level of cortisol.

o   Zona reticularis, the zona reticularis produces androgens, mainly dehydroepiandrosterone (DHEA) and DHEA sulfate in humans.

 

Adrenal Medulla

·         The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. The chromaffin cells of the medulla are the body's main source of the circulating catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine).

·         These water-soluble hormones, derived from the amino acid tyrosine, are part of the fight-or-flight response initiated by the sympathetic nervous system.

·         The adrenal medulla can be considered as specialized ganglion of the sympathetic nervous system, lacking distinct synapses, instead releasing secretions directly into the blood.

·         Noradrenaline is the postganglionic neurotransmitter of the sympathetic division of the autonomic nervous system.

·         Adrenaline and some noradrenaline are released into the blood from the adrenal medulla during stimulation of the sympathetic nervous system they are structurally very similar and this explains their similar effects. Together they potentiate the fight or flight response by:

• Increasing heart rate
• Increasing blood pressure
• Diverting blood to essential organs including the heart, brain and skeletal muscles by dilating their blood vessels and constricting those of less essential organs, such as the skin
• Increasing metabolic rate
• Dilating the pupils.

Adrenaline has a greater effect on the heart and metabolic processes whereas noradrenaline has more influence on blood vessels.

 

Arteries and veins

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

·         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 veins, 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.

Refer to Handout 16.1: Lymphatic drainage and Innervation of Adrenal Glands

Step 3: Functions of the Adrenal Gland (40 minutes)

·         The adrenal cortex produces three groups of steroid hormones from cholesterol. They are collectively called adrenocorticocoids (corticosteroids, corticoids). They are,

o   Glucocorticoids

o    Mineralocorticoids

o   Sex hormones (androgens).

·         The hormones in each group have different characteristic actions but due to their structural similarity the actions may overlap.

Glucocorticoids

·         Cortisol (hydrocortisone), is the main glucocorticoid but small amounts of corticosterone and cortisone are also produced.

·         They are essential for life, regulating metabolism and responses to stress. Secretion is controlled through negative feedback system involving the hypothalamus and anterior pituitary. It is stimulated by ACTH from the anterior pituitary and by stress

·         In non stressful conditions, secretion has marked circadia variations.

·         Glucocorticoids have widespread metabolic effects and these include:

o   gluconeogenesis (formation of new sugar from, for example, protein) and hyperglycaemia (raised blood glucose level)

o   lipolysis (breakdown of triglycerides into fatty acids and glycerol for energy production)

o   stimulating breakdown of protein, releasing amino acids, which can be used for synthesis of other proteins, e.g. enzymes, or for energy (ATP) production

o   Promoting absorption of sodium and water from renal tubules (a weak mineral corticoid effect).

In pathological and pharmacological quantities glucoocorticoids also have other effects including:

• anti-inflammatory actions
• suppression of immune responses
• delayed wound healing.

Mineralocorticoids (aldosterone)

·         Aldosterone is the main mineralocorticoid. Its functions are associated with the maintenance of water and electrolyte balance in the body.

·         Through a negative feedback system it stimulates the reabsorption of sodium (Na+) by the renal tubules and excretion of potassium (K+) in the urine. Sodium reabsorption is also accompanied by retention of water and therefore aldosterone is involved in the regulation of blood volume and blood pressure too.

·         The blood potassium level regulates the amount of aldosterone produced by the adrenal cortex. When the blood potassium level rises, more aldosterone is secreted. Low blood potassium has the opposite effect. Angiotensin also stimulates the release of aldosterone.

Renin-Angiotensin-Aldosterone System

·         When renal blood flow is reduced or blood sodium levels fall, the enzyme renin is secreted by kidney cells.

·         Renin converts the plasma protein angiotensinogen, produced by the liver, to angiotensin 1. Angiotensin converting enzyme (ACE), formed in small quantities in the lungs, proximal kidney tubules and other tissues converts angiotensin 1 to angiotensin 2, which stimulates secretion of aldosterone. It also causes vasoconstriction and increases blood pressure.

Sex Hormones

·         Sex hormones secreted by the adrenal cortex are mainly androgens (male sex hormones) and the amounts produced are insignificant compared with those secreted by the testes and ovaries in late puberty and adulthood.

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Activity:  Small Group Discussion (10 minutes)

ASK the students to divide into three groups.

GIVE   each group a task.

Group 1:    Describe alpha cell secretion in pancreas

Group 2:    Describe beta cell secretion in pancreas

Group 3:    Describe delta cell secretion in pancreas

GIVE five minutes to complete the task.

ASK each group to choose a volunteer who will present the answers on a flip chart, and other member of the class to add if they are having additional information.

SUMMARIZE the activity by clarifying on students responses.

Step 4: Hormones Secreted by Endocrine Pancreas (30 minutes)

Figure 3:  Anterior relations of the pancreas

Figure 4:  Posterior Relations of the Pancreas

·         The cells that make up the pancreatic islets (islets of Langerhans) are found in clusters irregularly distributed throughout the substance of the pancreas.

·         Unlike the exocrine pancreas, which produces pancreatic juice, there are no ducts leading from the clusters of islet cells. Pancreatic hormones are secreted directly into the bloodstream and circulate throughout the body.

·         There are three main types of cells in the pancreatic islets:

o   α  (alpha) cells, which secrete glucagon

o   β  (beta) cells, which secrete insulin

o   ∆ (delta) cells, which secrete somatostatin

·         The normal blood glucose level is between 3.5 and 8 mmol/litre (63 to 144 mg/100 ml, blood glucose levels are controlled mainly by the opposing actions of insulin and glucagon,

o   Glucagon increases blood glucose levels

o   Insulin reduces blood glucose levels.

Refer to Handout 16.2: Lymphatic drainage and Innervation of Pancreas

Figure 5:  Microstructure and Control of Function of the Endocrine Pancreas

·         The most numerous cells, types alpha and beta, secrete glucagon and insulin respectively. Alpha cells tend to be concentrated at the periphery of islets, and beta cells more centrally.

·         A third type, the delta cell, secretes somatostatin and gastrin, and like alpha cells, is peripherally placed within the islets.

·         A minor cell type, the F cell, secretes pancreatic polypeptide (PP), which is stored in smaller secretory granules. The autonomic transmitters acetylcholine (ACh) and noradrenalin affect islet cell secretion.

·         ACh augments insulin and glucagon release, noradrenalin inhibits glucose-induced insulin release and they may also affect somatostatin and PP secretion

Insulin

·         The main function of insulin is to lower raised blood nutrient levels, especially glucose but also amino acids and fatty acids.

·         When these nutrients, especially glucose, are in excess of immediate needs insulin promotes their storage by:

o   Acting on cell membranes and stimulating uptake and use of glucose by muscle and connective tissue cells

o   Increasing conversion of glucose to glycogen (glycogenesis), especially in the liver and skeletal muscles

o   Accelerating uptake of amino acids by cells, and the synthesis of protein

o   Promoting synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)

o   Decreasing glycogenolysis (breakdown of glycogen, into glucose)

o   Preventing the breakdown of protein and fat, and gluconeogenesis (formation of new sugar from, e.g., protein).

Glucagon

·         The effects of glucagon increase blood glucose levels by stimulating

o   Conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis)

o   Gluconeogenesis.

o   Somatostatin (GHRIH)

·         The effect of this hormone, also produced by the hypothalamus, is to inhibit the secretion of both insulin and glucagon in addition to inhibiting the secretion of GH from the anterior pituitary.

Refer to Handout 16.3: Function of Pancreas as Exocrine Gland

Step 5:  Functions of Local Hormones (5 minutes)

• A number of body tissues secret hormones that act locally, these are Histamine, serotonin, and Prostaglandins.
• Others are gastrointestinal hormones including Gastrin, Secretin and Cholecystokinin.
• Histamine is secreted by mast cells and Basophils during inflammation. It increases capillary permeability and causes vasodilatation.
• Prostaglandins have a wide range of physiological effects in:

o   The inflammatory response

o   Potentiating pain

o   Fever

o   Regulating blood pressure

o   Blood clotting

o   Uterine contraction during labour.

Step 6:  Key Points (5 minutes)

• The adrenal cortex produces three groups of steroid hormones from cholesterol. They are collectively called adrenocorticocoids (corticosteroids, corticoids). They are,

o   Glucocorticoids

o   Mineralocorticoids

o   Sex hormones (androgens).

·         There are three main types of cells in the pancreatic islets:

• α  (alpha) cells, which secrete glucagon
• β  (beta) cells, which secrete insulin
• ∆ (delta) cells, which secrete somatostatin

·         The main function of insulin is to lower raised blood nutrient levels, especially glucose but also amino acids and fatty acids. When these nutrients, especially glucose, are in excess of immediate needs insulin promotes their storage by:

o   Acting on cell membranes and stimulating uptake and use of glucose by muscle and connective tissue cells

o   Increasing conversion of glucose to glycogen (glycogenesis), especially in the liver and skeletal muscles

Step 7: Evaluation (5 minutes)

·         Describe the structure of Adrenal  gland

·         Describe the functions of Adrenal  gland

·         List the hormones secreted by endocrine pancreas

ASK students if they have any comments or need clarification on any points.

References

• Anne Waugh and Allison Grant (2001): Ross and Wilson Anatomy and Physiology in Health and Illness. Churchill Livingstone (UK)
• Kenneth S. Saladin. (2005): Human Anatomy: McGraw –Hill Companies .USA
• Moore, Keith, Agur, Anne (2007). Essential Clinical Anatomy, 3^rd Edition. Lippincott William & Wilkins
• Rod Seeley, Trent Stephens, Philip Tate. (2003):  Anatomy & Physiology: McGraw-Hill Companies. USA
• Williams & Wilkins Moore, Keith L.; Agur, Anne M. R. (2007):  Essential Clinical Anatomy, 3^rd Edition. Lippincott

Handout 16.1: Lymphatic drainage and Innervation of Adrenal Glands

Handout 16.2: Lymphatic drainage and Innervation of Pancreas

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