endocrinology -mr. panneh

THE ENDOCRINE SYSTEM

AND HORMONES

MR. ABDOU PANNEH

(RM ,FWACN,DHAF,NT, RN)

INTRODUCTION

• The endocrine system is part of the chemicalcommunication system of the body andcomplements the nervous system.

• The nervous system evokes rapid but short-termresponses whereas the former (endocrinesystem) causes slower and more sustainedeffects.

• The endocrine system consists of glands widelyseparated from each other with no directanatomical links.

• They are commonly referred to as the ductlessglands because the hormones they secrete passdirectly into the blood stream.

• A hormone is a chemical messenger which,having been formed in one organ or gland, iscarried in the blood to another organ (targetorgan) or tissue, probably quite distant, where itinfluences activity, growth, nutrition, etc.

HYPOTHALAMUS AND PITUITARY GLAND

• Although the hypothalamus is classified as apart of the brain and not as an endocrinegland, it has a direct controlling effect on thepituitary gland and an indirect controllingeffect on many others.

• The pituitary gland (master gland) and thehypothalamus act as a unit, regulating theactivity of most of the other endocrine glands.

• The pituitary gland is located just below thehypothalamus, to which it is attached by astalk. The pituitary gland consists of two mainparts:

The adenohypophysis (anterior lobe) is an upgrowth of tissues from the pharynx.

The neurohypophysis (posterior lobe) is adown growth from the brain.

Adenohypophysis

• There are several hormones secreted by theadenohypophysis:

Growth hormone (GH)

• GH is synthesised by the anterior pituitarygland.

• Its release is stimulated by GHRH (growthhormone releasing factor) secreted by thehypothalamus.

• GH promotes growth of the skeleton, muscles,connective tissue and organs such as kidneys,liver, adrenal glands, intestines, pancreas, etc.Increased secretion is stimulated by exercise,anxiety, sleep and hypoglycaemia.

• It stimulates growth and activity of the thyroidgland which secretes the hormones thyroxin(T4) and triiodothyronine (T3).

Thyroid Stimulating Hormone

(TSH)

• This hormone is synthesised by the anteriorpituitary gland and its release is stimulated bythyroid releasing hormone (TRH).

• Secretion is also regulated by a negativefeedback mechanism. When the blood level ofthyroid hormones is high secretion of TSH isreduced and vice versa.

Adrenocorticotrophic Hormone (ACTH)

• Corticotrophin releasing factor (CRF) from thehypothalamus promotes the synthesis andrelease of ACTH by the anterior pituitary.

• This stimulates the flow of blood to theadrenal cortex, increases the concentration ofcholesterol and steroids within the gland andincreases the output of steroid hormones,especially cortisol.

Prolactin

• This hormone has a direct effect on thebreasts immediately after parturition.

• Suckling stimulates prolactin secretion and theresultant high blood level is a factor inreducing the incidence of conception duringlactation.

Gonadotrophic hormones

• The anterior lobe secretes two gonadotrophic orsex hormones in females and males:

Female gonadotrophic hormones

• Follicle-stimulating hormone stimulates thedevelopment and ripening of the ovarian follicle.

• During its development the ovarian folliclesecretes its own hormone, oestrogen. As thelevel of oestrogen increases in the blood so FSHsecretion is reduced.

• Luteinising hormone promotes the finalmaturation of the ovarian follicle andovulation (discharge of the mature ovum).

• Its main function is to promote the formationof the corpus luteum which secretes thesecond ovarian hormone, progesterone.

• As the level of progesterone in the bloodincreases there is a gradual reduction in theproduction of the Luteinising hormone.

Male gonadotrophic hormones

• Follicle-stimulating hormone stimulates theepithelial tissues of the seminiferous tubulesin the testes to produce spermatozoa.

• Luteinising hormone stimulates the interstitialcells in the testes to secrete the hormonetestosterone.

Neurohypophysis

• The posterior lobe of the pituitary glandreleases the hormones oxytocin andantidiuretic hormone (ADH).

• These two hormones are synthesized by thecells of the hypothalamus and migrate alongnerve fibres to the posterior pituitary wherethey are stored in nerve endings.

• Each hormone is released in response to adifferent stimulus.

Oxytocin

• Oxytocin promotes contraction of uterinemuscle and contraction of the myoepithelialcells of the lactating breast, squeezing milk intothe large ducts behind the nipple.

• In late pregnancy the uterus becomes verysensitive to oxytocin. The amount secreted isincreased just before and during labour and bythe suckling baby.

Antidiuretic hormone/ vasopressin

• This hormone has two main functions:

• Antidiuretic effect – this hormone stimulatesthe kidneys to reabsorb more water.

• The amount of ADH secreted is influenced bythe osmotic pressure of the blood circulatingto the osmoreceptors in the hypothalamus.

• As the osmotic pressure rises the secretion ofADH increases and more water is reabsorbed.

• Conversely, when the osmotic pressure of theblood is low, the secretion of ADH is reduced,less water is reabsorbed and more urine isproduced.

• The blood level of ADH is increased indehydration and following haemorrhage.

• Pressor effect – In pharmacological doses,ADH stimulates contraction of smooth muscle,especially in blood vessel walls of the skin andabdominal organs, raising the blood pressure.

• It is not clear whether physiological amountshave a significant pressor effect.

THYROID GLAND

• The thyroid gland, which is situated at thefront of the neck, partially surrounding thetrachea, plays an important role in regulatingthe body’s metabolism.

• It consists of two lobes, one on either side ofthe thyroid cartilage. The lobes are joined by anarrow isthmus, lying in front of the trachea.

• The lobes are roughly cone-shaped, about5cm long and 3cm wide.

Function of the thyroid hormones

• Iodine is essential for the formation of thethyroid hormones, thyroxin (T4) andtriiodothyronine (T3).

• It is ingested in food and most of it is taken upby the gland and used in hormone formation.

• The release of thyroid hormones is regulatedby thyroid releasing hormone (TRH) from thehypothalamus.

• Thyroid releasing hormone stimulates theanterior pituitary gland to secrete thyroidstimulating hormone (TSH) which in turn actson the thyroid to secrete its hormones.

• The hormones produced by the thyroid glandact on cells to control the body’s metabolism,including the use of energy.

• They are also associated with the growth anddevelopment of tissues, especially thenervous system.

• Increased levels of T3 and T4 decrease thyroidstimulating hormone secretion and vice versa.

• When the supply of iodine is deficient, excessTSH is secreted and there is proliferation ofthyroid gland cells and enlargement of thegland.

• Secretion of T3 and T4 begins about the 3rd

month of fetal life and is increased at pubertyand in women during the reproductive years,especially during pregnancy.

• Calcitonin is secreted by the C-cells in thethyroid. It acts on bone and the kidneys toreduce the blood calcium level.

PARATHYROID GLANDS• There are four small parathyroid glands, two

embedded in the posterior surface of eachlobe of the thyroid gland.

• The function of the parathyroid glands is tosecrete the hormone parathryn (PTH).

Function of parathryn

• Secretion of PTH is regulated by the bloodlevel of calcium. When this falls, secretion ofPTH is increased and vice versa.

• The main function of PTH is to maintain theblood concentration of calcium within normallimits.

• This is achieved by influencing the amount ofcalcium absorbed from the small intestinesand reabsorbed from the renal tubules.

ADRENAL/SUPRARENAL GLANDS

• There are two adrenal glands, one situated onthe upper pole of each kidney enclosed withinthe renal fascia. They are about 4 cm long and3cm thick.

• The glands are composed of two parts whichdiffer both anatomically and physiologically.

• The outer part is the cortex and the inner part,the medulla. The cortex is essential to life butthe medulla is not.

Adrenal cortex

• The adrenal cortex produces three groups ofhormones:

a. Glucocorticoids

b. Mineralocorticoids

c. Androgens (sex hormones)

Glucocorticoids

• Cortisol (hydrocortisone) and corticosteroneare the main glucocorticoids.

• Secretion of glucocorticoids is stimulated byadrenocorticotrophic hormone (ACTH) fromthe anterior pituitary and by stress.

• Glucocorticoids have widespread effects onbody systems. The main functions include:

Regulation of carbohydrate metabolism

Promotion of the formation and storage ofglycogen.

Gluconeogenesis from protein, raising theblood glucose level.

Promotion of sodium and water reabsorptionfrom the renal tubules

In pathological and pharmacologicalquantities glucocorticoids have anantinflammatory action and suppress theresponse of the tissues to injury, delayinghealing.

Mineralocorticoids

• Aldosterone is the main mineralocorticoid. Itsfunction is associated with the maintenance ofthe electrolyte balance in the body.

• It stimulates the reabsorption of sodium bythe renal tubules and when the amount ofsodium reabsorbed is increased the amount ofpotassium excreted is increased.

• Aldosterone increases the reabsorption ofwater and sodium and the excretion ofpotassium by the kidneys.

• The amount of aldosterone produced isinfluenced by the sodium level in the blood. Ifthere is a fall in the blood sodium level morealdosterone is secreted and more sodiumreabsorbed.

Adrenal medulla

• The medulla is completely surrounded by thecortex. It produces the hormones adrenalineand noradrenaline.

• The adrenal medulla is not essential to life butits hormones greatly assist the body in itsresponse to adverse environmentalconditions.

• The main function of noradrenalin ismaintenance of blood pressure by causinggeneralized vasoconstriction, except of thecoronary arteries.

• Adrenaline is associated with potentiating theconditions needed for ‘fight or flight’, e.g.:

Constricting skin blood vessels

Dilating blood vessels of muscles, heart andbrain

Converting glycogen to glucose

Increasing the metabolic rate

Dilating the pupils

Dilating the bronchioles, allowing an increasein air intake

ISLETS OF LANGERHANS

• The cells which make up the islets ofLangerhans are found in clusters irregularlydistributed throughout the substance of thepancreas.

• Their secretion passes directly into thepancreatic veins and circulates throughout thebody.

• There are three main types of cells in the isletsof Langerhans:

• Alpha cells that secrete glucagon,

• β (beta) cells that secrete insulin,

• and delta cells that secrete somatostatin

• Insulin and glucagon influence the level ofglucose in the blood, each balancing theeffects of the other.

• Glucagon tends to raise the blood glucoselevel and insulin reduces it.

• The normal blood glucose level is between2.5and 5.3mmol/litre (45 to 95mg/100ml).

• When there has been excessive exercise or aninsufficient intake of carbohydrate foods itmay fall to the low end of the normal range orbelow.

• When this happens glucagon has the effect ofraising the blood glucose level by mobilisingthe glycogen stores in the liver.

• Insulin acts on all cell membranes, stimulating the uptake of glucose, amino acids and fats.

• In addition it is associated with:Conversion of glucose to glycogen

in the liver and musclesSynthesis of DNA and RNAStorage of fat as adipose tissuePrevention of gluconeogenesis and

the breakdown of protein and fat

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