Endocrinology

Chapter 9

Features of Endocrine system  

  Along with nervous system, endocrine system coordinates and directs the body activities

• (Homeostasis dynamic equilibrium of internal environment( blood and tissue fluid that surrounds the body cells)

• How? By regulating sugar, electrolyte balance in the body fluids such as blood, extra cellular fluid 

Comprises of glands, which secrete chemicals – Hormones

Hormones: Defined as chemical substances secreted by endocrine glands into – extracellular fluids from where

they enter – blood stream via capillaries– And regulate metabolic activity

of the body cells

The scientific study of hormones and endocrine organs - Endocrinology

Endocrine system

Difference between endocrine and exocrine gland secretions

• Exocrine Glands:• Exocrine glands secrete

their contents into a duct

• The duct transports the secretions to the surface where its action is seen

• E.g. sebaceous glands

 

• Endocrine glands:• Endocrine glands secreted

contents (hormones) are picked up by the blood capillaries

• Transported to the target organ or tissue through the blood

• And stimulate a specific response through the receptors present on the specific organ 

Difference between endocrine and exocrine gland secretions

Comparison of endocrine system to nervous system

• Similarity: It helps in regulating the activities of various organ systems like nervous system

• Differences:

• It conveys its message through blood instead of neurons

• The response of target organ is slow and for longer duration instead of quick and short duration

Chemistry of Hormones • Hormone derived from a Greek word meaning

– to arouse ( causing a change in cell’s physiology by increasing or decreasing cellular activity)

• Hormones are classified as:

– Amino acid–based: which includes• Proteins • Peptides • Amines

– Steroids— made from cholesterol, includes sex hormones (ovary & testes)

– Prostaglandins— made from highly active lipids from cell membranes

Mechanisms of Hormone Action

• Hormones affect only certain tissues or organs (target cells or target organs)

• Each target tissue has a specific receptor for a specific hormone

• Hormone-binding alters cellular activity

Mechanisms of Hormone Action

• Two mechanisms in which hormones act

– Direct gene activation

– Second-messenger system

Second-Messenger System• If the hormone is a proteinous in nature

• Hormone binds to a membrane receptor

( also known as primary messenger)

• And activates enzymes which activates cAMP from ATP

• cAMP acts as a secondary messenger

• cAMP stimulates a set of enzymes known as protein kinases

• This triggers a series of reactions known as

signal transduction mechanism

 

• Which will ultimately activate or inhibit the enzymatic activity

• Affects metabolic pathway

• Activate or shut down

• eg. Glycogen breakdown by hormone glucagon

   

Second-Messenger System

Direct Gene Activation

(Steroid Hormone Action)• If the hormone (lipid soluble hormone)

is steroid in nature

• Hormones diffuse directly through the plasma membrane of target cells

• Enter the nucleus

• Bind to a specific receptor protein within the nucleus

• Forms hormone-receptor complex

• And then binds to specific sites on the cell’s DNA

• Activate genes that result in synthesis of new proteins

• Directly stimulates or inhibits gene expression

• e.g. sex hormones

Control of Hormone Release

• Hormone secretion is regulated by negative feed back mechanism

• A stimulus or low hormone levels in the blood triggers the release of more hormone

• Hormone release stops once an appropriate level in the blood is reached

• So that concentration of hormone remains relatively constant in the blood with slight fluctuations

Negative feed back loop e.g. thyroxine hormone

in resp o nse to in te rn a l a nd e xte rna l s tim u li re lea se thyrotropin releasing factor

th yro tro p in (T S H )

T h yro x ine

T h yro id g la nd

P itu ita ry

H yp oth a lam us

Other types of regulation

• Hormonal Stimuli:

• Endocrine glands are activated by other hormones eg. Anterior pituitary hormones

               

Other types of regulation 

• Humoral Stimuli:

• Changing blood levels of certain ions stimulate hormone release

– Parathyroid hormone– Calcitonin– Insulin

• eg. Insulin is released in blood from pancreas to regulate glucose level

Other types of regulation• Neural Stimuli:• Stimulation by

nervous system. e.g. stress stimulates adrenal gland to secrete epinephrine and norepinephrine 

Endocrine glands• Hypothalamus• Pituitary gland• Thyroid Gland• Parathyroid gland• Adrenal glands• Pancreas• Pineal gland• thymus gland• Reproductive glands

Hypothalamus• Part neuronal and part endocrine

in function

• Located in the diencephalon below the thalamus

• It is the connecting link between the environment and the endocrine glands

• By perceiving the changes in the environment (internal/external)

• And controlling the activities of pituitary gland

Pituitary Gland • It is also known as hypophysis

• It is a small gland about the size of a pea

• It is attached to the hypothalamus of the brain by a stalk called infundibulum

• It is called the master gland because it releases hormones that affect the working of other glands such as thyroid, gonads etc.

• It is divided into two lobes:– anterior pituitary– posterior pituitary

• Six anterior pituitary hormones– Two affect non-endocrine targets

• Growth hormone• Prolactin

– Four stimulate other endocrine glands (tropic hormones)

• Thyroid-stimulating hormone (thyrotropic hormone)• Adrenocorticotropic hormone• Two gonadotropic hormones

Hormones of the Anterior Pituitary

• Characteristics of all anterior pituitary hormones– Proteins (or peptides)– Act through second-messenger systems– Regulated by hormonal stimuli, mostly

negative feedback

Hormones of the Anterior Pituitary

Hormones of the Anterior Pituitary

• Growth hormone– General metabolic hormone– Major effects are directed to growth of skeletal

muscles and long bones– Plays a role in determining final body size– Causes amino acids to be built into proteins– Causes fats to be broken down for a source of

energy

Hormones of the Anterior Pituitary

• Growth hormone (GH) disorders– Pituitary dwarfism results from hyposecretion

of GH during childhood– Gigantism results from hypersecretion of GH

during childhood– Acromegaly results from hypersecretion of GH

during adulthood

Hormones of the Anterior Pituitary

Giganticism

Hypersecretion causes excessive growth as shown in these identical twins.

Giganticism

12-year old boy with

his mother.

Acromegaly Hypersecretion after puberty causes acromegaly.

Acromegaly Face

Acromegaly• Jaws and ridge of frontal bone continues to

grow. Andre the Giant was a WWF

wrestler.

• Prolactin (PRL)– Stimulates and maintains milk production following

childbirth

• Adrenocorticotropic hormone (ACTH)– Regulates endocrine activity of the adrenal cortex

• Thyroid-stimulating hormone (TSH)– Influences growth and activity of the thyroid gland

Hormones of the Anterior Pituitary

• Gonadotropic hormones– Regulate hormonal activity of the gonads

• Follicle-stimulating hormone (FSH)– Stimulates follicle development in ovaries

– Stimulates sperm development in testes

• Luteinizing hormone (LH)– Triggers ovulation of an egg in females

– Stimulates testosterone production in males

Hormones of the Anterior Pituitary

Posterior Pituitary

• Hormones present in posterior pituitary gland are actually secreted by hypothalamus

• But stored in posterior pituitary

• Hormone release from posterior pituitary is regulated by nerve impulses coming from hypothalamus

 Posterior pituitary • (i)  ADH or antidiuritic hormone:

• It is an antiurination hormone • Important for retention of water by kidneys

and prevent dehydration

• By decreasing urine volume increases Blood volume

• ADH deficiency causes excessive urine , diabetes insipidus

• It is also called vasopressin because ADH increases vasoconstriction and leads to increase blood pressure

• (ii) Oxytocin: works on uterus

• Important for uterine contraction during delivery

• Important for milk ejection in lactating women

 Posterior pituitary

Thyroid Gland

• Found at the base of the throat

• Consists of two lobes and a connecting isthmus

• Produces two hormones– Thyroid hormone

– Calcitonin

Thyroid Hormone• Thyroid hormone

– Major metabolic hormone

– Composed of two active iodine-containing hormones

• Thyroxine (T4)—secreted by thyroid follicles

• Triiodothyronine (T3)—conversion of T4 at target tissues

• Iodine is important for thyroxine formation

• Thyroid hormone is transported through blood to the target tissue

    • Thyroid hormones increases the rate of body metabolism

(BMR)

• Amount of energy required for an organism to survive in a resting stage

• Varies from person to person depending on height, weight, age, stress etc

• It is important for growth and development (protein, lipid and carbohydrate metabolism of all tissues)

• What happens when the hormone is over-secreted or under-secreted?

Thyroid Hormone

Thyroid hormone disorders

Hyperthyroidism or Graves disease:

• Enlargement of thyroid gland • Excess of thyroid secretion production

caused by tumor• Eyes bulge• Patient is hyperactive, irritable, nervous

etc• BMR increases• Treatable by surgery and radioactive

Iodine

Thyroid Hormone

Grave’s Disease

Simple Goiter: ( Iodine deficiency and hormone deficiency)

• Thyroid over stimulated because of lack of usable thyroid hormone

– due to lack of iodine

• Results in hypothyroidism

• Enlarged thyroid gland

More Goiters

• Hypothyroidism (Lack of stimulation by TSH):

• Crinitism: in child hood. Slow growth, mental retardation low body metabolism

• In adult: called as myxedema

• Results in physical and mental sluggishness

• puffiness of face, fatigue, obesity, dry skin

Hypothyroidism in Adults Myxedema

• Calcitonin:

• Thyroid gland also secretes calcitonin

• Important for bone formation by inhibiting osteoclast activity and

• Leading to increase in osteoblast activity thus incorporating Calcium into bone.

Thyroid Gland

Parathyroid gland• located posterior to thyroid

gland

• 4 in number

• Produces the hormone PTH

( parathyroid hormone)

• regulate calcium levels in the blood.

• stimulate osteoclasts when ever calcium levels falls in blood

• PTH and calcitonin work against one another and regulate Ca+2 levels in the blood

Calcium homeostasis of blood9–11 mg/100 ml

RisingbloodCa2+

levels

Thyroid glandreleasescalcitonin

Osteoclastsdegrade bonematrix and releaseCa2+ into blood

PTH

Calcitonin Calcitoninstimulatescalcium saltdepositin bone

Parathyroidglands releaseparathyroidhormone (PTH)

Thyroidgland

Thyroidgland

Parathyroidglands

FallingbloodCa2+

levels

Imbalance

Imbalance

Hormonal Regulation of Calcium in Blood

Adrenal glands  • Two adrenal glands are located

superior to kidney• It has two parts.• A. Adrenal cortex• B. Adrenal medulla• Adrenal glands activities are

controlled by hypothalamus • By producing Adrenocorticotropic

(ACTH ) hormone

Adrenal cortex: • outer region of adrenal gland.• It secretes several hormones.

• These hormones are essential for human survival

• Minerocorticoids • (e.g. aldosterone) help kidneys to retain

Na+ and excrete K+ levels in blood

• This helps in conservation of water by kidney and maintains blood volume and blood pressure

Adrenal glands  

• Glucocorticoid (including cortisone and cortisol)

• It regulates glucose metabolism

• When glucose levels in blood are low and glycogen levels are down

• Releases Cortisol

• Stimulate liver to build glucose from amino acids, fatty acids and glycerol

• In the process, it maintains blood sugar level

• In addition, cortisol relieves inflammation and stress

• Thus cortisol is released under SANS stimulation

• Adrenal cortex also produces Sex hormones

– Produced in the inner layer of the adrenal cortex

– Small amounts are made throughout life

– Mostly androgens (male sex hormones) are made

– Some estrogens (female sex hormones) are also formed

 Adrenal medulla:

Functions as a part of sympathetic nervous system. It secretes Epinephrine (adrenaline) and norepinephrine (noradrenaline)

which are important for physical activity such as increase blood sugar levels, heart rate The release of these hormones is stimulated by emotions, injury, stress etc.

Pancreas• Is located along small intestine

and stomach

• It is made of pancreatic islets(islets of Langerhans)  Each islet is made of

cells, which secrete hormone called Glucagon.

• It works on liver and releases sugar into blood to be used for energy.

• cells secrete insulin

• Its target organ is liver, muscle and adipose tissue.

• It helps in uptake of excess sugar in blood

• Low levels of insulin results in diabetes mellitus 

• Cannot store sugar in the body• excretes sugar in urine • ( hyperglycemia)• Starts breaking fat and protein stores

leading to production of ketones ( acidosis)• Treatable by taking insulin shots

Pancreas

 

• In general, parasympathetic nervous stimulation increases insulin secretion and helps in storage of sugar into glycogen

• Sympathetic stimulation inhibits secretion of insulin and helps so that sugar is released into blood and used for

physical activity

Pancreas

Insulin-secretingcells of the pancreasactivated; releaseinsulin into theblood

Elevatedblood sugarlevels

Stimulus:rising bloodglucose levels(e.g., aftereating fourjelly doughnuts)

Rising bloodglucose levelsreturn blood sugarto homeostatic setpoint; stimulus forglucagon releasediminishes

Blood glucoselevels declineto set point;stimulus forinsulin releasediminishes

Stimulus:declining bloodglucose levels(e.g., afterskipping a meal)

Low bloodsugar levels

Glucagon-releasingcells of pancreasactivated;release glucagoninto blood; targetis the liver

Uptake of glucosefrom blood is en-hanced in mostbody cells

Liver breaks downglycogen stores andreleases glucose tothe blood

Liver takes upglucose and storesit as glycogen

Homeostasis: Normal blood glucoselevels (90 mg/100ml)

Imbalance

Imbalance

• Ovaries– Produce eggs– Produce two groups of steroid hormone

• Estrogens

• Progesterone

• Testes– Produce sperm– Produce androgens, such as testosterone

Gonads

• Estrogens– Stimulate the development of secondary female

characteristics – Mature female reproductive organs

• With progesterone, estrogens also– Promote breast development– Regulate menstrual cycle

Hormones of the Ovaries

• Progesterone– Acts with estrogen to bring about the menstrual

cycle– Helps in the implantation of an embryo in the

uterus– Helps prepare breasts for lactation

Hormones of the Ovaries

Other hormone producing tissues 

• Thymus gland:      Present above the heart in the thoracic area.      Produces thymosin hormone important for maturation of T lymphocytes (WBC)

Pineal Body     Cone shaped structure

present behind midbrain attached to roof of the third ventricle.      Important for the production of melatonin which regulates the sleep and wake cycles

• with the help of the rods and cones of retina

     Prostaglandins

     Made by most of the cells of the body      Important for promotion of inflammation,

• constriction of blood vessels and bronchial tubes ,• cramps caused by the contraction of uterine wall

during menstrual cycle.      Antiprostaglandins( aspirin, ibuprofen) block production of prostaglandins thus reducing inflammation etc.