[MOD3LEC] Part 2 Endocrine System

Hypothalamus [Major Endocrine Glands And Organs]

A major gland of the endocrine system that regulates primitive functions of the body

• Regulation of water balance

• Thermoregulation

• S*x drive

• Childbirth

Hormones that stimulate the secretion of anterior pituitary gland hormones are known as releasing hormones

Hormones that suppress the secretion of anterior pituitary gland hormones are called inhibiting hormones

Name hypothalamic releasing and inhibiting hormones that regulate the anterior pituitary gland (refer to page 123)

• Thyrotropin-releasing hormone (TRH)

• Corticotropin-releasing hormone (CRH)

• Gonadotropin-releasing hormone (GnRH)

• Growth hormone-releasing hormone (GHRH)

• Prolactin-inhibiting hormone (PIH)

• Somatostatin (type of inhibiting hormone)

All of these either promote or inhibit the release of specific anterior pituitary gland hormones

IMPORTANT NOTE: ALL RELEASING HORMONES ACT SPECIFICALLY TOWARDS THE ANTERIOR PITUITARY GLAND

Pituitary gland [Major Endocrine Glands And Organs]

A gland composed of two structures: the anterior pituitary gland and the posterior pituitary gland

This gland grows about 50% larger in pregnancy

Anterior pituitary gland [Structures Of The Pituitary Gland]

Constitutes about three-quarters of the pituitary gland’s size

• Has no nervous connection to the hypothalamus but is linked by a complex of blood vessels known as the hypophyseal portal system

• The hypothalamus controls the anterior pituitary gland by secreting hormones that enter the primary capillaries, which travel down venules, and diffuse out of the secondary capillaries into the pituitary tissue

Name anterior pituitary gland hormones

• Follicle-stimulating hormone (FSH) - found in the ovaries and testes

• Luteinizing hormone (LH) - stimulates ovulation

• Thyroid-stimulating hormone (TSH) - stimulates growth of the thyroid gland and the secretion of thyroid hormone

• Adrenocorticotropic hormone (ACTH) - target is the adrenal cortex

• Growth hormone (GH) - stimulates mitosis and tissue growth in the body

Posterior pituitary gland [Structures Of The Pituitary Gland]

Constitutes a quarter of the pituitary gland’s size

• Is actually nervous tissue, and not a true gland

• Its nerve fibers arise in the hypothalamus, pass down the infundibulum as a bundle known as the hypothalamo-hypophyseal tract, ending in the posterior lobe of the pituitary gland

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Name posterior pituitary gland hormones

• Antidiuretic hormone (ADH) - increases water retention and reduces urine volume, preventing dehydration

• Oxytocin (OT) - heavily related to childbirth and emotional bonding of mother and infant

Pineal gland (circadian rhythm) [Major Endocrine Glands And Organs]

A pine cone shaped gland that regresses rapidly after age 7 and is a tiny mass of fibrous tissue in the adult

• Undergoes involution, or the shrinkage of an organ

• Pineal secretion peaks between the ages of 1 and 5 and declines 75% by the end of puberty

• Plays a role in establishing circadian rhythms, or the synchronization of the body with daytime and nighttime

Thyroid (overall metabolism) [Major Endocrine Glands And Organs]

The largest adult gland to have a purely endocrine function

• Receives one of the body’s highest rates of blood flow per gram of tissue and has a dark reddish brown color

• Secretes the hormone thyroxine, also known as tetraiodothyronine

• Also produces the hormone triiodothyronine

Thyroid hormone refers to both tetraiodothyronine and triiodothyronine

Thyroid hormone is secreted in response to TSH (thyroid stimulating hormone). What occurs in the body as a result of these thyroid hormones being released?

Thyroid hormones main job is to increase the body’s metabolic rate. When it is released, the body undergoes changes like:

• Increased respiratory rate

• Increase in heart rate

• Stimulation of appetite

• Promotes alertness and quickness of reflexes

• Promotes growth hormone secretion

• Promotes the breakdown of carbs, fats, and protein for fuel

Thyroid also responds to rising levels of blood calcium by releasing calcitonin

Parathyroid glands (calcium levels) [Major Endocrine Glands And Organs]

Ovoid or egg-shaped glands found in the posterior surface of the thyroid

• Secrete parathyroid hormone (PTH) in response to hypocalcemia

• PTH raises blood calcium levels by decomposition of bone matrix, or the release of calcium from bones

Thymus (T cell maturation) [Major Endocrine Glands And Organs]

A gland that plays a role in the endocrine, lymphatic, and immune systems

• Is enormous in the fetus and infants

• Continues to grow until the age of 5 or 6 years old

• Undergoes involution similar to the pineal gland and becomes less functional as a gland

• Secretes hormones like thymopoietin, thymosin, and thymulin that stimulate the development of other lymphatic organs

The thymus is a site of maturation for T cells that help defend the immune system

Adrenal glands [Major Endocrine Glands And Organs]

A gland formed by the merging of two different structures, the adrenal medulla and the adrenal cortex

• Sits like a cap on the superior part of the kidneys

Adrenal cortex [Structures Of The Adrenal Gland]

A structure that surrounds the adrenal medulla on all sides

• Produces more than 25 steroid hormones, known all as corticosteroids

The five most important corticosteroids fall into three categories, being mineralocorticoids, glucocorticoids, and corticosterone

What are the three categories of corticosteroids? (refer to page 132)

• Mineralocorticoids - aldosterone

• Glucocorticoids - corticosterone and cortisol

• S*x steroids - dehydroepiandrosterone (DHEA) which is converted into more potent forms like testosterone or dihydrotestosterone

Adrenal medulla [Structures Of The Adrenal Gland]

The inner core of the adrenal glands that makes up 10% to 20% of the gland

• Both an endocrine gland and a ganglion of the sympathetic nervous system

• Sympathetic preganglionic nerve fibers penetrate through the cortex to reach chromaffin cells in the medulla

The chromaffin cells in the medulla are considered neuroendocrine cells. When stimulated by the nerve fibers, the chromaffin cells release catecholamines, which increase alertness and prepare the body for physical activity

Describe catecholamines

They are made up of about three-quarters epinephrine, one-quarter norepinephrine, and a trace of dopamine

Epinephrine is said to have a glucose-sparing effect, which inhibits the secretion of insulin. This causes the use of alternative fuels, while any blood glucose is left for use by the brain

Pancreas [Major Endocrine Glands And Organs]

An elongated, spongy gland located inferior and posterior to the stomach, most of it being retroperitoneal

• Most of it is an exocrine digestive gland

• Scattered throughout the exocrine tissue are endocrine cell clusters known as pancreatic islets (islets of Langerhans)

• These pancreatic islets secrete hormones of incredible importance such as in process of regulating glycemia

The main cell types of the pancreas are alpha cells (20%), beta cells (70%), and delta cells (5%)

Alpha cells [Cells Of The Pancreas]

Secrete glucagon between meals when the blood glucose concentration falls below 100 mg/dL

• Glucagon then stimulates gluconeogenesis and glycogenolysis, and as a result the release of glucose into circulation, raising the blood glucose level

• Stimulates fat catabolism in adipose tissue, releasing free fatty acids

• Promotes amino acid absorption and provides cells with the materials required for gluconeogenesis

Beta cells [Cells Of The Pancreas]

Secrete insulin during and immediately following a meal when blood nutrient levels are rising

• Principal targets are the liver, skeletal muscles, and adipose tissue

• In times of overabundance, insulin also stimulates cells to absorb glucose, fatty acids, and amino acids and to store or metabolize them

• Promotes the synthesis of glycogen, fat, and protein, thereby promoting the storage of nutrients for later use

Insulin released by beta cells antagonize glucagon, which is released from alpha cells

Delta cells [Cells Of The Pancreas]

Secrete somatostatin, or hormones that inhibit growth hormone (GH)

• The secretion of somatostatin occurs at the same time that insulin is released by beta cells

• Somatostatin inhibits the secretion of stomach acid

Gonads (cytogenic glands) [Major Endocrine Glands And Organs]

Similar to the pancreas, are glands that are both endocrine and exocrine

• Its exocrine products are whole cells, such as the eggs and sperm

• Its endocrine products are the gonadal hormones, most of which are steroids

Testes [Types Of Gonads]

A type of gonad consisting of minute seminiferous tubules that produce sperm

• Its endocrine secretions are testosterone, inhibin, and lesser amounts of weaker androgens and estrogens

• Inhibin comes from sustentacular (Sertoli) cells

Between the seminiferous tubules are interstitial cells that are the source of testosterone and the other sex steroids

Ovaries [Types Of Gonads]

A type of gonad that primarily secrete estradiol, progesterone, and inhibin

• Each egg develops in its own follicle, which is lined by a wall of granulosa cells and surrounded by a capsule known as the theca

• Theca cells synthesize the androgen androstenedione

• Granulosa cells converts androstenedione to estradiol, estriol, and estrone

REVIEW CARD Name thyroid hormones

Tetraiodothyronine, triiodothyronine

REVIEW CARD Name thymus hormones

Thymopoietin, thymosin, thymulin

REVIEW CARD What type of steroids do the adrenal glands produce? What three categories do they fall into?

The adrenal glands produce corticosteroids. They fall into the categories of mineralocorticoids, glucocorticoids, and s*x steroids

REVIEW CARD Provide the specific names of some mineralocorticoids, glucocorticoids, and s*x steroids

Mineralocorticoids - alodsterone

Glucocorticoids - cortisol, corticosterone

Sex steroids - dehydroepiandrosterone (DHEA), estradiol

REVIEW CARD What do alpha cells secrete? What do they stimulate?

Alpha cells secrete glucagon when the blood glucose concentration falls below 100 mg/dL

Glucagon stimulates gluconeogenesis and glycogenolysis, releasing glucose into circulation

It stimulates fat catabolism in adipose tissue, releasing free fatty acids

REVIEW CARD What do beta cells secrete? What do they stimulate?

Beta cells secrete insulin during and immediately following a meal

Insulin stimulates cells to absorb glucose, fatty acids, and amino acids, storing or metabolizing them to lower blood glucose levels

Its main targets are the liver, skeletal muscles, and adipose tissue

REVIEW CARD What do delta cells secrete? What do they inhibit?

Delta cells secrete somatostatin (growth hormone-inhibiting hormone) at the same time that insulin is being released by beta cells

Somatostatin inhibits the secretion of stomach acid

REVIEW CARD Name anterior pituitary gland hormones

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Growth hormone (GH)

Skin [Endocrine Functions Of Other Tissues And Organs]

Keratinocytes of the epidermis convert a cholesterol-like steroid into cholecalciferol (Vitamin D₃), using UV radiation from the sun

• The liver and kidneys further convert cholecalciferol into calcitrol

Liver [Endocrine Functions Of Other Tissues And Organs]

Involved in the production of at least five hormones

• Converts cholecalciferol from the skin into calcidiol

• Secretes a protein called angiotensinogen, which is conveted into angiotensinogen, which the kidneys and lungs transform into angiotensin II