The Endocrine System

Part 3: Integration & Control

The Endocrine System

Endocrinology: The study of the endocrine system.

Endocrine System: Communicates with & helps control body systems via hormones.Communication is slower than in the nervous

system & has longer-lasting effects. Hormones: Chemicals secreted directly

into the bloodstream to affect target cells or organs.

Glands

Exocrine Glands: Secrete hormones into ducts, which then drain the hormones into another structure. E.g., salivary and sweat glands.

Glands

Endocrine Glands: Secrete the hormones directly into the bloodstream. Typically rely on…Negative Feedback Systems: Releases

hormones to counteract the effects of other hormones with the goal of maintaining homeostasis.

Hormones

Three Chemical Classes of Hormones:Steroid HormonesAminesPeptide (Protein) Hormones

Local Hormones: Eicosanoids

Hormones

Steroid Hormones:Lipid-solubleHydrophobic (NOT water-soluble)Must bind to transport proteins to effect target

cells.Examples:

Aldosterone Estrogen Calcitriol Testosterone

Hormones

Amines aka Biogenic Amines: Require tyrosine (an amino acid) for

synthesis.Hydrophilic (water-soluble)Examples:

Epinephrine Dopamine Thyroid hormones

Hormones

Peptide Hormones aka Protein Hormones:Amino acid polymersHydrophilic (water-soluble)Examples:

Oxytocin ADH hGH

Hormones

Eicosanoids aka Local Hormones: Considered paracrine secretions Have very brief effect on many cells f the body.

Eicosanoids Include… Prostoglandins: Secreted by organs to serve various

functions. Leukotrienes: Control allergic & inflammatory

reactions. Examples:

Prostadcyclin: Inhibits blood clotting & vasoconstriction Thromboxanes: Override the effects of prostacyclin when

injured.

Hormone Effects

Hormone Receptors: Protein or glycoprotein molecules on target cells. Site of binding for hydrophobic hormones on the cell

nucleus. Binding allows alterations in the gene expression of

the cell, which triggers the receptor cell’s response.

Second-Messenger Systems: System for hormone binding for hydrophilic hormones. Causes the activation of enzyme molecules to

catalyze the desired reaction.

Hormone Effects

Up-Regulation: Hormones increase the number of receptors & therefore sensitivity.

Down-Regulation: Hormones decrease the number of receptors & therefore sensitivity.

Synergistic Effects: Hormones work together to produce a combined greater effect.

Permissive Effects: One hormone enhances the target cell’s response to another hormone.

Antagonistic Effects: Hormones oppose each other’s effects.

Feedback Systems

Negative Feedback Mechanisms: Maintains the body condition in question within a small “normal range” of its set point.MOST hormones work using negative

feedback!Examples:

Blood sugar range (80-120mg/ml) Body Temperature (36.5-38*C) Blood pH Your thermostat!

Endocrine Glands: Hypothalamus

Hypothalamus: Controls primary functions, including water balance, sleep & sex drive, & acts as a main controller for the ANS. Produces 9 Major hormones:

7 control the Pituitary gland Oxytocin (OT): Stored in Posterior Pituitary Gland. Antidiuretic Hormone (ADH): Stored in Posterior Pituitary

Gland.

Infundibulum: Stalk that connects the hypothalamus & pituitary glands.

Sella Turcica: Bony saddle-shaped structure on the superior surface of the sphenoid.

Endocrine Glands: Pituitary

Pituitary Gland aka Hypophysis: The “master gland: of the endocrine system, controlled by the hypothalamus. Two parts: Anterior Pituitary Gland: aka

Adenohypophysis: Posterior Pituitary Gland: aka

Neurohypophysis

Endocrine Glands: Pituitary

Anterior Pituitary Gland: Made up of..

Anterior Lobe Pars Tuberalis

Hypothalamo-Hypophyseal Portal System: Allows blood to flow from capillaries in the hypothalamus into the portal veins of the anterior pituitary. Transports the inhibitory & releasing hormones from

the hypothalamus to control the APG.

Endocrine Glands: Pituitary

Anterior Pituitary Hormones: 7 Major Hormones

Gonadotrophs: Hormones that target sex hormones.

Follicle-Stimulating Hormone (FSH): Stimulates follicle & egg development in females & sperm production in males, stimulates estrogen secretion.

Luteinizing Hormone (LH): Stimulates ovulation in females & testosterone secretion in males.

Endocrine Glands: Pituitary

Trophic Hormone Gonadotrophin: Thyrotrophs: Stimulates the seceretion of

thyroid hormones. Thyroid-Stimulating Hormone (TSH) aka

Thyrotropin.

Somatotrophs: Stimulates Human Growth Hormone (HGH) for body growth & development & metabolism regulation.

HGH also controls Insulinlike Growth Factors, important in regulating hypoglycemic & hyperglycemic reactions.

Endocrine Glands: Pituitary

Lactotrophs: Trigger mammary glands to produce milk via prolactin (PRL).

Can indirectly stimulate testosterone secretion in males.

Corticotrophs: Hormones that stimulate the adrenal cortex to secrete glucocorticoids.

Adrenocorticotropic Hormone (HCTH) aka Corticotropin: Stimulates adrenal cortex.

Melanocyte-Stimulating Hormone (MSH): Can darken skin when levels are too high.

Endocrine Glands: Pituitary

Posterior Pituitary Gland: Doesn’t synthesize hormones, but stores hypothalamus-produced hormones. Oxytocin (OT): Important in enabling positive-

feedback mechanism of labor & in the ejection of milk from mammary glands.

Antidiuretic Hormone (ADH): Decreases the production of urine from the kidneys to prevent dehydration.

Increases blood volume &U pressure due to water retention in kidneys, sweat glands, & blood vessels.

Endocrine Glands: Pineal

Pineal Gland: Mass of neural and secretory (pinealocytes) cells siitting on the roof of the brain’s 3rd ventricle. Melatonin: Only hormone produced – acts as an

antioxidant, regulates internal biological clock, circadian rhythms.

Production is INCREASED in darkness and DECREASED in bright light.

Seasonal Affective Disorder (SAD): Depression triggered by overproduction of melatonin during the winter’s lack of natural light.

Endocrine Glands: Thyroid

Thyroid Glands: Largest & most vascular endocrine gland; inferior to larynx with lobes on either side of the trachea. Thyroid Follicles: Sacs that make up the

majority of the thyroid gland and store the hormones produced.

3 Specialized Thyroid Hormones: Triiodothyronine Thyroxine Calcitonin

Endocrine Glands: Thyroid

3 Specialized Thyroid Hormones: Triiodothyronine aka T3: Increases Basal

Metabolic Rate (BMR) to increase the rate of oxygen consumption; increases ATP production, accelerates growth, & stimulates protein synthesis.

Contains 3 iodine atoms; produced by follicles & diffused into blood stream.

Released during cold, pregnancy, low metabolic rates, or low thyroid hormone levels.

Release is regulated by the hypothalamus.

Endocrine Glands: Thyroid

3 Specialized Thyroid Hormones: Thyroxine aka Tetraiodothyronine aka T4:

Produces the same basic effects as T3, but is less potent & released in much larger quantities.

Contains 4 iodine atoms; produced by the follicles & diffuses into the bloodstream.

Target cells may concert T3 to T4

Calcitonin: Lowers the level of calcium in the blood by inhibiting osteoclasts.

Produced by the parafollicular cells in between the follicles of the thyroid & triggered by high Ca2+ levels in the bloodstream.

Thyroid Disorders

Hypothyroidism: Lowered production of thyroid hormones. Can be passed down by the mother and result in stunted growth, low body temperature, and possible mental retardation.

Myxedema: Low metabolism, increased weight gain, & high blood pressure due to low levels of thyroid hormones.

Thyroid Disorders

Goiter: Pathological enlargement of the thyroid gland, typically caused by iodine deficiency.

Graves Disease: An autoimmune disease where antibodies cause the thyroid to grow abnormally. Can lead to lead to elevated heart rates and

metabolism.Triggered by a goiter becoming toxic.

Endocrine Glands: Parathyroid

Parathyroid Glands: Regulates calcium, magnesium, & phosphate ion levels in the blood via parathyroid hormone (PTH). Found behind the thyroid, attached to the

thyroid. PTH & Calcitonin work together to regulate

homeostasis of calcium in the bloodstream. Stimulates Calcitriol to aid in calcium

absorption from food.

Parathyroid Disorders

Hyperparathyroidism: Too much parathyroid hormone is produced, causing elevated levels of calcium in the bloodstream. Leads to softening of the bones & a risk for kidney

stone formation.

Hypoparathyroidism: Too little parathyroid hormone is produced, causing the spontaneous creation of action potentials. Leads to muscle twitches, spasms, & tetany.

Endocrine Glands: Thymus

Thymus Gland: Assists the immune system. Produces hormones responsible for

developing & regulating T-cells crucial to the autoimmune responses.

Thymosin Thymic Humoral Factor (THF) Thymic Factor (TF) Thymopoietin

Endocrine Glands: Adrenals

Adrenal Glands: Located on top of the kidneys. Adrenal Cortex: Outer layer

Zona Glomerulosa: Secrete mineralcorticoids to maintain homeostasis.

The enzyme renin to stimulate aldosterone (regulates sodium & potassium levels & controls blood pressure by increasing blood volume.)

Zona Fasciculata: Secretes glucorticoids to increase energy supplies, regulate metabolism, break down protein & tryglycerides, & help resist stress.

Cortisol (90% of glucorticosteroid activity) is released in response to stress. Release controlled by the hypothalamus; increases metabolism, lowers inglammation, depresses immune response.

Endocrine Glands: Adrenals

Zona Reticularis: Responsible for the synthesis & secretion of androgens (masculinizing hormones).

Dehydroepiandrosterone (DHEA) is important for female sex drive & can be converted to estrogen.

Adrenal Medulla: Inner Layer composed of autonomic nervous system ganglia.

Chromaffin cells: Ganglia that release stress hormones instead of neurochemicals that increase heart rate, blood pressure, blood flow, oxygen intake, & glucose production.

Epinephrine aka Adrenaline Norepinephrine aka Noradrenaline

The Pancreas

Pancreas: Functions as both an endocrine & exocrine gland.

Pancreatic Islets aka Islets of Langerhans: Function as endocrine glands; series of 4 different cells. Alpha Cells (A Cells): Secrete glucagon to

raise the blood sugar level by triggering glycoeogenesis in the liver.

Beta Cells (B Cells): Secrete insulin to lower the blood sugar level by increasing the synthesis of glycogen, protein, & fat.

The Pancreas

Delta Cells (D Cells): Secrete somatostatin to inhibit growth hormone & regulate the secretion of insulun & glucagon.

F Cells: Secrete Pancreatic Polypeptide to inhibit somatostatin secretion & the secretion of enzymes from the gallbladder & pancreas.

Gonads

Gonads: The ovaries (females) and testes (males). Ovaries: Produces estrogen, progesterone, inhibin, &

relaxin. Mammary gland production, the menstrual cycle, and

pregnancy are maintained by progesterone, estrogen, FSH, & LH.

Inhibin inhibits the secretion of FSH. Relaxin is important for delivery.

Testes: Produce testosterone, which regulates the production of sperm & helps develop masculine features (facial hair, deep voice, etc.)

Other Hormones

Atrial Natriuretic Factor (ANF): Produced by the heart when blood pressure is too high, causing the kidneys to increase urine output & lower the blood pressure.

Calcitriol: Produced by the kidneys and used to regulate calcium levels and works with FPO to stimulate red blood cell production.

Humon Chorionic Gonadotropin (HCG): Produced by the placenta to support pregnancy.

Stomach & small intestine secrete hormones that help regulate digestion.

General Adaptation Syndrome

General Adaptation Syndrome: The three stages the body goes through in “fight or flight” mode or during times of stress. Alarm (Fight or Flight) Reaction: Norepinephrine

and epinephrine are released to arouse the body. Blood glucose, aldosterone, and angiotension levels

increase. Resistance Reaction: If stress continues, cortisol is

increased to break down fats & proteins to glucose for instant energy.

Exhaustion: If the stress still continues, cortisol levels become too high and cause headache, ulcers, & immune compromise.