Endocrine System (rev 5-10)

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Endocrine System (rev 5-10)

The endocrine system is a collection of specialized cells, tissues and glands that produce and secrete chemical “messengers” called hormones that control many body functions.

Endocrine glands are ductless and secrete their hormones into interstitial fluid, lymph, and blood.

• Exocrine glands secrete its products into ducts that empty into the appropriate sites.

Hormones are bloodborne “information” units.– Come from endocrine glands– Circulate in the bloodstream– Act on specific cells in the body

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• Endocrine glands are stimulated in one of three ways:

1. By nervous impulses (usually from the hypothalamus)

2. By hormones (usually from the hypothalamus or anterior pituitary)

3. By humoral stimulation (usually a chemical [glucose, calcium] )regulated by the hormone which activates or inhibits hormone release

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Endocrine System Characteristics

• Access to every cell because hormones circulate in the blood

• Each hormone acts only on specific cells (target cells) because only the hormone’s target cells have the appropriate receptor to fit it;

• Endocrine control slower than nervous system• Endocrine and nervous systems interact i.e.

timing of growth and sexual maturation involves a complex sequence of changes in both endocrine and nerve signals; release of some hormones is dependent on input from sensory neurons.

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Classification of Hormones

• Hormones are classified as “steroid” or “nonsteroid” based on their structure and mechanism of action.

• Steroid hormones:

– Lipid soluble, chemically derived from cholesterol

– Enter target cells, bind to an intracellular receptor and activate genes that produce new proteins

– Slower acting than nonsteroid hormones; minutes to hours depending on how long it takes to produce a new protein

Steroid hormones pass through the plasma membrane and act in a two step process. Once inside the cell, steroid hormones bind to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates mRNA to produce new proteins.

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• Nonsteroid hormones:– Water soluble; derived from amino acid protein building

blocks– Bind to receptors on target cell membranes to convert

an inactive molecule within the cell into an active molecule. The activated molecule produces a chemical which is called a second messenger.

– Work through intermediate mechanisms to activate existing enzymes so that even a small amount of hormone can produce a significant cellular change

– Faster action than steroid hormones, seconds to minutes

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Homeostasis is generally maintained by a negative feedback loop.

In a negative feedback loop involving a hormone, the endocrine gland is the control center, the hormone represents the pathway between the control center and the effectors, the hormone’s target cells, tissues, or organs.

• An endocrine system negative feedback loop is a stable, self-adjusting mechanism for maintaining homeostasis of the controlled variable because any change in the controlled variable causes a response that reverses that change.

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Hypothalamus and the Posterior Pituitary Gland

The hypothalamus – plays an important role in the regulation of

homeostasis. (It monitors fluid and electrolyte balance, temperature, and carbohydrate metabolism.)

– Also produces hormones– Hypothalamic hormones stimulate the release of most

pituitary hormones

• Has specialized neuroendocrine cells which function as both nerve and endocrine cells; they generate nerve impulses and release hormones into the blood vessels; called the neuroendocrine reflex

• Monitors the pituitary gland

– Releasing hormones from the hypothalamus are actually neurosecretions

– Interaction between the hypothalamus and the pituitary demonstrates the relationship between the endocrine and the nervous systems

– Is physically connected to the hypothalamus by a thin strip of tissue called the infidibulum

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Pituitary Gland (also called hypophysis)– Called the “master gland” because it

secretes 8 different hormones which regulate many of the other endocrine glands;

– Consists of 2 lobes: posterior and anterior1. Posterior pituitary (lobe):

–Neuroglial-like supporting cells and nerve fibers

2. Anterior pituitary (lobe) (adenohypophysis)

–Glandular tissue

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Posterior pituitary:

– Connection to hypothalamus: hormones made in hypothalamus, stored in posterior pituitary

– Hormones: nonsteroidal; made by neuroendocrine cells

• Antidiuretic hormone (ADH): causes reabsorption of water in kidneys, regulates water balance in body

• Oxytocin: causes uterine contractions during labor and milk ejection

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Hypothalamus and the Anterior Pituitary Gland

• Anterior pituitary:– Releasing and inhibiting hormones from the

hypothalamus travel to anterior pituitary through the pituitary portal system--a special blood supply that runs directly between the hypothalamus and the anterior pituitary. Even though the amount of these hormones is tiny, the concentrations are enough to stimulate the anterior pituitary gland cells

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Hypothalamus and the Anterior Pituitary Gland

– Hormones: nonsteroidal:• Adrenocorticotropic hormone (ACTH):

stimulates adrenal cortex• ACTH-Adrenocorticotropic Hormone

–Stimulates the adrenal cortex to release glucocorticosteroids (steroid hormones involved in stress related conditions and the metabolism of glucose)

– TSH-Thyroid Stimulating Hormone• Stimulates the thyroid gland to produce

thyroid hormone

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Hypothalamus and the Anterior Pituitary Gland

– Follicle stimulating hormone (FSH): induces egg development and sperm development

– Luteinizing hormone (LH): promotes ovulation and testosterone. These hormones are absent until ages 10-13 years and their production stimulates sexual maturation and development of the secondary sexual characteristics

– Prolactin (PRL): stimulates the development of mammary gland cells and production of milk

– Growth hormone (GH): widespread effects on growth

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Pituitary Disorders

Endocrine disorders tend to be chronic conditions of hypersecretion (too much hormone) or hyposecretion (too little hormone)

• Syndrome of inappropriate ADH secretion (SIADH): hypersecretion of ADH, excessive water retention so the body is out of balance—symptoms: headache, vomiting

• Diabetes insipidus: hyposecretion of ADH, inability to save water appropriately; frequent urination

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Pituitary Disorders

• Gigantism: hypersecretion of growth hormone during childhood and adolescence; usual cause is a tumor of the anterior lobe of the pituitary. Person has relatively normal body proportions

• Acromegaly: hypersecretion of growth hormone in adulthood; overgrowth of hands, feet and face

• Pituitary dwarfism: hyposecretion of growth hormone during childhood; immediate treatment with growth hormone during childhood

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Pancreas: Endocrine FunctionsPancreas is both an endocrine gland and an

exocrine gland (secreting products into the digestive tract)

Endocrine cells are located in the Islets of Langerhans; clusters of cells throughout the pancreas

• Hormones: nonsteroidal: all work to regulate blood sugar– Glucagon: made by alpha cells; raises blood sugar– Insulin: made by beta cells, lowers blood sugar – Somatostatin: made by delta cells, inhibits secretion

of glucagon and insulin, regulates other hormones

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Disorders of the Endocrine System

Since the endocrine system is one of 2 primary systems for controlling body functions, any disruption can have a widespread effect

Diabetes mellitus: DM—poor control of blood sugar resulting in an inability to get glucose into cells so the glucose stays in the circulatory system and overwhelms the kidney’s ability to resorb the sugar so it gets excreted in the urine. Because of the high level, the person also excretes a large amount of water and thus goes to the bathroom extremely frequently.

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– Person becomes dehydrated and very thirsty, tired, has blurred vision, frequent infections, slow healing cuts, tingling in feet and hands.

When glucose can’t be metabolized correctly, the body will metabolize fat and proteins. This causes other problems—cardiovascular and neural diseases, renal failure, blindness, and potentially leg amputations.

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• There are 2 types of diabetes:– Type I -caused by the failure of the pancreas

to secrete enough insulin possibly caused by a virus that causes the immune system to attack the beta cells of the pancreas• Person is insulin dependent for the rest of

his/her life

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– Type II -thought to be insulin resistance—cells fail to respond adequately to insulin even when present• Usually occurs in adults over 40 years• Treatment: lifestyle changes• Oral pills: drugs to stimulate the pancreas

to secrete more insulin; drugs that increase the uptake of glucose by the liver and muscle cells

• New: inhalable insulin

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Adrenal Glands

Adrenal glands are located just above the kidneys– Have an outer layer—cortex and an inner

layer--medulla• Adrenal cortex:

– Secretion: mediated through hypothalamus-pituitary secretions

– Hormones: steroidal

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Adrenal Gland-Cortex– Glucocorticoids: cortisol assists glucagon in

maintaining glucose levels during prolong fasting by promoting using fats and amino acids (liver can use amino acids to make glucose) AND

• Suppresses inflammation after infection or injury

• Also secreted when we are under emotional stress

– Mineralocorticoids: aldosterone--primarily regulates minerals (sodium and potassium)

• Also helps maintain body water balance

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Adrenal Gland: Medulla

• Adrenal medulla: neuroendocrine organ (when these are released into blood stream, funtion as hormones; when work as neurotransmitters, are nervous system transmitters

– Secretion: sympathetic nervous system

– Hormones: nonsteroidal:

• Epinephrine and norepinephrine: enhance function of sympathetic nervous system (fight-or-flight response)

• Play role in metabolism and controlling blood pressure and heart activity

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Adrenal glands:• Addison’s disease: failure of adrenal

cortex to secrete sufficient cortisol and aldosterone

• Lack of cortisol decreases blood sugar levels

• Lack of aldosterone lowers blood sodium levels

• Symptoms: fatigue, weakness, abdominal pain, weight loss

• Treatment is medicine to replace hormones

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• Cushing’s syndrome: excessive cortisol production causing– Excessive production of glucose and retention

of too much salt and water.– Blood sugar rises and muscle mass

decreases because protein is used to make sugar. Some glucose is converted to fat but in specific areas of the body—the face, abdomen and back of the neck (a hump of fat can be seen by the neck)

– Symptoms include weakness, fatigue, edema and high blood pressure

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Thyroid Gland

Located below larynx at the front of the trachea.• Functionally linked to the parathyroid gland.

Both help regulate calcium balance.

Thyroid gland helps control metabolism.• Secretion: mediated through hypothalamus–

pituitary secretions• Steroidal hormones:

– Thyroxine (T4) and Triiodothyronine (T3): both regulate production of ATP from glucose, affect metabolic rate

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Thyroid GlandIodine deficiency causes decrease in production of

thyroid hormones• This causes goiter which is a result of the failure of the

feedback inhibition of production of TRH and TSH.– Hypothalamus secretes thyrotropin releasing hormone (TRH)

which stimulates your pituitary to release TSH. – TSH stimulates the thyroid to make thyroid hormones. – TSH causes increase in size of thyroid gland as a result of the

gland trying to produce more hormones which it can’t do because of lack of iodine

• Calcitonin: decreases rate of bone resorption – lowers blood calcium levels by stimulating the intake of

calcium by bone and increasing bone mass

• Especially important for the development of bones in children

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Disorders of the Endocrine System

Thyroid:– Hyperthyroidism: overactive thyroid gland;

hyperactivity, nervousness, agitation and weight loss• Graves’ disease: an autoimmune disease

in which the person’s antibodies stimulate the thyroid to produce too much thyroxine. Accompanied by protruding eyes (called exopthalmos, caused by fluid accumulation behind the eyes)

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Disorders of the Endocrine System

Thyroid:– Hypothyroidism: underactive thyroid gland

slows body growth, changes brain development, delays the onset of puberty• Children: cretinism—mental retardation and

stunted growth• Adults: myxedema—swelling under the

skin, lethargy, weight gain, low body temperature

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Parathyroid Glands

Located in the back of the thyroid gland• Hormone: parathyroid hormone (PTH),

nonsteroidal:– Removes calcium and phosphate from bone– Increases absorption of calcium by the

digestive tract– Increases retention by the kidneys of calcium

and excretion of phosphate • PTH increases the calcium concentration in the

blood and is secreted in response to lowered blood calcium levels

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Reproductive Hormones

The gonads (testes and ovaries) are responsible for the production of sperm and eggs and production of the sex hormones.

The testes, located in the scrotum, produce androgens, the male sex hormone.

• Hormone: in males: the androgen testosterone (steroidal):– Functions:

• Regulates development and normal functioning of sperm, male reproductive organs, male sex drive

• Development of male secondary sex characteristics and bone and muscle growth at puberty

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Ovaries

• Hormones (steroidal) and functions:– Estrogen: initiates development of secondary

sex characteristics, regulates menstrual cycle– Progesterone: regulates menstrual cycle

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Other Hormone Sources• Thymus: thymosin and thymopoietin, assist

maturation of T lymphocytes (non-steroidal hormone)

• Pineal gland: melatonin– Receives input from the eyes; melatonin secretion is

higher in the dark; important in synchronizing the body’s rhythms to the daily light/dark cycle (Circadian cycle or rhythm)

• Heart: atrial natriuretic hormone (ANH) (non-steroidal hormone); secreted by the atria to help regulate blood pressure by increasing the rate at which sodium and water are excreted in urine thus decreasing blood volume

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Other Hormone Sources

• Digestive system: gastrin, secretin, cholecystokinin; they stimulate activities of the stomach, pancreas and gallbladder

• Kidney: Erythropoietin stimulates RBC production– Renin stimulates aldosterone secretion and

constricts blood vessels

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Other Chemical Messengers

Function in ways similar to hormones but aren’t secreted directly into the bloodstream. – Actions of these are primarily local– Actions short because they are either quickly

destroyed or reabsorbed by the cells that produced them

• Histamine: inflammation– Mast cells release into local interstitial fluid in

response to tissue injury or allergen presence• Histamine increases local mucus secretion, dilates

blood vessels and increases the leakiness of capillaries

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• Prostaglandins: local control of blood flow; – Can constrict and dilate blood vessels depending on

what is needed; contribute to the inflammatory response; involved in blood clotting at injury site

• Nitric oxide: multiple functions– Regulates local blood flow, regulating smooth muscle

contraction in the digestive tract; fights bacteria, interferes with clotting mechanisms

• Growth factors: local acting to modify development of specific tissues; influence when a cell will divide