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Endocrine System

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Endocrine Glands

The endocrine system is made of glands and tissues that secrete hormones.

Endocrine glands are ductless organs, producing their messengers and secreting them directly into the bloodstream, whereas other glands (exocrine glands) produce their chemicals and excrete them into a duct (ex. digestive enzymes, sweat).

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Hormones are chemicals that influence metabolism of cells, the growth and development of body parts, and homeostasis.

Hormones can be classified as protein or steroids.

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Hormones (meaning “set in motion”) are chemical regulators produced by cells of endocrine glands in one part of the body that affect cells in other parts of the body. They can be divided into two types:

- target hormones – affect specific cells in the body (e.g. gastrin stimulates stomach cells)- nontarget hormones – have broad effects in the body (e.g. growth hormone – affects the growth of long bones)

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There is a close association between the endocrine and nervous systems.

Hormone secretion is usually controlled by either negative feedback or antagonistic hormones that oppose each other’s actions, and results in maintenance of a bodily substance or function within normal limits.

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The Endocrine System

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Chemical SignalsA chemical signal is any substance that

affects cell metabolism or behavior of the individual.

Chemical signals can be used between body parts, between cells, and between individual organisms (pheromones).

Underarm secretions may be slightly attractive and may be involved in synchronizing the menstrual cycles of women who live together.

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Chemical signals

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Chemical signals

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The Action of Hormones

Steroid hormones enter the nucleus and combine with a receptor protein, and the hormone-receptor complex attaches to DNA and activates certain genes.

Transcription and translation lead to protein synthesis.

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Hormones trigger changes in their target cells when they bind to receptor proteins on or within the cells.

A model of a hormone (A) bound to its protein receptor (B). Each hormone of the endocrine system has a unique molecular shape, which fits into a specific receptor protein on its target cells.

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Action of a steroid hormone

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Peptide hormones are usually received by a hormone receptor protein located in the plasma membrane.

Most often the reception of a peptide hormone leads to activation of an enzyme that changes ATP to cyclic AMP (cAMP).

cAMP, as a second messenger, then activates an enzyme cascade.

Calcium is also a common second messenger.

Hormones work in small quantities because their effect is amplified by enzymes.

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Action of a peptide hormone

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• Hormone production will be regulated in most cases by negative feedback systems. Once the desired outcome is reached, the outcome will inhibit the hormone release.

• Hormones are also classified as:– Tropic: have endocrine glands as their

target– Non-tropic:don’t have endocrine glands as

their target

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The Endocrine System

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Hypothalamus and Pituitary Gland

The hypothalamus regulates the internal environment through the autonomic system and also controls the secretions of the pituitary gland.

The pituitary has two portions: the anterior pituitary and the posterior pituitary.

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Posterior PituitaryThe posterior pituitary stores and releases

the antidiuretic hormone (ADH) and oxytocin produced by the hypothalamus.

ADH is secreted during dehydration and causes more water to be reabsorbed by the kidneys; the secretion of ADH is regulated by negative feedback.

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Posterior PituitaryADH is released when the blood plasma concentration is high

(and blood pressure is low). ADH stimulates the kidneys to absorb more water, which dilutes the blood plasma (and increases blood pressure).

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Posterior Pituitary

Oxytocin causes uterine contractions and milk release, and is controlled by positive feedback.

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Anterior PituitaryThe hypothalamus controls the anterior

pituitary by producing hypothalamic-releasing hormones and hypothalamic-inhibiting hormones.

The anterior pituitary produces six hormones.

Three of these six hormones have an effect on other endocrine glands:

1) Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce thyroid hormones;

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2) adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce cortisol;

3) the gonadotropic hormones (FSH and LH) stimulate the gonads to produce sex cells and hormones.

In these three instances, the blood level of the last hormone exerts negative feedback control over the secretion of the first two hormones.

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The next three anterior pituitary hormones do not effect other endocrine glands.

After childbirth, prolactin (PRL) causes mammary glands to produce milk.

Growth hormone (GH) promotes skeletal and muscular growth.

Melanocyte-stimulating hormone (MSH) causes skin color changes in fishes, amphibians, and reptiles.

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Hypothalamus and the pituitary

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Effects of Growth Hormone

The quantity of GH is greatest during childhood and adolescence; GH promotes bone and muscle growth.

Pituitary dwarfism results from too little GH during childhood.

Giants result from too much growth hormone during childhood.

If growth hormone is overproduced in an adult, it causes acromegaly.

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Effect of growth hormone

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Acromegaly

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Adrenal Glands

Adrenal glands sit atop the kidneys and have an inner adrenal medulla and an outer adrenal cortex.

The hypothalamus uses ACTH-releasing hormone to control the anterior pituitary’s secretion of ACTH that stimulates the adrenal cortex.

The hypothalamus regulates the medulla by direct nerve impulses.

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The adrenal glands release several hormones involved in the body’s response to stress.

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The adrenal medulla secretes epinephrine and norepinephrine, which bring about responses we associate with emergency situations.

On a long-term basis, the adrenal cortex produces glucocorticoids similar to cortisone and mineralocorticoids to regulate salt and water balance.

The adrenal cortex also secretes both male and female sex hormones in both sexes.

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Adrenal glands

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GlucocorticoidsCortisol promotes breakdown of muscle

proteins to amino acids; the liver then breaks the amino acids into glucose.

Cortisol also promotes metabolism of fatty acids rather than carbohydrates, which spares glucose.

Both actions raise the blood glucose level.

High levels of blood glucocorticoids can suppress immune system function.

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Mineralocorticoids Aldosterone causes the kidneys to

reabsorb sodium ions (Na+) and excrete potassium ions (K+).

When blood sodium levels and blood pressure are low, the kidneys secrete renin; the effect of the renin-angiotensin-aldosterone system is to raise blood pressure.

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Regulation of blood pressure and volume

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Malfunction of the Adrenal Cortex

Addison disease develops when the adrenal cortex hyposecetes hormones.

A bronzing of the skin follows low levels of cortisol, and mild infection can lead to death; aldosterone is also hyposecreted, and dehydration can result.

Cushing syndrome develops when the adrenal cortex hypersecretes cortisol.

The trunk and face become round; too much aldosterone results in fluid retention.

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Addison disease

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Cushing syndrome

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PancreasThe pancreas is between the kidneys and the

duodenum and provides digestive juices and endocrine functions.

Pancreatic Islets of Langerhans secrete:- insulin, from the beta cells, which lowers the blood glucose level

- insulin makes cells more permeable to glucose- glucagon, from the alpha cells, which increases the blood glucose level

- glucagon causes the conversion of glycogen to glucose

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Pancreas

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Regulation of blood glucose level

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Diabetes MellitusThe most common illness due to

hormonal imbalance is diabetes mellitus.

Diabetes is due to the failure of the pancreas to produce insulin or the inability of the body cells to take it up.

Hyperglycemia symptoms develop, and glucose appears in the urine.

Diabetes is diagnosed using a glucose tolerance test.

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Glucose tolerance test

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TYPE I – insulin-dependent• caused by lack of insulin production in

pancreas, hereditary but may skip generations

• treated with insulin injections and rigid blood monitoring

• since insulin is a protein it would be digested if taken orally

• must monitor both hypoglycemia (need glucagon or glucose) and hyperglycemia (need insulin)

• in research: islet transplants, gene therapy (thought to have found gene)

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TYPE II- insulin-independent

• caused by decreased insulin production, or too much glucose produced by the liver (not enough compensation by pancreas), insulin resistance

• gestational diabetes, during pregnancy, mother develops symptoms – at a greater risk for type II later in life

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• diabetes insipidous, which has nothing to do with insulin, but ADH production in the pituitary – a tumour or injury causes ADH or response to ADH, causing frequent urination (up to 30 L per day). Treat with ADH nasal spray

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• only type I requires daily insulin injections, type II and GDM are treated by changing diet & sometimes sulfonamides

• symptoms of type I and II– fatigue (not enough glucose inside cells to provide an

energy source – must use fat & protein)– excessive thirst & urination (glucose in urine pulls out water

by osmosis)– increased appetite (& weight loss – type I)– increased susceptibility to infection

• *** in type II, since it onsets slowly, there may be no symptoms initially

• diabetes causes many complications due to fluctuations in blood sugar and ketoacidosis (products of fat breakdown which are toxic to the body)

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Thyroid and Parathyroid Glands

The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx.

The four parathyroid glands are embedded in the posterior surface of the thyroid gland.

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The thyroid gland secretes hormones that regulate cell metabolism, growth, and development.

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Thyroid Gland

The thyroid gland requires iodine to produce thyroxine (T4) which contains four iodine atoms, and triiodothyronine (T3) which contains three iodine atoms.

Thyroid hormones increase the metabolic rate, and stimulate all body cells to metabolize and use energy at a faster rate.

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Thyroxine secretion is regulated by the release of thyroid-stimulating hormone (TSH) from the anterior pituitary. TSH is regulated by negative feedback by thyroxine on the hypothalamus and pituitary.

1. Releasing hormone from hypothalamus stimulates anterior pituitary

2. Anterior pituitary releases TSH into bloodstream

3. TSH targets thyroid

4. Thyroid secretes thyroxine into bloodstream

5. High levels of thyroxine cause negative feedback, shutting down production of TSH

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Effects of Thyroid Hormones

Iodine is required to synthesize thyroxine

If too little iodine in the diet thyroid swells (goiter)

• swelling is due to the continued stimulation by TSH (no thyroxine made), causes increase in thyroid size in an attempt to make more thyroxine

- iodized salt helps prevent simple goiters.

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Simple goiter

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If the thyroid is overactive (Grave’s disease) an exophthalmic goiter develops.

• too much thyroxine is present hyperthyroidism (Grave’s disease)– jittery, weight loss, fast heart rate, feel

warm, mood swings, hair loss, bulging eyes

– treated by removing a portion of the thyroid gland (surgically or chemically)

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Hypothyroidism in childhood produces cretinism; in adulthood it causes myxedema.

• too little thyroxine is present hypothyroidism– cold, fatigue, dry skin, hair loss, weight

gain, sleep a lot (myxedema)– in children, leads to abnormal mental and

physical development , growth retardation (cretinism)

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Cretinism

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CalcitoninThe thyroid gland also produces

calcitonin, which helps lower the blood calcium level when it is too high.

The primary effect of calcitonin is to bring about the deposit of calcium in the bones; it does this by temporarily reducing the activity and number of osteoclasts.

When the blood level of calcium is returned to normal, the release of calcitonin is inhibited.

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Parathyroid GlandsParathyroid glands secrete

parathyroid hormone (PTH), which raises the blood calcium when it is insufficient, and decreases the blood phosphate level.

PTH acts by stimulating the activity of osteoclasts, thus releasing calcium from bone, and stimulates the reabsorption of calcium by the kidneys and intestine.

Insufficient parathyroid hormone will cause serious loss of blood calcium and cause tetany.

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Regulation of blood calcium level

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Other Endocrine GlandsTestes and Ovaries

Testes, located in the scrotum, produce the male hormone testosterone.

Ovaries in the female produce estrogens and progesterone.

Secretions from the gonads are controlled by the anterior pituitary hormones.

These sex hormones maintain the sex organs and secondary sex characteristics.

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The effects of anabolic steroid use

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Prostaglandins

Prostaglandins are produced within cells from arachidonate, a fatty acid.

Prostaglandins act close to where they are produced.

They cause uterine muscle contraction and are involved in the pain of menstrual cramps; aspirin is effective against the pain by countering prostaglandins.

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Chapter Summary

Hormones are chemical signals that affect the activity of target glands or tissues.

Endocrine glands are ductless and distribute hormones by the bloodstream.

The hypothalamus is a part of the brain that controls the functioning of the pituitary gland.

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The anterior pituitary produces several hormones, some of which control other endocrine glands.

Growth hormone is produced by the anterior pituitary; giants are due to overproduction of growth hormone during childhood, and pituitary dwarfs are due to underproduction of growth hormone.

The thyroid produces two hormones that speed metabolism and another hormone that lowers the blood calcium level.

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The distinct parathyroid glands produce a hormone that raises blood calcium level.

Adrenal glands produce hormones that help us respond to stress.

Malfunction of the adrenal cortex leads to the symptoms of Addison disease and Cushing disease.

The pancreas secretes hormones that regulate the blood glucose level.

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Diabetes mellitus occurs when cells are unable to take up glucose and it spills over into the urine.

The gonads produce sex hormones that control secondary sex characteristics.

Many other tissues, although not traditionally considered endocrine glands, secrete hormones.

Hormones influence the metabolism of their target cells.