Operation of the Endocrine System
Biology 3201Unit 1
Endocrine System Notes
What is the Endocrine System?
The endocrine system is comprised of the hormone – producing glands and tissues of the body.
Hypothalamus→ controls the endocrine hormone system (as well as the autonomic nervous system). Maintains the body’s internal environment
Functioning of the Endocrine System
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The endocrine system functions along with the nervous system to help maintain homeostasis.
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Endocrine system functions slower than nervous system but gives a more sustained effect.
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The endocrine system carries out its functions based upon messages received from the
Hypothalamus.
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The hypothalamus monitors the blood and sends hormones from glands into the blood when needed.
 It is important to note that hormones do not ‘seek’ out particular organs, but rather organs await the arrival of the hormone. Cells that react to a particular hormone have specific receptors for that particular hormone.
 Specific hormones attach to specific receptors in cells. This happens in a “lock and- key” fashion.
 Organs that contain receptors for specific hormones are called target organs.
For example : Vigorous exercise triggers endocrine glands in the brain to release several different hormones to regulate oxygen consumption, basal metabolic rate and the metabolizing carbohydrates and fat for energy. As a result, the rate and depth of breathing increases, heart rate and muscle contraction increases and energy stores are quickly mobilized. Therefore, fuel is available for an increase in skeletal muscle, heart, and brain activity, while at the same time maintaining normal physiological processes.
The endocrine system also controls : blood sugar, metabolism, growth, reproductive development and function and other physiological activities.
Components of the Endocrine System
The endocrine system is composed of glands and tissues.
2 Types of Glands
A. Endocrine - Glands that do not have ducts. Hormones are dumped directly into the bloodstream for transport to target cells.

B. Exocrine - glands produce chemicals which travel to target organ through channels or ducts
.i.e. sweat glands tear glands, salivary glands, mammary glands and secrete substances such as sweat, saliva, tears, milk, or digestive enzymes
Hormones: These are chemicals that circulate throughout the blood and exert some measure of control over most every organ and tissue in the body.
Referred to as ‘chemical messengers’ these chemicals travel throughout the body via the blood stream and regulate overall metabolism, maintenance of homeostasis, reproduction and vital body functions.
Target organ : organ that contains receptors for a particular hormone
TYPES OF HORMONES
Hormones are either Steroidal or Non-steroidal.
1. Steroid Hormones – Hormones manufactured from a fatty substance called Cholesterol.
These substances are “fat soluble”.
 Able to pass through cell membrane of cells .
Ex: Cortisol, Progesterone, testosterone

How Steroid Hormones Work
 These hormones enter a cell and binds to a protein receptor in the cell. This creates a hormone-receptor complex.
 The hormone-receptor complex enters the nucleus where it activates a specific gene in the DNA.
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Activated gene produces an enzyme (protein) that initiates a chemical reaction within the cell.
2. Non-Steroid Hormones – Hormones composed of proteins, peptides or amino acids.
These hormones are NOT fat soluble. They are unable to enter cells because they are not soluble in the cell membrane.
Ex: Adrenaline, ACTH, LH, FSH, ADH
How Non-steroid Hormones Work
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A Hormone (called a ‘first messenger’) binds to receptors on surfaces of target cell.
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The binding causes ATP to be changed into Cyclic AMP(cAMP).
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Cyclic AMP (Second messenger) causes chemical reactions to occur within the cell.
Summary of Steroidal .vs. Nonsteroidal Hormones
Hormone Solubility in Cell
Membrane
Location of
Receptors
End Result
Steroidal Soluble Within cell Gene produces a protein
Nonsteroidal
Antagonistic Hormones
Insoluble Surface of cell cAMP causes chain reactions
These are hormones that work against each other or they have opposite effects on the body.
Ex: Insulin and glucagon
Parathyroid Hormone and Calcitonin
Tropic Hormones
These are hormones that regulate the hormone production of many other glands.
Ex: Thyroid Stimulating hormone, ACTH

Endocrine/Hormone Disorders
Problems with most endocrine glands are either caused by Hyposecretion or Hypersecretion of a hormone.
Hyposecretion: An under secretion of a hormone within the body.
Hypersecretion: An over secretion of a hormone within the body.
Feedback Mechanisms and Operation of Hormones
Hormones regulate endocrine function on the basis of feedback mechanisms.
There are two types of feedback mechanisms.
1. Negative feedback Mechanism/loop
A mechanism that works to “reverse” or
“decrease” changes in the body.
Ex: Hypothalamus – Pituitary Feedback
Mechanism
Operation of the Hypothalamus-Pituitary
Feedback mechanism
2. Positive Feedback Mechanism  A mechanism that serves to “increase” the effect of an action.
Ex: Oxytocin Feedback loop.

Glands of the Endocrine System
1.
Pituitary Gland
• Located at the base of the brain. Called the “Master Gland”
• Connected to the Hypothalamus
• Nervous system sends signals via the hypothalamus to exert control over the pituitary.
• Comprised of two glands � Posterior Pituitary and Anterior Pituitary.
Anterior Pituitary
• Located toward front of head.
• Makes up largest portion of pituitary gland.
• Produces 6 different hormones.
Hormone Name/Acronym
Somatotropin or Human
Growth Hormone -- HGH
Hormones of the Anterior Pituitary
Target Cell/organ Action
Regulates growth and development of
Bones, Tissues body tissues.
Increases intestinal absorption of calcium increasing cell division and growth in bones and cartilage.
Adrenocorticotropic
Hormone (ACTH)
Prolactin (PRL)
Adrenal Cortex
Mammary Glands
Stimulate protein synthesis and lipid metabolism
Causes the adrenal cortex to produce cortisol and aldosterone.
Stimulates milk production (lacto genesis)
Stimulates development of mammary tissue.
Increased prolactin levels during pregnancy suppress the production of LH which stops ovulation.
Thyroid Gland Stimulates the release of Thyroxin Thyroid Stimulating
Hormone (TSH)
Follicle Stimulating
Hormone (FSH
Lutenizing Hormone (LH)
Ovaries, Testes
Ovaries, Testes
Causes the maturation of a follicle within an ovary.
Starts development of sperm in males
Causes follicle to release an egg.
Forms a corpus luteum.
Stimulates production of testosterone in males

Hormone Name
Oxytocin
Hormones of the Posterior Pituitary
Acronym Target
Cell/Organ
Uterine Muscles
Mammary Glands
Causes muscles to contract to help expel baby.
Action
Causes mammary glands to contract to release milk
Antidiuretic Hormone
(ADH) or Vasopressin
Blood, Arteries &
Collecting Ducts of kidneys
Regulates sodium levels in blood
Causes damaged blood vessels to constrict to reduce blood loss
Causes kidneys to retain water.
Note: ADH and Oxytocin are created by the HYPOTHALAMUS and STORED in the posterior pituitary.
More On Oxytocin:
Stimulates contraction of the smooth muscles in the uterus during childbirth. Towards the end of the pregnancy, the baby’s head pushes against the opening of the uterus. Pressure receptors in the uterus send impulses to the hypothalamus. This triggers the release of oxytocin from the posterior pituitary. The presence of oxytocin stimulates the uterine muscles to contract. Each contraction causes additional impulses from the pressure receptors and the release of even more oxytocin. This process ends with the birth of the baby (positive feedback loop ).
Problems associated with the Pituitary Gland a. Dwarfism
Cause: Insufficient production of HGH during childhood.
Effect: Causes an abnormally short stature.
Treatment: Injections of HGH

b. Gigantism
Cause: Excess production of HGH prior to puberty.
Effect: abnormal growth of long bones in the skeleton.
Treatment: microsurgery of pituitary, radiation treatment of gland tissue. c. Acromegaly
Cause: Excess HGH production during adult years.
Effect: Causes thickening of bone tissue. Causes abnormal growth of head, hands and feet. Also causes spinal deformities.
Treatment: Surgical removal of tumor causing too much HGH to be released. Radiation of tumor. Injection of HGH blocking drug. d. Diabetes Insipidus
Cause: Insufficient production of ADH.
Effect: increased thirst and dehydration, Production of high amounts of dilute urine, Enlarged urinary bladder.
Treatment: Injections of ADH to reverse effects.
2. Thyroid Gland
• Located in the throat
• Butterfly shaped
Non-steroid
Function  produces Thyroxin and Calcitonin
Note: Iodine is needed in diet in order to produce Thyroxin.
Function of Thyroxin
Thyroxin increases metabolism rate and oxygen consumption.
Thyroxin is released from the thyroid gland when stimulated by TSH from the pituitary.
TSH --------- > Thyroid Gland -------- > releases Thyroxin
TSH and Thyroxin work on a negative feedback loop.
Thyroxine regulates the body’s metabolic rate and oxygen consumption, especially in the heart, skeletal muscles, liver &kidneys. The Hypothalamus monitors the level of thyroxine levels.
When the levels drop, the Hypothalamus produces TSH which stimulates the thyroid gland to produce and secrete more thyroxine. High levels of thyroxine have an opposite effect, the
Hypothalamus reduces its level of TSH which, in turn, causes a decrease in thyroxine production/release (see figure 13.13 on page 432). This is an example of a negative feedback loop.

Function of Calcitonin
• Moves calcium from the blood and into the bones. This lowers amount of calcium in the blood. ( causes calcium levels to drop in blood by reabsorbing it into bones)
3. Parathyroid gland
→ found near the thyroid gland ( Located on the surface of thyroid gland.)
 Produces PTH (Parathyroid Hormone)
→ produces parathyroid hormone (PTH) which causes bones to release calcium into the blood from bones
 Calcium is an extremely important mineral in human physiology - it is involved in blood clotting, growth, bone/teeth strength, nerve & muscle function.
→ calcitonin and parathyroid hormone work together to maintain calcium levels in the blood
(see fig. 13.16, p. 434)
 High calcium levels stimulate an increase in calcitonin - this triggers increased calcium absorption into bone cells, and decreased calcium absorption by the kidneys & small intestine.
Low calcium levels stimulate an increase in PTH - this triggers bone cells to release calcium and increased absorption of calcium by the kidneys and small intestine (see figure 13.16, page 434)
Problems associated with Thyroid Gland
A. Hypo secretion of Thyroxin
1. Myxedema
• Condition where adults become obese, lose appetite and are often tired because of under production of thyroxin.
Treatment: drugs that have Thyroxin in them
2. Congenital Hypothyroidism or Cretinism
• Condition in children where they are often short/stalky and often cognitively delayed.
Treatment: drugs having thyroxine
3. Goiter
• A condition caused by a lack of iodine in the diet. This causes a decreased production of
Thyroxin.
• Patients end up with swollen thyroid gland. Looks like a large growth on the neck.
Treatment: increase iodine intake.
Governments have added iodine to salt for years (Iodized salt) to counteract this problem.

B. Hypersecretion of Thyroxine i. Grave’s Disease
• Condition where there is an increased metabolism (“hyperness”) in a person.
• Increased sweating
• Muscle weakness
• Protruding eyes
Treatment: Surgical removal of thyroid gland, Thyroid blocking drugs.
4. Pancreas (Islets of Langerhans)
• Located near the small intestine.
• Contains specialized endocrine cells called Islets of Langerhans.
• Islets of Langerhans is made up both Alpha and Beta cells.
• Secretes Insulin and Glucagon
Function of Insulin (Created by the Beta cells of the Islets of Langerhans)
• Causes the conversion of Glucose into Glycogen that is stored in the liver.
• Causes excess glucose to be changed into fats.
• Helps regulate blood-sugar levels.
Function of Glucagon (Created by the Alpha cells of the Islets of Langerhans)
• Causes the conversion of Glycogen (liver) into Glucose that is released into the blood as needed.
Operation of Insulin and Glucagon in sugar (glucose) regulation.
Insulin and glucagon are antagonistic hormones. They work opposite each other to maintain proper blood sugar levels.
They work by either increasing or decreasing the amount of glucose (sugar) in the blood.
If the amount of glucose in the blood is high then insulin is secreted by the pancreas. This causes cells to uptake the glucose from the blood and the blood glucose level decreases. When the blood glucose level decreases glucagon is secreted and this causes the liver to convert glycogen into glucose. Glucose is released into the blood stream and the blood glucose level rises.

Disorders of the Pancreas a. Type 1 Diabetes (Diabetes Mellitus)
• Known as Juvenile or Insulin Dependent diabetes. Occurs in individuals under 20 yrs old.
Cause: Beta cells of Islets of Langerhans do not produce enough insulin.
Result: abnormally high levels of sugar (glucose) in blood.
Symptoms: Fatigue, thirst, weight loss, frequent urination, blindness
Treatment: Insulin injections (usually for rest of life ) b. Type 2 Diabetes
• Known as “adult onset diabetes”. Usually Occurs in individuals over 40 yrs old.
• Insulin is produced by pancreas but cells “do not recognize” it. Cells do not use the sugar found in the blood.
Treatment: Strict control of carbohydrate intake to reduce amount of sugar in blood.
The Discovery of Insulin
Insulin was discovered by two Canadian scientists: Fredrick Banting and Charles Best. They worked with dogs and first discovered the link between the pancreas and diabetes in that dogs that had their pancreas removed developed symptoms of diabetes. In addition, they found the islet cells of the pancreas were responsible for preventing diabetes.
Later on, they did an experiment. They extracted the islet secretions from a healthy dog and put them in the system of a diabetic dog (removed pancreas). As expected, the diabetic dog’s blood sugar did go back to normal.
After several tests and modifications, this extract was given to a human patient with success. It was named insulin after the Latin word for island.
5. Pineal Gland
• Produces Melatonin
 small cone-shaped structure located deep within the brain
Function of Melatonin
• Causes a feeling of “sleepiness” and aids with sleep.
 melatonin has a circadian rhythm, a daily biological cycle or regular pattern. Melatonin levels increase as the day goes on, producing the feeling of sleepiness
6. Thymus Gland
• Located between the lobes of the lung in the upper chest.
• Produces thymosin
Function of Thymosin
• Causes the production and maturation of lymphocytes into T-cells.

7. Adrenal Glands
• Located on top of the kidneys.
• Composed of two layers
Outside  Adrenal Cortex (outside of gland)
Inside  Adrenal Medulla (inside of gland)
• Both the Adrenal Medulla and Adrenal Cortex are controlled by the Hypothalamus.
Adrenal Cortex
Releases three major hormones.
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Aldosterone (mineralocorticoids)  Regulates blood pressure and salt levels in the blood.
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Cortisol (Glucocorticoids)  Reduces inflammation and causes glucose to be made by the liver.
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Sex Hormones
Males hormones called “Androgens”
Females hormones called “Estrogens”
Adrenal Medulla
Releases two major hormones: Adrenaline (epinephrine) and Noradrenalin (norepiniphrine) 
Released during times of stress. The “Fight or Flight” hormone. Increases heart rate, respiration rate, release of glucose by liver etc.
Produces a mixture of 85% adrenaline ( epinephrine )& 15% noradrenaline ( norepinephrine ).
These 2 hormones have only structural differences & produce the same effects.
Both hormones act to increase heart rate and blood pressure
Some people suffer from severe allergies to things generally deemed harmless (i.e. fish, peanuts, etc). When exposed to these things, a cascade of reactions results - the immune system interprets these foreign antigens as a threat and immediately releases histamines, which dilate the arteries and allow fluid & proteins to flow out of the capillaries. This causes a rapid drop in blood pressure & blood supply to the organs. The person is danger of dying from this type of anaphylactic shock. An injection of the hormone adrenaline mimics the actions of the sympathetic nervous system increasing heart rate, which serves to restore normal blood pressure
Also involved in the “fight or flight” response

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Disorders Related to the Adrenal glands
Cushing’s syndrome
- caused by excessive ACTH which raises the level of cortisol/aldosterone
- symptoms include high blood pressure, high blood sugar, muscle wasting/weakness and edema (fluid accumulation)
- Can be treated by drugs to block the production of the hormones causing the problems the cause may also be a tumor requiring surgery
Addison’s Disease
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- autoimmune disease where adrenal cortex is destroyed deficiency in cortisol/aldosterone symptoms include low blood pressure, low blood sugar, weight loss
8. Gonads  Ovaries and Testes a. Ovaries – found in females
Produces estrogen and progesterone
Estrogen  controls secondary sex characteristics in females & Prepares the uterus for pregnancy
Promotes the development of ‘female’ sex characteristics such as widening of hips, development of breasts (puberty), on-set of menstrual cycle
Progesterone  Maintains uterus during pregnancy b. Testes – found in males
Produces testosterone  Controls secondary sex characteristics in males.
Anabolic steroids are an artificial form of hormone testosterone. Taking this hormone promotes increased muscle development leading to increased body mass. This usually has a positive effect on performance. There is, however, numerous side effects including liver/kidney failure, high blood pressure, lowered sperm count/sterility, ceasing of menstrual cycle.