The Endocrine System

Healthy Living thru Chemistry

I. Overview of the endocrine system

A. Endocrine system regulates body metabolism and maintains homeostasis through the release of chemical messengers called hormones.

1. Endocrine glands are ductless glands.

a. Their secretion is released directly into the blood stream.

b. Glands that release their product through a duct are called exocrine glands.

Ex. == salivary glands, pancreatic gland

2. Hormones are classified chemically into three types.

a. amino acid based molecules - proteins, peptides, amines

b. steroid molecules - made from cholesterol

c. prostaglandins - made from lipid-like molecules

B. Mechanism of hormonal actions

1. A given hormone only affects a specific tissue or organ - called the target cell, target tissue, target organ

2. Hormone (Greek word - to arouse) - hormones exert their effect by changing the normal activity of their target

3. Hormones exert their effect in one of 4 ways

a. They may change the permeability or electrical potential of a cell membrane.

b. They may cause the synthesis of proteins in the cell.

c. They may activate or inactivate enzymes.

d. They may stimulate mitosis

4. Hormones trigger changes in their target cells by two mechanisms.

a. Steroidal hormones can pass through the cell membrane.1. enter the cell

2. enter the nucleus

3. bind to receptor proteins

4. cause the synthesis of active proteins

b. Non-steroidal hormones cannot pass through the cell membrane

1. cannot enter the cell2. enter the nucleus

3. this binding activates an enzyme

4. the enzyme catalyzes the reactions that produce the effect

5. The release of most hormones is controlled by a negative feedback system

a. A stimulus (disrupts homeostasis) stimulates release of hormone

b. rising levels of the hormone inhibits further release of the hormone

6. Stimuli for hormone release fall into three categories.

a. hormonal stimuli - release of one hormone stimulates the release of another hormone Ex. - hormones released by the hypothalamus stimulate many anterior pituitary hormones to be released

b. humeral stimuli (humor = old term for body fluids) - changing levels of a chemical in the blood stimulate release of the hormone

Ex - rising blood glucose levels stimulate the release of insulin

c. neural stimuli - nerve fibers stimulate the release of hormones

Ex. - activation of the sympathetic nervous system causes the release of epinephrine

II. Major Endocrine Glands and their Products

A. Pituitary gland - grape sized

hangs by a stalk from the hypothalamus

has two functional lobes - anterior and posterior

1. The Anterior Pituitary produces the following hormones.

a. Growth hormone - (GH) - stimulates the growth of skeletal muscles and long bones

b. Prolactin (PRL) - stimulates milk production (lactation) in mothers.

c. Adrenocortical Stimulating Hormone (ACTH) - regulates the release of hormones by the adrenal cortex

d. Thyroid Stimulating Hormone (TSH) - stimulates the release of Thyroid hormone by the thyroid gland

e. Follicle Stimulating Hormone (FSH) - stimulates the development of eggs and sperm

f. Luteinizing Hormone (LH) - stimulates the release of an egg from the follicle - ovulation; stimulates the development of the ruptured follicle into the corpus luteum

g. The anterior pituitary is sometimes called the master endocrine gland because controls the activity of so many other glands.

h. The anterior pituitary is controlled by hormones released by the hypothalamus

2. The Posterior Pituitary gland produces 2 hormones

a. Oxytocin - stimulates uterine contractions during childbirth; stimulates the release of milk in nursing mothers

b. Antidiruetic Hormone (ADH) - stimulates the kidneys to reabsorb water from the urine and so decreases the volume of fluid lost in the urine; also raises blood pressure due to fluid retention

B. Thyroid Gland

1. The thyroid gland produces 2 hormones.

a. Thyroid hormone -

controls the rate at which glucose is turned into energy

controls the body's metabolism requires iodine

b. Calcitonin - lowers blood level of calcium by causing it to be deposited in the bones

D. Adrenal Gland- located above the kidney

- there are two functional parts - the cortex and the medulla

- The Adrenal Cortex produces 3 groups of steroidal hormones

1. Aldosterone - stimulates the kidneys to reabsorb sodium and flush out potassium causing blood levels of sodium to rise and blood levels of potassium to fall;

water follows sodium therefore

Aldosterone causes water retention and can affect blood pressure;

When blood pressure drops the kidneys release rennin - stimulates

the release of aldosterone

2. Glucocortoids have the following effects:

- promote normal metabolism

- resistance to stress

- stimulates glucose production

- reduces inflammation

- reduces edema, reduces pain

3. Sex Hormones -

Androgens in males

Estrogen in females

The Adrenal Medulla produces 2 hormones

1.      epinephrine

2. norepinephrine

3.      They both mediate the fight or flight response –

raise blood pressure

increase respiration

increase heart rate

dilate the air waysincrease blood glucose levels

E. Pancreatic Islet Cells – the Islets of Langerhans

These cells produce 2 hormones

       Insulin is produced by the beta cells

- released in response to increased glucose levels in the blood

- increases a cells transport of glucose across the cell membrane

- it lowers blood glucose levels

b. Glucagon is produced by the alpha cells.

- released in response to lowered blood glucose levels

- stimulates the liver to break down glycogen to form glucose

- increases blood glucose levels

F. Pineal Gland – located in the brain

The function of this gland is not well understood.

The gland does produce Melatonin

Melatonin levels vary with each 24 hour period of time.

Melatonin may play a role in our bio rythyms.

 

G. Thymus – present in the fetus and neonate. Not present in adults

1. The thymus produces a hormone that stimulates the maturation of lymphocytes

H. The Gonads – testis and ovary

1. The ovary produces female hormone – estrogen. It produces secondary sex characteristics.

2. The ovary also produces progesterone – it maintains normal pregnancy.

3. The testis produces male hormone – testosterone. It produces secondary sex characteristics.

. I. The Placenta

1.      Produces Human Chorionic Gonadotropin – stimulates the corpus luteum to continue to produce estrogen and progesterone during the first third of the pregnancy.

2.    During the second and third trimester the placenta produces estrogen and progesterone.

3. The placenta produces Human Placental Lactogen – it prepares the breasts for lactation.

4. The placenta produces Relaxin – it causes the bones of the pubis to relax in preparation for birth.