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Chapter 13 - The Endocrine System
13.1-The Endocrine System and Homeostasis
Chemical Regulation
In regulating homeostasis, the body uses the electrochemical impulses of the nervous system in
combination with chemical messengers of the endocrine system. The endocrine system regulates
growth, metabolism, reproduction, homeostasis, etc . and acts slower than nervous regulation, but its
effects may last weeks, months, years.
Glands
 Are organs made up of epithelial cells; specialize in the production and secretion of required
substances
 Types of Glands: exocrine glands - secrete substances into ducts which take them to where
they are needed (example: salivary glands
endocrine glands - secrete substances directly into the bloodstream which
will take them to where they are needed (example: thyroid gland)
 Secretions are called hormones; they are chemical signals which affect target organs
Hormones
Hormones are carried by the bloodstream throughout the body, but only affect target organs.
They act as chemical messengers and regulate the target tissue to either increase or decrease its
actions. There are 2 main classes of hormones:
1)Steroid hormones
 made up of lipid-like carbon rings derived from cholesterol molecules.
 able to pass through cell membranes and are more likely to be stored in the body . (examples:
estrogen, progesterone, testosterone).
2) Protein Type hormones
 Non-steroid hormones (a.k.a. protein type) are made of amino acids. They can not pass through
cell membranes because they are too large. They are water soluble so they are less likely to be
stored in the body and can be excreted in urine.(examples - adrenaline, non-adrenaline, growth
hormone, thyroxine, &insulin).
Prostaglandins - Local hormones that do not enter the bloodstream but may influence other hormones
Regulation of Hormone Secretion
Hormones are regulated by the nervous system and/or chemical stimuli (other hormones). A
feedback system is used in which one change causes another change which in turn affects the original
change.
Positive Feedback
In positive feedback, the original change is reinforced. For example, an increase [A] causes an increase in
[B] which causes A] to increase. This amplifies the original change in A. Positive feedback doesn’t
promote homeostasis and is not commonly used. (example of positive feedback: release of oxytocin
during labor)
Negative Feedback
In negative feedback, the original change is opposed. For example, an increase [A] causes an increase in
[B] which causes a decrease in [A]. This type of feedback maintains homeostasis by keeping [A] relatively
constant. Most hormone levels are regulated using negative feedback.
Mechanisms of Hormone Regulation
1) One messenger model – utilized by steroid type hormones with can pass through the cell membrane
by binding to a protein carrier in blood to create a hormone-receptor complex. The steroid hormone
can enter the cell’s nucleus, bind to specific regions of DNA to create its effect.
2) Two Messenger Model – utilized by protein type hormones which cannot enter the cell. Target cells
have a receptor on the cell membrane which binds to the hormone causing a reaction on the inner side
of the cell membrane. This produces a second messenger called cAMP from a molecule of ATP which
creates a cascade reaction of enzyme to create the desired effect.
3.2 - Endocrine Glands
Hypothalamus - connected directly to the to the posterior pituitary gland through a network of
blood vessels and secretes chemicals known as releasing factors which influence the anterior pituitary
gland to release its hormones
Pituitary gland- often referred to as the “master gland” because it controls the activities of
many other endocrine glands. Divided into 2 lobes:
Anterior Lobe of Pituitary Hormones:
1) Human Growth Hormone (HGH) -Protein type hormone which stimulates calcium absorption in small
intestine , stimulates cell division to promote growth of bone and cartilage, stimulates protein synthesis
and lipid metabolism . Conditions associated with this gland: achondroplasia, giantism, acromegaly
2)Thyroid Stimulating Hormone (TSH) – protein type hormone which stimulates production and release
of thyroxine in the thyroid gland .
3) Adrenocorticotrophic Hormone (ACTH)- stimulates the production and release of hormones from the
cortex layer of the adrenal glands
4)Follicle Stimulating Hormone (FSH)- stimulates egg development of ovaries and sperm production in
testes
5)Luteinizing Hormone (LH)- releases eggs in ovaries and stimulates production of sex hormones in
both males and females
6)Prolactin- protein type hormone which stimulates milk production in female after birth and the
development of mammary gland tissue
Posterior Lobe of Pituitary Hormones:
1) Antidiuretic Hormone (ADH)- controls the levels of sodium ions in the blood
2) Oxytocin- In females, it stimulates uterine contractions during labor and the release of milk from
mammary glands. In males, it thought to contribute to emotional bonding
Thyroid Gland – Located in the neck region below the larynx at the front of the trachea. Produces
thyroxine which is a protein type hormone that regulates cellular metabolism. Need iodine to produce
thyroxine. Stimulated by TSH from the pituitary gland. Conditions associated with this gland: goiter,
hyperthyroidism (Graves disease) and hypothryroidism
Parathyroid Gland - 4 tiny, rice sized glands located at the back of the thyroid gland. Secrete
Parathyroid Hormone (Parahormone) which regulates blood calcium levels. If levels of this hormone are
too low, result may be tetany (violent contractions of skeletal muscles). If levels of this hormone are too
high, result may be osteoporosis (brittle bones due to bone density loss)
Calcitonin- hormone release is stimulated by high blood calcium ion levels and it causes bone tissue to
take up calcium ions and deposit it in bone tissue as well increasing the calcium ion secretion in the
kidneys .
Pancreas Gland - located near the stomach and small intestine which works as an exocrine (release
digestive enzymes via a duct to the small intestine) and an endocrine gland (releases its hormones
directly to the bloodstream). Clusters of cells called the Islets of Langerhans are scattered throughout
the pancreas and contain 2 cell types: alpha cells which secrete the hormone glucagon and beta cells
which produce the hormone insulin.
 Insulin – protein type hormone secreted by beta (β) cells due to high blood glucose levels.
Influences cells to be receptive to the uptake of glucose molecules and excess glucose to be
stored by the liver as glycogen.
 Glucagon –protein type hormone secreted by alpha (α) cells due to low blood glucose levels.
Causes the release of glycogen from the liver for its conversion to glucose. Its actions are
antagonist to insulin.
 Conditions associated with this gland: Type 1 and Type II diabetes
13.3 - The Adrenal Glands
2 small glands found on top of/anterior to each kidney which contains 2 endocrine layers, the cortex and
medulla.
Adrenal Cortex (outer layer of gland) -Secretes the hormones aldosterone and cortisol
1) Aldosterone (mineralcorticoid)- a steroid hormone responsible for maintaining mineral balance in
the blood (i.e. it will increase sodium ion reabsorption and potassium ion secretion in the kidneys and
stimulates sodium ion absorption in the large intestine - the increased sodium in the blood causes the
hypothalamus to secrete ADH which increases blood water levels and thus increase the blood volume,
increased blood pressure results)
2) Cortisol (glucocorticoid) - a steroid hormone which is triggered to be released by ACTH (from anterior
pituitary gland) – regulates carbon, glucose, and fats levels. Regulates response to long term stress.
The adrenal cortex also produces small amounts of male (androgens) and female (estrogens) sex
hormones.
Adrenal Medulla (inner layer of adrenal gland)
Adrenaline (epinephrine) and Noradrenaline (norepinephrine)- gland secretes a mixture of the two
(85% adrenaline, 15% noradrenaline) and is used to respond to sudden stress (i.e. fight or flight
response) by increasing heart rate and blood pressure, vasodilation of coronary and pulmonary blood
vessels, glycogen catabolism in liver, diversion of blood from digestive tract and skin surface to muscles
Conditions associated with the adrenal gland – Cushing’s syndrome, Addison’s disease, anaphylactic
shock .
OTHER GLANDS:
Thymus Gland- located near the heart. More prominent in children; disappears in adults.
Produces Thymosin which stimulates production and maturation of T cells (lymphocytes)- immunity.
Pineal Gland – located in the brain. Produces melatonin which aids in pigmentation and sleep