Download Chapter 51 The Endocrine System

Chapter 51 The Endocrine System
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Consists of glands that transmit chemical messengers throughout the body.
These chemical messengers are called hormones and they circulate in the
bloodstream.
Types of Glands
1. Exocrine glands – secrete non-hormonal chemicals into ducts, which transport
the chemicals to specific locations inside and outside the body.
a. Examples: sweat glands, mucous glands, salivary glands, other digestive
glands.
2. Endocrine glands – ductless glands that are located throughout the body. They
secrete hormones into the bloodstream through the fluid that surrounds their
cells.
Types of Hormones
- Hormones influence the activity of distant cells. There are 2 categories of
hormones.
1. Amino acid-based hormones – epinephrine, proteins, peptides, and amino acids.
2. Steroid hormones (lipid/fat hormones) – estrogen and testosterone (synthesized
from cholesterol).
Hormone Action
- Each hormone only affects 1 specific kind of cell. This is called a target cell.
- Target cells have receptors that recognize and bind to the hormone. Receptors
are proteins that are located on the inside and outside surfaces of the cell.
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Amino-Acid based Hormones
Because these hormones cannot diffuse across the member, a 2-messenger
system is generally used.
The hormone acts as a first-messenger, by binding to a specific receptor on the
surface of the target cell. This forms a hormone-receptor complex, which
activates a second-messenger.
The second-messenger relays and amplifies the hormone signal.
o In many cases, the hormone-receptor complex transforms ATP into
cyclic- AMP. c-AMP acts a second messenger by activating other
enzymes and proteins in the target cell.
Steroid Hormones
Steroid hormones act by diffusing through the membranes of their target cells
and bind to receptors in the cytoplasm. This new hormone-receptor complex
activates existing enzymes or initiates the synthesis of new enzymes or proteins.
Prostaglandins
- Are a group of hormone-like lipids that also regulate cell activities.
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They are not produced by specific endocrine glands. They are produced in small
amounts by many cells throughout the body.
They don’t get transported by the blood but rather act locally, inducing
relaxation of smooth muscles, regulating blood pressure, contraction of the
intestinal wall and uterus, and stimulation of the body’s inflammatory response.
Endocrine Glands
1. Pituitary Gland – located at the base of the brain. Generally called the “Master
Gland” of the endocrine system.
a. Two lobes – Posterior and Anterior. Regulated by the hypothalamus
using neurosecretory cells, which are stored in the pituitary.
i. Posterior – produces oxytocin and Antiduretic hormone.
Oxytocin is responsible for uterine contraction. ADH helps
regulate the concentration of solutes in the blood my controlling
the amount of water secreted by the kidneys.
ii. Anterior – secrete releasing hormones, which stimulate endocrine
cells of the anterior pituitary lobe to produce and secrete
hormones. Other cells also produce release-inhibiting hormones,
which prohibit production and secretion of anterior-pituitary
hormones. Some cells produce Growth Hormone (GH), which
controls skeletal and muscular growth, while other cells secrete
prolactin, which stimulates the production of breast milk during
lactation.
2. Thyroid Gland – located near the lower part of the larynx. The anterior
pituitary releases Thyroid Stimulating Hormone (TSH), which in turn causes
the thyroid to release thyroxine and triiodothyronine. These two hormones are
derived from the same amino acid and need iodine to be synthesized. They
maintain normal heart rate, blood pressure, and body temperature. The thyroid
gland also releases calcitonin, which stimulates the transfer of calcium ions from
the blood to bone.
a. Hyperthyroidism – over activity. Symptoms include weight loss, high
blood pressure, high heart rate, and high body temperature.
b. Hypothyroidism – under activity. Symptoms include lethargy, weight
gain, and low heart rate and body temperature, cretinism (form of mental
retardation), and goiter (a swelling of the thyroid gland because of lack of
iodine).
3. Adrenal Gland – located above each kidney. The medulla and the cortex
function as separate endocrine glands.
a. Adrenal Medulla – produces epinephrine and norepinephrine. These
hormones are responsible for the body’s “fight-or-flight” response. When
stressed, the medulla secretes epinephrine into the blood stream. This
causes the liver to breakdown glycogen, which will be used for extra
energy. The pupils enlarge, heart rate increases, and more blood
circulates to the muscles, brain, and heart.
b. Adrenal cortex – responds to adrenocorticotropic hormone (ACTH),
which is secreted by the pituitary. This secretion stimulates the cortex to
secrete cortisol, which regulates metabolism of carbohydrates and
proteins, and aldosterone, which helps maintain the salt-to-water balance
in the body.
4. Gonads – the ovaries and testes. They secrete the sex hormones, which causes
body changes around puberty. Puberty is when the secondary sex
characteristics start to appear.
a. Females – the anterior pituitary secretes lutenizing hormone (LH) and
follicle-stimulating hormone (FSH). These stimulate the secretion of
estrogen and progesterone by the ovaries. These hormones cause the
monthly release of the egg in preparation for pregnancy. Estrogen is
also responsible for the secondary sex characteristics in females.
b. Males – LH stimulates the testes to secrete androgens (ex. Testosterone).
This causes the appearance of the secondary sex characteristics. Along
with FSH, testosterone also stimulates the production of sperm.
5. Pancreas – contains endocrine cells called the Islets of Langerhans. These cells
secrete 2 amino-acid based hormones that regulate the level of sugar in the
blood. Insulin lowers the blood sugar level by causing cells to absorb the
glucose. Glucagon stimulates the release of glucose into the bloodstream by the
liver cells.
a. Diabetes mellitus – a condition of abnormally high blood glucose
concentration.
i. Type I – severe childhood disorder in which the Islet cells die.
Treated by daily injections of insulin into the blood. Sometimes
treated with an Islet transplant.
ii. Type II – usually occurs after 40 and is less severe. Caused by
insufficient insulin production or unresponsive target cell
receptors. Usually hereditary and can be controlled with diet and
exercise. This condition can cause hypoglycemia, a disorder in
which glucose is stored, rather than used by the body cells. This
can cause lethargy, dizziness, nervousness, and in extreme cases,
death.
6. Thymus – located beneath the sternum and between the lungs. Consists of Tcells, which play a role in the immune response. It secretes and amino-acid
based hormone called thymosin, which stimulates the formation of T-cells.
7. Pineal Gland – Located near the base of the brain. Secretes the hormone
melatonin. This hormone regulates the sleep-wake cycle.
8. Parathyroid Glands – embedded in the back of the thyroid gland, two in each
lobe. Secretes parathyroid hormone, which increases the concentration of
calcium ions in the blood.
9. Digestive organs – endocrine cells of the stomach secrete gastrin, a hormone
that stimulates other stomach cells to release digestive enzymes and HCl.
Endocrine cells of the small intestine secretes secretin, a hormone that
stimulates the release of various digestive fluids from the pancreas and bile from
the liver.
Feedback Mechanisms – the last step in a series of events controls
the first step.
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Homeostasis
Controlled by the endocrine system. For example, insulin and glucagon work
together to maintain a balanced blood glucose level. These hormones are
considered antagonistic hormones because their actions have opposite effects.
Most hormone systems use negative feedback, in which the release of an initial
hormone stimulates release and production of other hormones or substances that
subsequently inhibit further release of the initial hormone.
In positive feedback, release of an initial hormone stimulate production of other
hormones, which stimulate further release of the initial hormone.