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					                                           Chapter 20
                                        Endocrine System
20.1 Endocrine Glands
The endocrine consists of glands and tissues that secrete hormones.
Hormones*


Endocrine glands*


Hormones can categorized as either peptides or steroids.

Hormones and Homeostasis
Unlike exocrine glands (salivary glands), endocrine glands are ductless; they release hormones
directly into the bloodstream for distribution throughout the body.
Table 20.1 lists endocrine glands and hormones. The glands and their locations in the body are
shown in figure 20.3.
The endocrine system is involved in homeostasis.
There are 2 mechanisms that control the effect of endocrine system.
The most common mechanism is a negative feedback mechanism. The gland is sensitive to the
level of hormone in the blood or the condition it is regulating. Once the level of hormone
increases or the desired effect is achieved, the release of the hormone is then dampened.
The presence of an antagonistic hormone is a way the effect of a hormone is controlled. The
thyroid and parathyroid glands work opposite to each other in regulating the level of calcium in
the blood.

20.2 Hypothalamus and Pituitary Gland
The hypothalamus gland regulates the internal environment trough the autonomic system by
aiding in control of heartbeat, temperature and water balance. The hypothalamus also controls
the hormonal secretions of the pituitary gland. The pituitary gland and the hypothalamus are
directly linked by a stalklike structure. The pituitary has 2 parts: the anterior and posterior
pituitary.

Posterior Pituitary
Neurosecretory cells in the hypothalamus release oxytocin and antidiuretic hormone through
axons where they are stored in the posterior pituitary in axons endings.
ADH promotes the reabsorption of water from collecting ducts of nephrons. The neurons in the
hypothalamus are sensitive to osmolarity of the blood. Negative feedback is the mechanism that
stimulates this response. The effect of the hormones stops the release of the hormone. Diabetes
insipidus results from an inability to produce ADH.
Oxytocin causes the uterus to contract more forcefully during labor. This is a result of positive
feedback; one uterine contraction brings another more forceful contraction.
Oxytocin *


Anterior Pituitary
A portal system*

There is a portal system connecting the hypothalamus and the pituitary gland. It produces
hypothalamic-releasing and hypothalamic-inhibiting hormones. These hormones exhibit
stimulatory or inhibitory control of the anterior pituitary gland.
Three of the six hormones released by the anterior pituitary have an effect on other glands. The
three hormones are:
             thyroid stimulating hormone (TSH)- thyroid hormones
             adrenocorticotropic hormone (ACTH)- cortisol
             gonadotropic hormone (FSH,LH)- gametes and sex hormones
The other three hormones act directly, they do not affect other glands:
             Prolactin (PRL)- stimulates milk production
             Melanocyte (MSH)-stimulating hormone- skin color changes
             Growth hormone (GH)- promotes skeletal and muscular growth

Effect of Growth Hormone
Produced during childhood and adulthood.
Too little GH – *

Too much GH- *


If an excess of GH is produced in adulthood, then the result is acromegaly. The bones in the
hand, feet, and face grow.


20.3 Thyroid and Parathyroid Gland
These glands are attached to the trachea just below the larynx.
The parathyroid is attached to the posterior of the thyroids.

Thyroid Gland
It is composed of a number of follicles; each made of thyroid cells filled with thyroxine (T4), and
triiodothyronine (T3).

Effects of Thyroid Hormones
The thyroid actively acquires iodine to produce its hormones. If it does have access to iodine, the
thyroid will become enlarged due to constant stimulation by the pituitary gland to release its
hormones.
*Goiter

Thyroid hormones increase the metabolic rate. They stimulate all organs to increase the
metabolic rate. More glucose is produced and more energy is used.
Hypothyroidism: Cretinism occurs when the thyroid fails to develop properly*
In adults, hypothyroidism results in the condition of myxedema. Symptoms include weight gain,
loss of hair, slower pulse rate, lowered body temp, and thickness and puffiness of the skin.

Hyperthyroidism: Graves’ disease. The thyroid is enlarged. Eyes protrude because of edema in
socket tissues and swelling of eye muscles. Called exophthalmic goiter. Symptoms include
irritability, hyperactive, nervous, and insomnia.
A tumor on the thyroid can result in this condition as well. Surgery or radiation to destroy the
thyroid is used as a treatment. Radioactive iodine is used. The prognosis is excellent.

Calcitonin
The thyroid gland secretes calcitonin, which regulates the amount of calcium ions in the blood.
The primary effect is the deposit of calcium in the bones. This is accomplished by reducing the
number and activity of the osteoclasts. When the blood calcium levels return to normal, the
hormone is no longer secreted.

Parathyroid Glands
Produce parathyroid hormone- causes blood phosphate levels to decrease and blood calcium
level to increase.
Low levels of calcium in the blood stimulates the parathyroid to release hormone that causes
bones to release calcium into the blood by promoting the work of osteoclasts
Insufficient PTH brings about tetany- muscles shake from continuous contractions due to a drop
in calcium level.
Figure 20.9 for nice diagram of feedback mechanism control of hormones.
20.4 Adrenal Glands
The adrenal glands are on top of the kidneys. The inner adrenal is called the adrenal medulla, the
outer layer is known as the adrenal cortex. No connection between the 2 layers
The hypothalamus controls the activity of the adrenals. It controls the activity of the medulla by
initiating nerve impulses that travel to the medulla and cause it to release its hormone.
When the hypothalamus releases ACTH releasing hormone, which stimulates the anterior
pituitary to release ACTH to stimulate the cortex.
Stress causes the hypothalamus to stimulate the adrenal glands.
Epinephrine(adrenaline) and norepinephrine (noradrenaline) are produced in the medulla and
they bring about physiological changes needed for an emergency response.
The hormones produced by the cortex provide a sustained response to stress. These hormones are
mineralocorticoids (salt and water balance) and glucocorticoids (carbohydrate, fat and protein
metabolism).
The adrenal cortex secretes a small amount of male and female sex hormones in both sexes.

Glucocorticoids

Cortisol is the most important of the glucocorticoids. It promotes the hydrolysis of proteins to
amino acids. The liver converts the amino acids to glucose. Cortisol also favors the metabolism
of fattty acids rather than carbohydrates. Works opposite to insulin and increases blood glucose
levels. Also counteracts the inflammatory response of bursitis or arhtritis. Very high levels can
suppress the body’s defense system.

Mineralocorticoids

Aldosterone is the most important mineralocorticoid. Promotes renal absorption of Na+ and the
renal excretion of K+.
Renin is an enzyme in the kidneys that causes a cascade of events that promotes the release of
aldosterone. It is called the renin-angiotensin-aldosterone system. The contrary hormone is atrial
natriuretic hormone (ANH) and it inhibits the release of aldosterone from the adrenals. Causes
Na+ to be excreted along with water and blood pressure returns to normal.

Malfunction of the Adrenal Cortex
Hyposecretion- Addison’s disease. The presence of ACTH causes a bronzing of the skin. The
lack of cortisol results n the ability to replenish blood glucose during stressful situations. The
lack of aldosterone causes a loss of water and Na+ and hypotension results. Severe dehydration is
possible. Potentially fatal condition.
Hypersecretion causes Cushing’s syndrome. The excess of cortisol causes diabetes mellitus as
muscle protein is metabolized. Excess aldosterone causes edema due to reabsorption of water.
This leads to a basic pH and hypertension. Masculinization of females may occur.
20.5 Pancreas
The pancreas lies in the abdomen between the kidneys near the duodenum of the small intestine.
It is composed of two types of tissues:
             Exocrine tissue- produces and secretes digestive juices that are sent to the small
                intestine by way of ducts.
             Endocrine tissue- called pancreatic islets that produce and secrete insulin and
                glucagon.
Insulin is secreted when the blood glucose level is high. It stimulates the uptake of glucose by
liver, muscle, and adipose cells. In the liver and muscles, glucose is converted to glycogen.
Adipose cells metabolize glucose to supply glycerol for the formation of fat.
Glucagon is secreted when glucose levels are low. Stimulates the liver to break down glycogen
to glucose and to use fat and and protein as energy sources instead of glucose. Adipose cells
break down fat to glycerol and fatty acids so that the liver will use them to produce glucose. All
these mechanisms are ways of increasing blood glucose levels.

Diabetes Mellitus
This is a disease where body cells do not take up and/or metabolize glucose.
As blood glucose levels rise, water and glucose is excreted in the urine. The person experiences
thirst. Protein and fat are metabolized for energy. The metabolism of fat causes excessive
presence in the blood and acidosis that can lead to coma and death.
Two types of diabetes mellitus:
              Type I- insulin dependent- The pancreas is not producing insulin. This is thought
                 to be caused by an agent that causes cytotoxic T cells to destroy the islets that
                 produce insulin.
             *transplant?




              Type II- non-insulin dependent: Occurs in obese and inactive people. Muscles and
               the liver do not respond to insulin, as they should. Exercise and diet can control
               this type of diabetes. The alternative is prescription medication that stimulates the
               pancreas to produce more insulin and enhance the metabolism of glucose by the
               liver and muscle cells.

20.6 Other Endocrine Glands
Testes and Ovaries
Testes are male gonads and are located in the scrotum; ovaries are female gonads and are located
in the pelvic cavity.
The testes produce androgens and the ovaries produce estrogens and progesterones.
The hypothalamus and the pituitary glands control the secretion of these organs.
Testosterone is essential for development and functioning of sex organs in males. It is also
necessary for maturation of sperm
Increased release of testosterone causes the primary and secondary sex characteristics to appear
in the male. This is marked by the "growth spurt". It is responsible for muscular strength of
males and this why some athletes use anabolic steroids. Testosterone also stimulates oil and
sweat glands of the body. Increased chance of acne as a result. Another effect of testosterone is
baldness.
Estrogen and progesterone are the female sex hormones. Estrogens are responsible for
appearance of primary and secondary sex characteristics. It is responsible for fat distribution and
the reason why females have a wider pelvis.
Estrogens and progesterones are needed for breast development and regulation of the uterine
cycle.

Thymus Gland
This gland is at its largest size and most active stage during childhood. Lymphocytes that
originate in the marrow and pass through the thymus become T lymphocytes.
Thymoxin*


Pineal Gland
It is located in the brain and produces melatonin, primarily at night. This hormone is involved in
our circadian rhythms.
This can cause problems for people who do shift work or fly across time zones.
It appears that melatonin also regulates sexual development. In children whose pineal gland has
been destroyed, they experience early puberty.

Hormones from Other Tissues

Some organs secrete hormones. The heart, stomach, small intestine are a few.
Leptin
This is a protein hormone produced by adipose cells that acts on the hypothalamus to signal
satiation.


Growth Factors

A number of differebt types of organs and cells produce peptide growth factors:

Granulocyte and macrophage colony stimulating factor:*

Platelet-derived growth factor*


Epidermal growth factor and nerve growth factor


Tumor angiogenesis

Prostaglandins:
20.7 Chemical Signals
A Chemical signal*


They can be characterized in 3 ways:
            Environmental signals that act at a distance between organisms- pheromones.
            Environmental signals that act at a distance between body parts-include hormones
            Environmental signals that act locally between adjacent cells- neurotransmitters
                and local hormones, i.e. histamine.
The Action of Hormones
Hormones fall into 2 basic categories:
            nonsteroid hormone: amino acid, peptide or a protein
            Steroids hormone- same complex of 4 rings with different side chains. Work at
                low concentration- effect is amplified by cellular mechanisms.
Organs have specific receptors for hormones; the hormone doesn’t select only certain organs.
Steroids are lipids and can move through the cell membrane. When they are inside the nucleus,
they bind to receptor proteins. The hormone-receptor complex binds to DNA and activates
certain genes. This leads to mass production of a cellular enzyme.
Nonsteroids cannot pass through the membrane, as they are not lipids. They bind to receptor
proteins on the surface of the cellular membrane. This starts a cascade effect through the cell.
The hormone is the first messenger, whatever is formed by this binding is the messenger and the
second messenger sets in motion an "enzyme cascade". It is a cascade because 1 enzyme will
activate another. Enzymes work repeatedly and every step in a cascade leads to more reactions.
The effect of the hormone is amplified.

				
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