Anatomy

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Anatomy Physiology

• Lesson 8

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THE ENDOCRINE SYSTEM

• Two body systems are responsible for sending and
  receiving sensory information and coordinating
  body responses. These are the nervous system and
  the endocrine system. Together, they are
  sometimes referred to as the neuro-endocrine
  system.
• The endocrine system regulates body activities by
  releasing hormones (chemical messengers) into the
  bloodstream, where they are carried throughout
  the entire body.
• Hormonal responses may be almost instantaneous,
  or may occur days later.
• There is a wide variety of hormonal effects.

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HORMONE FUNCTIONS

• Regulate the chemical composition and volume of
  the internal environment (extracellular fluid).
• Help regulate metabolism and energy balance.
• Help regulate contraction of smooth and cardiac
  muscle fibers and secretion by glands.
• Help maintain homeostasis, despite disruptions,
  such as infection, trauma, emotional stress,
  dehydration, starvation, hemorrhage, and
  temperature extremes.

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HORMONE FUNCTIONS

• Regulate certain activities of the immune system.
• Play a role in the smooth, sequential integration
  of growth and development.
• Contribute to the basic processes of
  reproduction, including gamete production,
  fertilization, nourishment of the embryo and
  fetus, delivery, and nourishment of the newborn.

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EXOCRINE ENDOCRINE GLANDS

• The body contains two kinds of glands
• Exocrine glands secrete their products into body
  ducts, which carry the products into body
  cavities, the lumen of an organ, or the outer
  surface of the body.
• Sudoriferous glands, sebaceous glands, mucous
  glands, and digestive glands.
• Endocrine glands secrete their products
  (hormones) into the extracellular space around
  the secretory cells. The secretions diffuse into
  capillaries and are carried throughout the body
  by the circulatory system.

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ENDOCRINE SYSTEM

• The endocrine system is composed of the bodys
  endocrine glands. These include
• The pituitary, thyroid, parathyroid, adrenal, and
  pineal glands
• There are also many organs that have cells which
  secrete hormones, but are not exclusively
  endocrine organs. These include
• The hypothalamus, thymus, pancreas, ovaries,
  testes, kidneys, stomach, liver, small intestine,
  skin, heart, and placenta.

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ENDOCRINE SYSTEM
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HORMONES

• Hormones can have very powerful effects, even
  when present in very low concentrations.
• There are approximately 50 different hormones
  produced in the human body. Most of these only
  affect a few types of cells.
• The specific cells which are affected by a
  hormone are called target cells.
• Hormones influence their target cells by binding
  to proteins or glycoproteins in the cell membrane
  called receptors. Only the target cells for a
  certain hormone have receptors that will
  recognize and bind to that hormone.

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HORMONES

• Hormones that pass into the blood and act on
  distant target cells are called circulating
  hormones or endocrines.
• Circulating hormones may linger in the blood for
  minutes to hours, exerting their effects for a
  prolonged period of time.
• Eventually, circulating hormones are inactivated
  by the liver and excreted by the kidneys.
• Hormones that act locally without first entering
  the bloodstream are called local hormones.
• Local hormones that act on neighboring cells are
  called paracrines.
• Local hormones that act on the same cell that
  secreted them are called autocrines.
• Local hormones are usually inactivated very
  quickly.

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HORMONES

• Chemically, there are four principal classes of
  hormones
• Steroids are lipids derived from cholesterol.
• Produced in the adrenal cortex, kidneys, testes,
  and ovaries.
• Include aldosterone, cortisol, androgens,
  calcitriol, testosterone, extrogens, and
  progesterone.
• Biogenic amines are structurally simple
  molecules, often synthesized by modifying
  tyrosine (an amino acid).
• Produced in the thyroid gland, adrenal medulla,
  mast cells, blood platelets, and pineal gland.
• Include thyroid hormones (T3 and T4),
  epinephrine, norepinephrine, histamine,
  serotonin, and melatonin.

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HORMONES

• Peptides and proteins consist of chains of 3-200
  amino acids. Some also have carbohydrate groups
  attached, making them glycoprotiens.
• Produced in the hypothalamus, anterior pituitary
  gland, pancreas, parathyroid glands, thyroid
  gland, stomach, small intestine, and kidneys.
• Include hypothalamic releasing and inhibiting
  hormones, oxytocin, antidiuretic hormone,
  anterior pituitary gland hormones, insulin,
  glucagon, somatostatin, pancreatic polypeptide,
  parathyroid hormone, calcitonin,
  digestion-regulating hormones, and
  erythropoietin.
• Eicosanoids are derived from arachidonic acid, a
  20-carbon fatty acid. They act as local
  hormones, and sometimes also as circulating
  hormones.
• Produced by all cells except red blood cells.
• Include prostaglandins and leukotrienes.

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MECHANISMS OF HORMONE ACTION

• The response to a hormone is dependent upon both
  the hormone and the target cell. The same
  hormone can have very different effects on
  different target cells.
• Possible hormone effects include synthesis of new
  molecules, changing the permeability of the
  plasma membrane, stimulating transport of a
  substance into or out of the target cell,
  altering the rate of a metabolic reaction, or
  causing contraction of smooth or cardiac muscles.

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MECHANISMS OF HORMONE ACTION

• Lipid-soluble hormones (including steroids and
  thyroid hormones) are able to pass through the
  plasma membrane of the target cell, so their
  receptors are located within the cell.
• These hormones affect cell function by altering
  gene expression.

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MECHANISMS OF HORMONE ACTION

• Water-soluble hormones (including catecholamine,
  peptide, and protein hormones) cannot pass
  through a plasma membrane, so must attach
  themselves to integral protein receptors that
  extend into the interstitial fluid. Since these
  hormones can only deliver their messages to the
  plasma membrane, they are called first
  messengers. A second messenger is required to
  relay the message into the interior of the cell.
  
• These hormones affect cell function by activating
  plasma membrane receptors, initiating a cascade
  of intracellular events.

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CONTROL OF HORMONE SECRETION

• Secretion of hormones is regulated to prevent
  oversecretion or undersecretion.
• Hormone secretion is controlled by signals from
  the nervous system, chemical changes in the
  blood, or other hormones.
• Usually, negative feedback systems regulate
  hormonal secretions.

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THE PITUITARY GLAND

• The pituitary gland or hypophysis is attached to
  the hypothalamus at the base of the brain.
• The hypothalamus is the major integrating link
  between the nervous and endocrine systems.
• Although the pituitary gland is sometimes called
  the master endocrine gland, it is actually
  subject to control by the hypothalamus, which
  receives input from other regions of the brain.
• Together, the hypothalamus and pituitary gland
  regulate virtually all aspects of growth,
  development, metabolism, and homeostasis.
• The pituitary gland can be divided into the
  posterior pituitary and anterior pituitary.

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THE POSTERIOR PITUITARY

• The posterior pituitary works as a unit with the
  hypothalamus.
• Although the posterior pituitary does not
  synthesize its own hormones, it does store and
  release oxytocin (OT) and antidiuretic hormone
  (ADH) produced in the hypothalamus.
• OT controls uterine contractions during delivery
  and milk ejection during breastfeeeding.
• ADH causes retention of body water, controlling
  the bodys water-balancing mechanism

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THE ANTERIOR PITUITARY

• The anterior pituitary secretes hormones that
  control a wide range of bodily activities.
• The hypothalamus regulates the anterior pituitary
  by producing releasing hormones that stimulate
  release of anterior pituitary gland hormones and
  inhibiting hormones that suppress release of
  anterior pituitary gland hormones.
• The Anterior pituitary has five princple types of
  cells which secrete seven major hormones.

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ANTERIOR PITUITARY CELLS

• Somatotrophs produce human growth hormone (hCG)
  or somatotropin, which stimulates general body
  growth and regulates certain aspects of
  metabolism.
• Thyrotrophs produce thyroid stimulating hormone
  (TSH), which controls secretions and other
  activities of the thyroid gland.
• Gonadoptrophs produce follicle-stimulating
  hormone (FSH) and luteinizing hormone (LH).
  Together FSH and LH stimulate the secretion of
  estrogen and progesterone and the maturation of
  oocytes in the ovaries and the secretion of
  testosterone and sperm production in the testes.
• Lactotrophs procude prolactin (PRL), which
  initiates milk production in the mammary glands.
• Corticotrophs produce adrenocorticotropic hormone
  (ACTH) and melanocyte-stimulating hormone (MSH).
  ACTH stimulates the adrenal cortex to secrete
  glucocorticoids. MSH affects skin pigmentation.

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THE THYROID GLAND

• The thyroid gland is shaped like a butterfly,
  located in the front center of the neck, just
  inferior to the larynx.
• The thyroid is responsible for regulating
  metabolism, growth, and development (including
  bone, nerve, and muscle tissues).
• The thyroid is the only endocrine gland that
  stores large quantities of its secretory product
  (usually about a 100 day supply).
• Disorders associated with malfunctioning of the
  thyroid gland include cretinism, myxedema,
  Graves disease, and goiter.

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THE THYROID GLAND

• The thyroid gland produces three hormones
  Thyroxine or tetraiodothyronine (T4),
  triiodothyronine (T3) (which are called thyroid
  hormones) and calcitonin.
• Calcitonin helps to regulate calcium homeostasis.
• T3 is the more potent of the thyroid hormones,
  but T4 is more abundant.
• Both T3 and T4 are synthesized from iodine and
  the amino acid tyrosine, under the influence of a
  negative feedback loop controlled by the
  hypothalamus.
• Most thyroid hormones becomes bound to blood
  proteins, making them unavailable for the body to
  use. Only about 1 of thyroid hormones remain in
  the free, unbound form which can be utilized by
  the body.

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THE PARATHYROID GLANDS

• The four parathyroid glands are located on the
  posterior surface of the thyroid gland.
• The parathyroid glands produce three forms of
  parathyroid hormone (PTH) or parathormone.
• PTH responds to Ca2 levels in the blood to
  regulate the homeostasis of calcium and phosphate
  ions. When Ca2 levels fall in the blood, PTH
  prompts the release of calcium from the bones.
• Too little Ca2 results in tetany.

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THE ADRENAL GLANDS

• The paired adrenal (suprarenal) glands lie
  superior to the kidneys.
• The adrenal glands are composed of the outer
  adrenal cortex and the inner adrenal medulla.
• The adrenal cortex produces a total of about 40
  different hormones which are collectively known
  as corticosteroids.
• The complete loss of adrenocorticol hormones
  leads to death within a few days to a week, due
  to dehydration and electrolyte imballances.

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THE ADRENAL GLANDS

• The adrenal cortex produces three classes of
  hormones mineralocorticoids (affect mineral
  homeostasis, help control water and mineral
  balance), glucocorticoids (affect glucose
  homeostasis, help regulate normal metabolism and
  stress resistance), and androgens (male sex
  hormones).
• Aldosterone accounts for about 95 of
  mineralocorticoid activity. It regulates salt
  excretion by the kidneys and is involved in the
  use of carbohydrates.
• Cortisol (hydrocortisone), corticosterone, and
  cortisone are the glucocorticoids. They aid in
  the production and storage of glucose, reduce fat
  inflammation following trauma, and are essential
  to immune function. Cortisol accounts for about
  95 of glucocorticoid acctivity.

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THE ADRENAL GLANDS

• The adrenal medulla contains hormone-producing
  cells called chromaffin cells.
• Chromaffin cells are under direct control of the
  ANS (autonomic nervous system), so hormone
  release can occur very quickly.
• Chromaffin cells produce epinephrine (adrenaline)
  and norepinephrine (noradrenalin or NE), which
  are involved in the fight or flight response,
  help resist stress, and help regulate blood
  pressure.

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THE PANCREAS

• Since the pancreas is classified as both and
  endocrine organ and an exocrine organ, it will
  also be discussed again with the digestive
  system.
• The pancreas is a flat organ located posterior
  and slightly inferior to the stomach.
• About 99 of pancreatic cells are arranged in
  clusters called acini. These exocrine cells
  produce digestive enzymes.
• Located between the acini are tiny groups of
  endocrine tissue called pancreatic islets or
  islets of Langerhans.

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THE PANCREAS

• Each pancreatic islet contains four types of
  hormone-secreting cells Alpha or A cells, beta
  or B cells, delta or D cells, and F cells.
• Alpha cells secrete glucagon, which increases
  blood glucose levels.
• Beta cells secrete insulin, which decreases blood
  glucose levels.
• Delta cells secrete somatostatin (identical to
  growth hormone inhibiting hormone secreted by the
  hypothalamus), which inhibits insulin release and
  may slow the absorption of nutrients from the GI
  tract.
• F cells secrete pancreatic polypeptide, which
  inhibits secretion of somatostatin, contraction
  of the gallbladder, and secretion of pancreatic
  digestive enzymes.

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THE OVARIES AND TESTES

• The ovaries and testes are paired oval organs
  referred to as gonads.
• The ovaries are the female gonads, located in the
  pelvic cavity. They secrete estrogens and
  progesterone, which are responsible for the
  development and maintenance of female sexual
  characteristics, as well regulating the female
  reproductive system in conjunction with
  gonadotropic hormones from the pituitary gland).
  The ovaries also produce relaxin, which soften
  connective tissues in preparation for childbirth.
• The testes are the male gonads, located in the
  scrotum. They secrete testosterone, which is
  responsible for male sexual characteristics, and
  inhibin, which controls sperm production by
  inhibiting follicle stimulating hormone.

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THE PINEAL GLAND

• The pineal gland is attached to the roof of the
  third ventricle of the brain. It is covered by a
  capsule formed by the pia mater and consists of
  masses of neuroglia and secretory cells called
  pinealocytes.
• The physiological role of the pineal gland
  remains unclear.
• The pineal gland secretes melatonin, which is
  thought to promote sleepiness and help regulate
  the bodys biological clock. In animals that
  breed during specific seasons, melatonin
  apparently alters their capacity for
  reproduction, but it has not been shown to have a
  similar effect on humans.

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THE THYMUS

• The thymus gland was previously discussed in the
  lymphatic system lesson.
• Hormones produced by the thymus gland are
  thymosin, thymic humoral factor (THF), thymic
  factor (TF), and thymopoietin. These hormones
  promote the proliferation and maturation of T
  cells (white blood cells which destroy microbes
  and foreign substances). Thymic hormones may
  also help to retard the aging process.

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OTHER ENDOCRINE TISSUES

• The GI tract synthesizes several digestive
  hormones, including gastrin, gastric inhibitory
  peptide (GIP), secretin, and cholecystokinin
  (CCK).
• The placenta produces the pregnancy-related
  hormones human chorionic gonadotropin (hCG),
  estrogens, progesterone, relaxin, and human
  chorionic somatomammotropin.
• When deprived of oxygen, the kidneys release
  erythropoietin, which stimulates red blood cell
  production. The kidneys also produce calcitriol,
  the active form of vitamin D.
• The atria of the heart produce atrial natriuretic
  peptide (ANP), which helps regulate blood
  pressure.