The Endocrine System

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The Endocrine System
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Objectives

• List and describe the major functions of the
  endocrine system
• Define hormone
• List and describe the various classes of hormones
  and their actions on cells of there target
  tissues
• List and identify the location, the structure,
  the hormonal secretions, and the action on the
  affected target tissues of the major endocrine
  organs.
• List and describe diseases and disorders of the
  endocrine system.

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Hormones

• Second messenger system of the body
• Uses chemical messages (hormones) that are
  released into the blood
• Hormones control several major processes
• Reproduction
• Growth and development
• Mobilization of body defenses
• Maintenance of much of homeostasis
• Regulation of metabolism
• Hormones are produced by specialized cells of
  endocrine glands
• Cells secrete hormones into extracellular fluids
• Blood transfers hormones to target sites
• These hormones regulate the activity of other
  cells called target tissues

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How Hormones Affect Target Tissues

• Hormones affect only certain tissues or organs
  (target cells or organs)
• Target cells must have specific protein receptors
  on their cell membranes outer surface for the
  specific hormone to bind
• Hormone binding influences the working of the
  cells
• Changes in plasma membrane permeability or
  electrical state
• Synthesis of proteins, such as enzymes
• Activation or inactivation of enzymes
• Stimulation of mitosis

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Classes of Hormones

• Amino acid-based hormones
• Proteins
• Peptides
• Amines
• Steroids made from cholesterol
• Prostaglandins made from highly active lipids

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Steroid Hormonal Action

• Diffuse through the plasma membrane of target
  cells
• Enter the nucleus
• Bind to a specific protein (chaperone protein)
  within the nucleus
• Bind to specific sites on the cells DNA
• Activate genes that result in synthesis of new
  proteins

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Steroid Hormonal Action
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Protein Hormonal Action

• Hormone binds to a membrane receptor
• Hormone does not enter the cell
• Sets off a series of reactions that activates an
  enzyme
• Catalyzes a reaction that produces a second
  messenger molecule (c AMP)
• Oversees additional intracellular changes to
  promote a specific response

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Protein Hormonal Action
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Control of Hormone Release

• Hormone levels in the blood are maintained by
  negative feedback
• A stimulus or low hormone levels in the blood
  triggers the release of more hormone
• Hormone release stops once an appropriate level
  in the blood is reached.
• Endocrine glands are activated by other hormones
• Changing blood levels of certain ions stimulate
  hormone release
• Nerve impulses stimulate hormone release and most
  are under control of the sympathetic nervous
  system

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Control of Hormone Release
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Major Endocrine Glands
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Pituitary Gland

• Size of a grape
• Hangs by a stalk from and is connected internally
  to the hypothalamus
• Protected by the sphenoid bone
• Has two functional lobes
• Anterior pituitary glandular tissue
• Posterior pituitary nervous tissue

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Pituitary Gland Anterior Lobe
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Pituitary Gland Posterior Lobe
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Thyroid Gland

• Found at the base of the throat
• Consists of two lobes and a connecting isthmus
• Produces two hormones
• Thyroid hormone Major metabolic hormone.
  Composed of two active iodine-containing hormones
• Thyroxine (T4) secreted by
  thyroid follicles
• Triiodothyronine (T3) conversion
  of T4 at target tissues
• Calcitonin Decreases blood calcium levels by
  causing its deposition on bone. Antagonistic to
  parathyroid hormone

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Thyroid Gland
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Parathyroid Glands

• Tiny masses on the posterior of the thyroid
• Secrete parathyroid hormone
• Stimulate osteoclasts to remove calcium from bone
• Stimulate the kidneys and intestine to absorb
  more calcium
• Raise calcium levels in the blood

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Thyroid and Parathyroid Calcium Regulation
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Adrenal Glands

• Two glands, each subdivided into two major
  regions
• Cortex outer glandular region in three layers
• Medulla inner neural tissue region
• They are located on top of the kidneys

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Adrenal Cortex Hormones

• Mineralocorticoids (mainly aldosterone)
• Produced in outer adrenal cortex
• Regulate mineral content in blood, water, and
  electrolyte balance
• Target organ is the kidney
• Production stimulated by renin and aldosterone
• Production inhibited by atrial natriuretic
  peptide
• Glucocorticoids (including cortisone and
  cortisol)
• Produced in the middle layer of the adrenal
  cortex
• Promote normal cell metabolism
• Help resist long-term stressors
• Released in response to increased blood levels of
  ACTH
• Sex hormones
• Produced in the inner layer of the adrenal cortex
• Androgens (male) and some estrogen (female)

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Adrenal Cortex
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Adrenal Medulla Hormones

• Produces two similar hormones (catecholamines)
• Epinephrine
• Norepinephrine
• These hormones prepare the body to deal with
  short-term stress

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Adrenal Gland Role in Responding to Stress
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Pancreas Pancreatic Islets

• The pancreas is a mixed gland which secretes
  digestive juices as well as hormones
• The islets of the pancreas produce hormones
• Insulin allows glucose to cross plasma
  membranes into cells and is created in the beta
  cells of the Islets of Langerhans
• Glucagon allows glucose to enter the blood and
  is created in the alpha cells of the Islets of
  Langerhans
• These hormones are antagonists that maintain
  blood sugar homeostasis

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Pancreatic Islets
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Blood Glucose Regulation
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Pineal Gland and Thymus Gland

• Pineal Gland
• Found on the third ventricle of the brain
• Secretes melatonin
• Helps establish the bodys wake and sleep cycles
• May have other as-yet-unsubstantiated functions
• Thymus Gland
• Located posterior to the sternum
• Largest in infants and children
• Produces thymosin
• Matures some types of white blood cells (T-cells)
• Important in developing the immune system

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Ovaries

• Estrogens
• Produced by Graafian follicles in the ovary or
  the placenta
• Stimulates the development of secondary female
  characteristics
• Matures female reproductive organs
• Helps prepare the uterus to receive a fertilized
  egg
• Helps maintain pregnancy
• Prepares the breasts to produce milk
• Progesterone
• Produced by the corpus luteum
• Acts with estrogen to bring about the menstrual
  cycle
• Helps in the implantation of an embryo in the
  uterus

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Testes

• Interstitial cells of testes are
  hormone-producing
• Produce several androgens
• Testosterone is the most important androgen
• Responsible for adult male secondary sex
  characteristics
• Promotes growth and maturation of male
  reproductive system
• Required for sperm cell production

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Other Organs Which Produce Hormones

• Parts of the small intestine
• Parts of the stomach
• Kidneys
• Heart
• Many other areas have scattered endocrine cells
• Placenta
• Produces hormones that maintain the
  pregnancy
• Some hormones play a part in the delivery of
  the baby
• Produces HCG in addition to estrogen,
  progesterone, and other hormones

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Diseases and Disorders of the Endocrine System

• Pituitary Gland
• An over active pituitary gland often results in
  the over production of growth hormone resulting
  in pituitary gigantism. If a person has reached
  maturity and begins to secrete GH a condition
  called acromegaly occurs.
• A pituitary gland that doesnt produce enough
  growth hormone results in dwarfism.

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Diseases and Disorders of the Endocrine System

• Diabetes Diabetes is due to the failure of
  glucose to be absorbed by the tissues and the
  level in the blood becomes extremely high leading
  to renal, neurological, and cardiovascular
  damage. There are three types
• 1. Diabetes insipidus due to a problem with the
  adrenal cortex (rare)
• 2. Diabetes mellitus Here the pancreas
  does not produce insulin due to destrcuction of
  Beta cells (Type I) or receptors for insulin on
  the cells begin to breakdown or do not respond to
  insulin (Type II).
• 3. Gestational diabetes- occurs during
  pregnancy due to mothers body supporting two
  individuals. May disappear after pregnancy or
  remain in the form of Type II.

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Diseases and Disorders of the Endocrine System

• Hypothyrodism Due to an underactive thyroid
  gland and lack of thyroxine. Leads to low
  metabolic rate, weight gain and lethargy. If
  the thyroid is inactive in children they do not
  grow and suffer mental retardation. This
  condition is called cretinism. If due to lack of
  iodine necessary for the production of thyroxine
  a goiter will form.

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Diseases and Disorders of the Endocrine System

• Hyperthyroidism Graves Disease is due to an
  overactive thyroid gland. Appears to be
  genetically transmitted results in nervousness,
  irritability, increased heart rate and elevated
  blood pressure and extreme weight loss. All of
  these symptoms are the result of the over
  production of thyroxine.

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Diseases and Disorders of the Endocrine System

• Cushings Disease (Rare) Is related to the
  adrenal cortex in which there is an over
  production of cortisol (the stress hormone).
  Individuals who are affected have a rounded face,
  collection of body fat in the upper body with the
  limbs remaining small, and stretch marks form.
  Bones become osteoporotic, weak muscle, fatigue,
  elevated blood pressue, and blood glucose levels
  are common symptoms of this disorder.