Endocrine System

• Hormones


  Chemical substances secreted by endocrine cells into the extracellular fluids that regulate the metabolic activity of other cells in the body
• Types of hormones


  • Amino acid-based molecules
  • Steroids
• Steroid hormones


  Made from cholesterol, including sex hormones made by the gonads and hormones produced by the adrenal cortex
• Nonsteroidal hormones


  Amino acid derivatives
• Prostaglandins


  Hormones that act locally, made from highly active lipids released from nearly all cell membranes
• Target cells/organs


  Cells or organs that a given hormone can affect, due to having specific protein receptors for that hormone
• Direct gene activation


  1. Steroid hormone diffuses through plasma membrane

  2. Enters nucleus

  3. Binds to specific hormone receptor

  4. Hormone-receptor complex binds to DNA

  5. Activates transcription of mRNA

  6. mRNA translated to produce new proteins
• Second-messenger system


  1. Hormone binds to receptor on cell membrane

  2. Activated receptor sets off reactions that activate an enzyme

  3. Enzyme catalyzes reactions that produce second-messenger molecules

  4. Second-messengers promote typical response of target cell
• Stimuli for hormone release

  • Hormonal
  • Humoral
  • Neural
• Hormonal stimuli


  Endocrine organs are stimulated by other hormones, leading to rhythmic hormone release
• Humoral stimuli


  Changes in blood levels of ions and nutrients stimulate hormone release
• Neural stimuli


  Nerve fibers stimulate hormone release, e.g. sympathetic stimulation of adrenal medulla
• Major endocrine glands


  • Pituitary
  • Pineal
  • Thyroid
  • Parathyroid
  • Thymus
  • Adrenal
  • Pancreas
  • Gonads (ovaries and testes)
• Hypothalamus


  Part of the nervous system, also considered a major endocrine organ as it produces hormones
• Pituitary gland


  Controls the activity of many other endocrine glands, so called the "master endocrine gland"
• Anterior pituitary


  Glandular tissue, releases hormones controlled by releasing and inhibiting hormones from the hypothalamus
• Posterior pituitary


  Nervous tissue, stores and releases hormones made by the hypothalamus
• Oxytocin


  Released by posterior pituitary, stimulates uterine contractions and milk ejection
• Antidiuretic hormone (ADH)


  Released by posterior pituitary, inhibits urine production and increases blood pressure
• Hyposecretion of ADH


  Leads to diabetes insipidus - excessive urine output and thirst
• Anterior pituitary hormones


  • Growth hormone
  • Prolactin
  • Follicle-stimulating hormone
  • Luteinizing hormone
  • Thyrotropic hormone
  • Adrenocorticotropic hormone
• Growth hormone (GH)


  Promotes growth of skeletal muscles and bones, breaks down fat, maintains blood sugar
• Hyposecretion of GH in childhood

  Leads to pituitary dwarfism
• Hypersecretion of GH in childhood


  Leads to gigantism
• Hypersecretion of GH after growth ends

  Leads to acromegaly - enlargement of facial bones, hands and feet
• Prolactin


  Stimulates and maintains milk production in the breasts after childbirth
• Gonadotropic hormones


  Follicle-stimulating hormone and luteinizing hormone, regulate the gonads (ovaries and testes)
• Hyposecretion of FSH or LH


  Leads to sterility in males and females
• Thyrotropic hormone (TSH)


  Influences the growth and activity of the thyroid gland
• Adrenocorticotropic hormone (ACTH)


  Regulates and stimulates the adrenal cortex to release its hormones
• Pineal gland


  Secretes the hormone melatonin, which plays a role in the sleep-wake cycle
• Thyroid hormones


  Thyroxine (T4) and triiodothyronine (T3), control metabolic rate and tissue growth/development
• Iodine deficiency


  Leads to goiter - enlargement of the thyroid gland
• Hyposecretion of thyroid hormones in childhood

  Leads to cretinism - dwarfism and intellectual impairment
• Hyposecretion of thyroid hormones in adults

  Leads to myxedema - physical and mental sluggishness
• Hypersecretion of thyroid hormones

  Leads to Graves' disease - high metabolic rate, intolerance to heat, rapid heartbeat, weight loss, nervousness, and exophthalmos (protruding eyes)
• Calcitonin


  Hormone produced by the thyroid that decreases blood calcium levels by depositing calcium in bones
• Parathyroid hormone (PTH)


  Regulates blood calcium levels by stimulating bone resorption to release calcium
• PTH


  Is a hypercalcemic hormone, increases blood calcium levels
• Calcitonin


  Is a hypocalcemic hormone, decreases blood calcium levels