CHAP 8 Anatomy

• Endocrine system


  • Second controlling system of the body
  • Uses chemical messengers (hormones) that are released into the blood
  • Hormones control several major processes including reproduction, growth and development, mobilization of body defenses, maintenance of homeostasis, and regulation of metabolism
• Hormones


  • Produced by specialized cells
  • Secreted into extracellular fluids
  • Transferred by blood to target sites
  • Regulate the activity of other cells
• Types of hormones


  • Amino acid-based (proteins, peptides, amines)
  • Steroids made from cholesterol
  • Prostaglandins made from highly active lipids that act as local hormones
• Hormone action


  • Hormones affect only certain tissues or organs (target cells or target organs)
  • Target cells must have specific protein receptors for hormone binding
  • Hormones alter cellular activity by changing plasma membrane permeability, activating/inactivating enzymes, stimulating/inhibiting cell division, promoting/inhibiting secretion, or turning on/off transcription of certain genes
• Hormone action mechanisms


  1. Direct gene activation (used by steroid hormones and thyroid hormone)

  2. Second-messenger system (used by protein and peptide hormones)

  
• Negative feedback


  Hormone levels in the blood are maintained mostly by negative feedback, where a stimulus or low hormone levels trigger the release of more hormone until an appropriate level is reached
• Stimuli for control of hormone release

  • Hormonal
  • Humoral
  • Neural

  
• Hormonal stimuli


  Endocrine organs are activated by other hormones, e.g. hormones of the hypothalamus stimulate the anterior pituitary to secrete its hormones

  
• Humoral stimuli


  Changing blood levels of certain ions and nutrients stimulate hormone release, e.g. parathyroid hormone and calcitonin are produced in response to changing blood calcium levels, insulin is produced in response to changing blood glucose levels
• Neural stimuli


  Nerve fibers stimulate hormone release, most are under the control of the sympathetic nervous system, e.g. sympathetic stimulation of the adrenal medulla to release epinephrine and norepinephrine
• Major endocrine organs


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

  • Pea-sized gland that hangs by a stalk from the hypothalamus in the brain
  • Has two functional lobes: anterior pituitary (glandular tissue) and posterior pituitary (nervous tissue)

  
• Hypothalamus

  • Produces releasing hormones and inhibiting hormones that are released into portal circulation to the anterior pituitary
  • Also makes oxytocin and antidiuretic hormone (ADH) that are carried to the posterior pituitary for storage
• Posterior pituitary

  • Does not make the hormones it releases
  • Stores hormones made by the hypothalamus, including oxytocin and antidiuretic hormone (ADH)

  
• Hormones released by posterior pituitary

  • Oxytocin
  • Antidiuretic hormone (ADH)
• Oxytocin

  • Stimulates contractions of the uterus during labor, sexual relations, and breastfeeding
  • Causes milk ejection (let-down reflex) in a breastfeeding woman
• Antidiuretic hormone (ADH)

  • Inhibits urine production by promoting water re-absorption by the kidneys
  • In large amounts, causes constriction of arterioles, leading to increased blood pressure
• Hormones secreted by anterior pituitary

  • Growth hormone (GH)
  • Prolactin (PRL)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Thyrotropic hormone (TH)
  • Adrenocorticotropic hormone (ACTH)

  
• Growth hormone (GH)

  General metabolic hormone that plays a role in growth of skeletal muscles and long bones, determines final body size, causes amino acids to be built into proteins, and causes fats to be broken down for energy
• Prolactin (PRL)

  Stimulates and maintains milk production following childbirth
• Gonadotropic hormones (FSH and LH)

  Regulate hormonal activity of the gonads: FSH stimulates follicle development in ovaries and sperm development in testes, LH triggers ovulation and stimulates testosterone production in males

  
• Thyrotropic hormone (TH)

  Influences growth and activity of the thyroid gland

  
• Adrenocorticotropic hormone (ACTH)

  Regulates endocrine activity of the adrenal cortex
• Pineal gland

  • Hangs from the roof of the third ventricle of the brain
  • Secretes melatonin, which is believed to trigger the body's sleep/wake cycle and coordinate the hormones of fertility
• Thyroid gland

  • Found at the base of the throat, inferior to the Adam's apple
  • Consists of two lobes and a connecting isthmus
  • Produces thyroid hormone and calcitonin

  
• Thyroid hormone


  Major metabolic hormone that controls the rate of oxidation of glucose to supply body heat and chemical energy, and is needed for tissue growth and development

  
• Pineal gland

  • Secretes melatonin
  • Believed to trigger the body's sleep/wake cycle
  • Believed to coordinate the hormones of fertility in humans and to inhibit the reproductive system until maturity occurs
• Thyroid gland

  • Found at the base of the throat, inferior to the Adam's apple
  • Consists of two lobes and a connecting isthmus

  
• Thyroid gland produces two hormones: Thyroid hormone and Calcitonin

  
• Thyroid hormone

  • Major metabolic hormone
  • Controls rate of oxidation of glucose to supply body heat and chemical energy
  • Needed for tissue growth and development
  • Composed of two active iodine-containing hormones: Thyroxine (T4) and Triiodothyronine (T3)

  
• Hyperthyroidism

  Too much thyroid hormone
• Calcitonin

  • Thyroid gland releases calcitonin when blood calcium level is high
  • Calcitonin decreases blood calcium levels by causing calcium deposition on bone (triggers osteoblasts)
  • Antagonistic to parathyroid hormone

  
• Parathyroid glands

  • Tiny masses on the posterior of the thyroid
  • Secrete parathyroid hormone (PTH) when blood calcium is low
  • Most important regulator of calcium ion (Ca2+) homeostasis of the blood
  • Stimulates osteoclasts to remove calcium from bone
  • Hypercalcemic hormone (increases blood calcium levels)
  • Stimulates the kidneys and intestine to absorb more calcium
• Thymus

  • Located in the upper thorax, posterior to the sternum
  • Largest in infants and children; decreases in size throughout adulthood
  • Produces a hormone called thymosin that stimulates the development and maturation of T cells (white blood cells) for the immune system

  
• Adrenal glands

  • Sit on top of the kidneys
  • Two regions: Adrenal cortex (outer glandular region that produces corticosteroids) and Adrenal medulla (inner neural tissue region)
• Mineralocorticoids (mainly aldosterone)

  • Regulate mineral (salt) content in blood, particularly sodium and potassium ions
  • Regulate water and electrolyte balance
  • Target organ is the kidney
  • Increased absorption of Na+ and water; increased K+ excretion leads to increased blood volume and blood pressure
• Release of aldosterone

  1. Stimulated by: Humoral factors (fewer sodium ions or too many potassium ions in the blood), Hormonal stimulation (ACTH), Renin and angiotensin II in response to a drop in blood pressure

  2. Inhibited by: Atrial natriuretic peptide (ANP), a hormone produced by the heart when blood pressure is too high
• Glucocorticoids (including cortisone and cortisol)

  • Promote normal cell metabolism
  • Help resist long-term stressors by increasing blood glucose levels (hyperglycemic hormone)
  • Have anti-inflammatory properties
  • Released in response to increased blood levels of ACTH
• Sex hormones

  • Small amounts are made throughout life
  • Most of the hormones produced are androgens (male sex hormones), but some estrogens (female sex hormones) are also formed
• Epinephrine (adrenaline) and Norepinephrine (noradrenaline)

  • Hormones produced by the adrenal medulla
  • Prepare the body to deal with short-term stress ("fight or flight") by increasing heart rate, blood pressure, blood glucose levels, and dilating small passageways of lungs