ch8. endocrine system

• Endocrine signalling


  Signalling molecules or hormones, typically produced by cells of the endocrine organs, act on target cells at a distance
• Paracrine signalling


  Signalling molecules affect cells that are nearby
• Autocrine signalling


  Signalling molecules released and sensed by the same cell
• Types of gland


  • Endocrine gland
  • Exocrine gland
• Endocrine gland


  Gland/group of cells that releases a hormone which travels in the blood and acts on distant target cells
• Exocrine gland


  Gland that makes chemicals secreted/excreted via a duct
• Classes of hormones


  • Polypeptide
  • Amino acid
  • Steroid
• Antagonistic hormones


  One hormone opposes another hormone
• Synergistic hormones


  Hormones have an additive effect
• Permissive hormones


  One hormone is needed for a second hormone to work
• Hormones help to maintain homeostasis - when everything is at balance in your body
• Hypothalamus


  Located in the brain, produces hypothalamic-releasing or inhibiting hormones
• Pituitary gland


  Located in the brain, divided into anterior and posterior parts, responds to the hypothalamus
• Scenario 1 - Balancing the salt concentration in the blood

  1. Blood too concentrated

  2. Neurons in hypothalamus sense salt balance is too high

  3. Neurones secrete ADH

  4. ADH travels to posterior pituitary and is released into blood

  5. ADH travels to kidneys and stimulates them to release less water into urine

  6. More water resorbed into blood

  7. Salt concentration of blood decreases

  8. Neurons in hypothalamus sense salt balance back to normal

  9. Production of ADH is shut off

  10. Kidneys go back to normal function
• Ovarian cycle


  • Follicular phase
  • Luteal phase
• Uterine cycle


  • Secretory phase
  • Proliferative phase
  • Menstruation
• Follicle stimulating hormone (FSH)


  Stimulates the follicle to mature
• Luteinizing hormone (LH)


  Stimulates the formation of a luteal spot in the ovary
• A spike in FSH and LH stimulates ovulation - when the egg is released from the follicle
• The corpus luteum then regresses until it is a small spot on the ovary
• During follicular phase in ovary


  There is the proliferative phase in the uterus
• After ovulation, the corpus luteum signals via progesterone


  It is time to start preparing the uterus for its normal non-pregnant state
• If fertilization occurs, the embryo produces human chorionic gonadotrophin (hCG) which maintains the corpus luteum
• The elevated levels of estrogen and progesterone prevent ovulation by acting on anterior pituitary
• Birth


  1. Sensory receptors in the uterus are activated by the stretching

  2. Posterior pituitary releases oxytocin

  3. Oxytocin acts on uterus and leads to uterine contractions, which ultimately lead to birth
• Milk let-down


  1. When the baby is born and starts suckling at the breast, sensory receptors sense this

  2. Posterior pituitary releases oxytocin

  3. Oxytocin acts on mammary glands to stimulate them to let down their milk
• Hormones produced in anterior pituitary gland

  • Thyroid stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Prolactin
  • Growth hormone
• Tropic hormones


  TSH, ACTH, FSH and LH - act on other endocrine glands
• Non-tropic hormones


  Prolactin, MSH and GH - do not act on other endocrine glands
• The anterior pituitary is called the 'MASTER GLAND' as it stimulates other endocrine glands
• Endocrine disorders


  • Pituitary dwarfism
  • Gigantism
• The endocrine system can both respond and also stop responding via a feedback loop to inhibit the production of hormone
• Parts of the adrenal gland

  • Adrenal medulla
  • Adrenal cortex
• Short-term stress response


  1. Starts in hypothalamus

  2. Nerve impulses go to spinal cord and directly innervate adrenal medulla via sympathetic fibers

  3. Epinephrine and nor-epinephrine released

  4. Heart rate and blood pressure increased, blood glucose increased, muscles energised
• Long-term stress response


  1. Hypothalamus stimulates anterior pituitary, which produces ACTH

  2. ACTH travels in blood to adrenal glands, stimulates adrenal cortex to release glucocorticoids and mineralocorticoids

  3. Glucocorticoids increase metabolism of protein and fat and reduce inflammation

  4. Mineralocorticoids regulate water balance, blood volume, and blood pressure
• Mineralocorticoids


  e.g. aldosterone, target the kidney where they promote the absorption of sodium ions and excretion of potassium ions, helping to regulate volume and pressure of blood
• Low blood pressure


  1. Kidneys sense drop in sodium levels and secrete renin

  2. Renin triggers conversion of angiotensinogen into angiotensin I, then angiotensin I is converted to angiotensin II

  3. Angiotensin II makes blood vessels constrict and stimulates adrenal cortex to release aldosterone

  4. Aldosterone signals to kidney to reabsorb sodium ions and water, and blood pressure increases
• High blood pressure


  1. Heart stretches more as it's pumping

  2. Cardiac cells secrete atrial natriuretic hormone (ANH)

  3. ANH goes to the kidneys, where it stimulates the excretion of sodium ions and water

  4. This leads to a drop in blood pressure and homeostasis is restored
• Islets of Langerhans


  Clumps of cells in the pancreas that produce insulin and glucagon
• Blood glucose regulation


  1. After a meal, blood glucose levels rise

  2. Pancreatic beta cells make insulin

  3. Insulin stimulates liver, muscles and adipose tissue to take up glucose

  4. Blood glucose levels drop, and homeostasis is restored

  5. When blood glucose levels are low, pancreatic alpha cells make glucagon

  6. Glucagon instructs the liver to break down glycogen and the adipose tissue to break down fat

  7. Glucose levels rise, and normal blood glucose level (homeostasis) is restored