The Endocrine System

[Ready for Fall 2026]

Modified: 2026-08-28 3:03 PM CDST

• One way to think of the endocrine system is as another integrating system.
  • By integrating system, I mean a system that helps to organize behavior and physiology.
  • The nervous system is also an integrating system.
  • The endocrine system works at a much slower pace than the nervous system, however.
  • We will consider the endocrine system's mechanism and its effects.

• Endocrine or ductless glands are specialized tissues that secrete hormones directly into the bloodstream.
  • Hormones are relatively simple chemicals that are absorbed by other tissues, known as target sites, and used by those tissues.
    • Examples of endocrine action include physical growth and the menstrual cycle.
  • In humans, physical growth is mediated by a hormone, growth hormone or somatotrophic hormone.
    • It promotes growth from birth to the late teen years.
    • In adulthood, it ceases to work, and growth stops too.
    • During adolescence, growth hormone and the sexual hormones combine to produce the adolescent growth spurt.
    • Such combinations are called synergistic.
  • A synergism is when two or more elements combine and produce greater effects in combination than they do singly. Synergistic effects are also seen when certain drugs are taken in combination.
• Endocrine Glands
  • Most of the action of the endocrine system discovered via medical research
  • Many medical conditions caused by overactive or underactive endocrine glands
• Pituitary (the Master Gland)
  • Connected to hypothalamus
  • Uses releasing factors to turn on other endocrine glands
• Adrenals
  • secrete adrenaline and noradrenaline
  • regulate mood, energy level, and stress coping
• Pancreas
  • secretes insulin and glucagon
  • regulates sugar levels
  • involved in metabolic processes (body weight, obesity)
• Ovaries
  • in the female
  • secrete estrogen and progesterone
  • involved in reproduction and secondary sexual characteristics
• Testes
  • in the male
  • secrete testosterone
  • involved in reproduction and secondary sexual characteristics
• Thyroids
  • secrete thyroxins
  • involved in metabolism, sexual desire
• Parathyroids
  • secrete parathormone
  • involved in body's calcium levels
• Action
  • Look at the analogy below
    • The thermostat is the pituitary gland
    • The furnace is the endocrine gland
    • It's winter, so it's cold outside
    • When house gets too cold, thermostat tells furnace to burn
    • When house gets to warm, thermostat tells furnace to quit burning
    • Look at the temperature, it goes up and down around the mean set by the themostat
  • This a negative feedback system
• 
• Look at the diagram again and let's put in the endocrine parts instead
  • The hypothalamus (not shown) detects that a hormone level is too low
    • It tells the pituitary gland (the thermostat) to secrete a releasing factor that will activate a particular endocrine gland
    • The gland begins to secree and its hormone level rises
  • When it gets high enough, the situation reverses
    • The hypothalamus tells the pituitary to quit secreting its releasing factor
    • The endocrine gland quits secreting its hormone.
  • Over time that hormone's level drops
  • When that level is too low, the whole process repeats, as the graph shows
• Examples
  • Growth: In USA takes 17 to 19 years
  • Menstrual cycle: usually 28 days with variations