The Endocrine System

Modified: 2020-06-10

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
• 
• 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 leve 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