Endocrine System 2

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What Are Hormones?

Hormones have several general functions:
  1. Help regulate:
    • Chemical composition and volume of internal fluids
    • Metabolism and energy balance
    • Contraction of smooth and cardiac muscle fibres
    • Glandular secretions
    • Some immune system activities
  2. Control growth and development
  3. Regulate operation of reproductive systems
  4. Help establish circadian rhythms
  5. Mobilise body's defence against stressors
Hormones are able to change the physical or biological properties of a target cell by:
  1. Altering membrane permeability
  2. Activating or inactivating key enzymes
  3. Changing genetic activity
Hormones differ from the nervous system in that, they:
  • Offer indirect communication
  • Are slow acting and produce a slow response
  • Respond only to chemical stimuli
  • Are capable of secreting (although the nervous system is capable of this, too)
  • Are delivered to target cells by the blood
  • Have an effect on cells far from the site of release
  • Can target virtually all cells in the body
  • Have an onset of action seconds, hours or days after release
Endocrine glands produce and secrete hormones. They are ductless, well-vascularised (because hormones travel through the blood) and secrete hormones into the extracellular fluid.

Circulating hormones affect distant target cells. Autocrines and paracrines are local hormones in the interstitial fluid which do not circulate in the blood.

There are three classes of hormones based on chemical structure. These are:
  1. Amino acid derivatives
    • These hormones are small molecules structurally related to amino acids. They are synthesised from the amino acids tyrosine and tryptophan. 
  2. Peptide hormones
    • These hormones are chains of amino acids that are synthesised as pro hormones. Pro hormones are inactive molecules converted to active hormones before or after secretion.
    • There are two groups of peptide hormones; glycoproteins and short polypeptide chains & small proteins.
  3. Lipid derivatives
    • These hormones are divided into two groups; eicosanoids are derived from arachidonic acid and steroid hormones are derived from cholesterol.
Hormones are either free, meaning they are functional for less than an hour, or transport protein-bound. Free hormones diffuse out of the bloodstream, are absorbed and then broken down by proteins. Transport protein-bound hormones are thyroid and steroid hormones and remain in circulation much longer.

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