•Cells of the body require specific conditions to
survive and function
•Maintenance of body conditions in a stable
steady state is called homeostasis

  • Body homeostasis is chiefly controlled by
     Nervous control
     Chemical control
  • Endocrine secretion – substance released by cell into
    blood stream that affects distant cells
  • Exocrine secretion – substance released by cell into a
    duct that leads to epithelial surface (onto skin or into
    gut). Action doesn’t depend on receptors in target tissue.
  • Endocrine and exocrine secretions are glandular
    secretions; they come from specialized secretory cells that
    are clumped together to form a gland.

Chemical Messengers

Chemical messenger systems regulates multiple activities of

  1. Neurotransmitters– released by axon terminals of neurons–
    act locally to control nerve cell functions
  2. Endocrine hormones– released by glands or specialized cells
    into circulating blood– act on target cells
  3. Neuroendocrine hormones– secreted by neurons into the
    circulating blood– acts on target cells
  1. Paracrines– secreted by cells into extracellular
    fluid and affect neighboring target cells of a
    different type
  2. Autocrines– secreted by cells into the ECF and
    affect the function of the same cells that
    produced them
  3. Cytokines– secreted by cells into ECF &
    function as autocrines, paracrines, or endocrine
    hormones. Examples of cytokines– interleukins
    & lymphokines


  • Neuroendocrine hormones
     Hormones of posterior pituitary
  • Endocrine Hormones
     Hormones of anterior pituitary, thyroid
    hormones, adrenocortical hormones
  • Exocrine Hormones


  • Role of hormones
    Growth– growth hormone
    Metabolism and development– growth hormone
    & thyroid hormone
    Water and electrolyte balance– aldosterone,
     Reproduction– sex hormones
     Behavior– thyroid hormone

Secretion of Hormone

  • Variable
  • May secrete within minutes after the stimulus
  • Action starts within seconds and may last till
  • Short duration of action– epinephrine &
  • Long duration of action– thyroxine
  • Concentration and secretion of hormone is usually
  • From picograms to micrograms

Control of Hormone Secretion

  • Plasma concentrations of hormones depends upon various
  • Mostly controlled by negative feedback mechanisms that
    ensure a proper hormone activity at target tissue
  • After a stimulus causes release of the hormone
  • Response resulting from the action of the hormone tend to
    suppress its further release
  • Thus negative feedback effect to prevent oversecretion or
    overactivity at the target tissue

Control of Hormone Secretion

  • Control is not always the secretory rate of hormone, it
    can be degree of activity of the target tissue
  • When target tissue activity rises to an appropriate level
    will feedback signals to decrease secretion of hormone
  • Feedback regulation can occur at all levels
    Gene transcription and translation steps involved in
    the synthesis of hormones
    Steps involved in processing hormones or releasing
    stored hormones

Control of Hormone Secretion

  • Positive feedback occurs when action of hormone requires
    additional secretion
  • Example–luteinizing hormone (LH) surge
  • Occurs as a result of stimulatory effect of estrogen on anterior
    pituitary before ovulation
  • LH acts on ovaries to stimulate more secretion of estrogen,
    which in turn causes more secretion of LH
  • Concentration is achieved and negative feedback is exerted

Variations in Hormone Release

  • Cyclical Variations also occur in hormone release.
  • Periodic variations occur in hormone release influenced by
     Seasonal changes
     Various stages of development and aging
     Diurnal (daily) cycle
     Sleep
  • For example, growth hormone is increased in early period
    of sleep and reduced in later stages of sleep
  • Cyclical variations occur due to changes in activity of neural
    pathways in controlling hormone release


  • Water-soluble hormones (peptides and catecholamines)
     Dissolved in plasma
     Diffuse out of the capillaries, into the interstitial fluid, and
    ultimately to target cells
  • Steroid and thyroid hormones,
     Circulate in the blood mainly bound to plasma proteins easily
    diffuse across the capillaries and reach target sites
     10 percent of steroid or thyroid hormones exist free in solution
  • Large amounts of hormones bound to proteins serve as reservoirs
  • Binding of hormones to plasma proteins slows clearance from the


  • Determined by
     Rate of secretion of hormone
     Rate of removal of hormone
  • Hormone is cleared by
     Rate of destruction in target tissues
     Binding with the tissues
     Metabolism by liver
     Removal by kidneys

•Water soluble hormones
Degraded by enzymes in blood and tissues
Cleared by liver/kidneys
•Lipid soluble hormones
Protein bound
Slowly cleared
Thyroid and steroid

Metabolic Clearance Rate= Disappearance of hormone from plasma / Concentration of Hormone

Measurement of Hormones

  • Hormone concentration can be measured by
  1. RIA (Radioimmunoassay)
  2. ELISA (Enzyme-Linked Immunosorbent

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