Summary of drinking

1-A

• Regulatory system
  • System variable (the variable that is regulated)
  • Set point (the optimum value of the system variable)
  • Detector (measures the system variable)
  • Correctional mechanism (to change it)
• Physiological regulatory systems also require a satiety mechanism, to anticipate the effects of the correctional mechanism. Part of the reasoning is that drinking and eating, and their effects are not instantaneous.

1-B

• There are three main fluid compartments in the body (not including cerebrospinal)
  • Intracellular (67%)
  • Interstitial (26%)
  • Intravascular (7%)
• Inter-relationships of the compartments
  • intracellular and intravascular -- NO EXCHANGE
  • intracellular and interstitial -- water only is exchanged
  • interstitial and intravascular -- both sodium and water can be exchanged
• Interstitial fluid affects both compartments, thus it must be closely regulated.
• Interstitial fluid becomes hypertonic (concentrated)
  • What happens: cells lose water
• Interstitial fluid becomes hypotonic (dilute)
  • What happens: cells gain water, blood plasma loses water
• Intravascular fluid must remain as constant as possible

1-C

• Osmometric thirst occurs when our interstitial fluid is hypertonic
  • This will bring water out of the cells
  • Caused by the body's water being evaporated or eating a salty meal
  • This is detected by osmoreceptors in the AV3V
  • The structure in the AV3V related to osmometric thirst is the OVLT
  • This stimulates the median preoptic nucleus, and drinking begins
• Volumetric thirst can occur when the body loses water through evaporation
  • Pure volumetric thirst is caused by blood loss, vomiting, and diarrhea
  • Blood flow to the kidneys determines the release of renin, which catalyzes angiotensinogen into angiotensin, a hormone
  • Angiotensin stimulates the SFO, which sends signals to start drinking
    • Angiotensin also increases blood pressure and simulates the secretion of hormones in the adrenal and pituitary areas
    • These additional hormones inhibit the secretion of water and sodium by the kidneys, and also start a "salt" appetite
  • The atrial baroreceptors also stimulate thirst, checking by means of "atrial stretch" (detecting lower blood volume)

1-D

• The AV3V region is responsible for both types of thirst
  • Volumetric Thirst:

    Path 1	Path 2
    Atrial baroreceptors	Angiotensin
    Then it goes to the nucleus of the solitary tract	Then it goes to the Subfornical organ (SFO)
    Next the signal travels to the Median Preoptic Nucleus	Next the signal travels to the Median Preoptic Nucleus
    Finally drinking is initiated	Finally drinking is initiated

    
    

  • Osmometric Thirst:
    • Only one path for osmometric thirst
      • Solute concentration of food is measured
      • OVLT has osmoreceptors that detect the concentration change
      • Next the signal travels to the Median Preoptic Nucleus
      • Finally drinking is initiated
    
    
    
    
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