PSYC 460 -- Dr. King -- Notes on Thirst

Slide 1: Ingestive Behavior (i.e., eating and drinking), Chapter 11 

Slide 2: "The constancy of the internal milieu is a necessary condition for a 
free life." -Claude Bernard (1813-1878) 

Slide 3: Homeostasis 
   A. the process by which the body's substances and characteristics are 
      maintained at their optimum levels 
   B. physiological regulatory mechanisms 
      1. our cells are still bathed in something very much like seawater 
      2. to which we add oxygen and nutrients 
      3. and from which we remove toxins 

Slide 4: A Regulatory Mechanism 
   A. see Fig. 11.1 

Slide 5: Four Essential Features of a Regulatory Mechanism 
   A. system variable - what's being controlled or regulated (temperature,
      glucose level) 
   B. set point - the optimal value of the system variable 
   C. detector - signals deviation from the set point 
   D. correctional mechanism - capable of altering the level of the system
      variable 

Slide 6: Negative Feedback 

Slide 7: Part I: Thirst 

Slide 8: How This Works in the Case of Drinking 
   A. see Fig. 11.2 
   B. Satiety mechanism is necessary because the other negative feedback
      pathway is too slow! 

Slide 9: Some Facts About Fluid Balance 
   A. fluid compartments of the body 
      1. intracellular fluid 
      2. extracellular fluid 
         a. intravascular fluid - blood (7%)
         b. interstitial fluid - between cells (26%)
         c. cerebrospinal fluid - ignored (less than 1%)

Slide 10: Fluid Balance (cont.) 
   A. These compartments are coupled by semi-permeable barriers. 
   B. diffusion and osmosis - nature hates a concentration gradient! 
      1. diffusion - Solute moves down its concentration gradient.
      2. osmosis - Water moves up a concentration gradient. 

Slide 11: Fluid Balance: Tonicity 
   A. isotonic fluid - same conc. of solutes as a cell 
   B. hypertonic fluid - higher conc. of solutes than a cell (draws water out
      of cells) 
   C. hypotonic fluid - lower conc. of solutes than a cell (water drawn into
      cell) 

Slide 12: Some Fancy Greek Words 
   A. hypovolemia - low blood volume 
   B. hypervolemia - high blood volume 
   C. The volume of the blood must be closely regulated because of the mechanics
      of the operation of the heart. 

Slide 13: Those Pesky Bodily Fluids Again 
   A. two important characteristics of body fluids 
      1. solute concentration of the intracellular fluid - if this is wrong,
         water will be drawn out of cells or "pushed" into cells 
      2. blood volume 
   B. It is possible to change one without changing the other. 
      1. Therefore, two thirst mechanisms are required! 

Slide 14: Two Types of Thirst 
   A. preliminary considerations 
      1. we will find that two aspects of our behavior must be regulated 
         a. water intake 
         b. sodium intake 
      2. normally we take in much more of these than we need 
      3. lost of water from either the intracellular or intravascular 
         compartment stimulates drinking 
         a. osmometric thirst - intracellular volume 
         b. volumetric thirst - blood (intravascular) volume 

Slide 15: Osmometric Thirst 
   A. occurs when the solute concentration of the interstitial fluid increases 
      1. either because its volume decreases, or... 
      2. we add minerals to it 
   B. water is drawn out of cells by osmosis causing the cells to shrink 

Slide 16: Osmoreceptors 
   A. Verney (1947) - proposed that some neurons alter their rate of firing
      depending upon their state of hydration (see Figure 11.5)

Slide 17: Osmoreceptors 
   A. water loss through evaporation from the skin (not sweating, which also
      loses salt) - osmometric + volumetric thirst 

Slide 18: Osmoreceptors 
   A. eating a salty meal - pure osmometric thirst 
      1. causes salt to be absorbed from the digestive system into the blood
         plasma 
      2. which becomes hypertonic 
      3. which causes water to be drawn out of the interstitial fluid into the
         blood 
      4. which causes the interstitial fluid to become hypertonic as well 
      5. which draws water out of cells 

Slide 19: Where Are These Osmoreceptors? 
   A. in the (get ready for it!) lamina terminalis of the circumventricular
      region surrounding the 3rd ventricle 
      1. organum vasculosum of the lamina terminalis (OVLT) 
      2. subfornical organ (SFO) 
   B. these areas are located outside the blood- brain barrier 

Slide 20: The Circumventricular Organs 
   A. there is a figure similar to one of these in the book

Slide 21: Egan et al. (2004) 
   A. Neural Correlates of the Emergence of Consciousness of Thirst 
      (Proceedings of the National Academy of Science, v.100) 
      1. used fMRI to confirm this in humans 
         a.  thirst activated the circumventricular area and the anterior
             cingulate gyrus 
         b. drinking reduced activity in the cingulate gyrus but not the
            circumventricular area 
         c. because the subjective sense of thirst was gone, but it takes 
            20-30 min. for the water to be absorbed into the general circulation
         d. region of ACC activated by thirst somewhat overlaps the area
            activated by painful stimuli 

Slide 22: Volumetric Thirst 
   A. the case of Little Billy (see p. 307 in the text) - Billy "compulsively"
      ate 1-2 lbs. of salt per day; the hospital staff denied him salt; by the
      next morning he was dead 
   B. blood loss, vomiting, and diarrhea cause hypovolemia without causing loss
      of intracellular fluid (because the lost fluid is isotonic) 
   C. also leads to sodium loss and a salt appetite 

Slide 23: Kidneys 
   A. whose job it is to filter the blood and to excrete excess water and
      sodium 
   B. the nephron (about 1 million of these per kidney)

Slide 24: Kidneys (cont.) 
   A. about 45 gal/day (375 lbs.) of water, sodium, and wastes are filtered out
      of the blood 
   B. if some of that water were not reclaimed, we'd all turn to dust pretty
      quickly! 
   C. sodium is actively pumped back into the ECF 
   D. water follows sodium due to osmotic pressure 
   E. 99% of water re-enters the body, and wastes are concentrated in the urine

Slide 25: Kidneys (cont.) 
   A. body cannot retain water unless there is sufficient sodium present 
      1. sweat is salty - sweating causes sodium loss and unless it is
         reingested (e.g., as sodium tablets) the body becomes dehydrated 
      2. sodium reuptake in the kidneys is regulated by the hormone aldosterone,
         which turns on the sodium pump in the nephrons 

Slide 26: Kidneys (cont.) 
   A. Billy had a damaged adrenal gland (cortex) - led to hypoaldosteronism 
   B. tumors in the adrenal gland may result in hyperaldosteronism 
      1. hypernatremia 
      2. high blood pressure 
      3. hypokalemia - K driven out electrostatically by Na 
         a. fatigue 
         b. headaches 
         c. can resemble depression 
   C. adrenal cortex -------------> kidneys --->  Na pump on ---> water retained
                      aldosterone

Slide 27: Kidneys (cont.) 
   A. diabetes insipidus - person compulsively drinks water (polydipsia) and
      urinates a lot (polyuria) 
      1. finding - damage to the posterior pituitary gland 
      2. normally secretes antidiuretic hormone (ADH) or vasopressin 
      3. vasopressin increases the permeability of the (distal) renal tubule and
         increases reuptake of water (the proximal tubule is always permeable) 
      4. failure of vasopressin leads to diabetes insipidus and intense thirst 

Slide 28: (Back To) Volumetric Thirst 
   A. decreased flow of blood to the kidneys results in the release of an enzyme
      called renin 
   B. renin converts angiotensinogen to angiotensin 
   C. and angiotensin... 

Slide 29: Angiotensin II (see Fig. 11.7)
   A. stimulates adrenal cortex to release aldosterone (which turns on the
      sodium pump in the kidney) 
   B. indirectly stimulates the release of vasopressin from the pituitary
      (which leads to increased retention of water by the kidneys) 
   C. causes vasocontriction (which increases blood pressure) 
   D. produces a salt appetite 
   E. produces drinking 

Slide 30: Atrial Baroreceptors 
   A. the atria of the heart contain receptors that detect stretch 
   B. they can detect if the atria are filling completely (normal blood volume) 
   C. information from these receptors to the brain can also stimulate thirst 

Slide 31: Neural Mechanisms 
   A. nucleus of the solitary tract (medulla) - sensory info from the atrial
      baroreceptors 
   B. subfornical organ - loaded with angiotensin receptors (which does not
      cross the BBB) 
   C. OVLT - osmoreceptors sensitive to solute concentration in the blood 
      1. median preoptic nucleus - receives input from all of the above 
      2. creates craving and behavior 
      3. humans with brain damage affecting this area experience adipsia and
         must remember to voluntarily drink at regular intervals even though
         they don't feel the need to 

Slide 32: Next: On to Hunger