Updated for 1999
Lecture Notes - Week 5

 PHYSIOLOGICAL REGULATORY MECHANISMS - maintains the constancy of some internal characteristic of the organism in the face of external variability

It contains 4 essential features:

1. system variable - characteristic to be regulated, such as body temperature

2. set point - optimal value of the system variable (98.6F)

3. detector - monitors the value of the system variable (such as an internal thermometer)

4. correctional mechanism - restores the system variable to the set point (such as shivering when cold, sweating when hot)

negative feedback - process by which the effect produced by an action serves to diminish or terminate that action (such as thermostat turning on heater when room temperature falls below set point and then the thermostat turns the heater off when the desired room temperature is reached)

satiety mechanisms - monitor activity of correctional mechanism, not the system variable, stopping the activity in anticipation of the replenishment that will occur later (thatís why we stop drinking after a few glasses of water when dehydrated, even before the fluid has reached our cells)


1 intracellular: fluid portion of the cytoplasm of cells

3 extracellular:

intravascular fluid (blood plasma)

cerebrospinal fluid

interstitial fluid ("seawater" around our cells)

Fluid compartments are separated by semipermeable barriers:

1. walls of capillaries separate interstitial fluid from blood plasma

2. cell membranes separate interstitial fluid from intracellular fluid


1. normally interstitial fluid is isotonic (the same concentration of solutes) with the intracellular fluid - water doesnít move in or out of cell

2. if interstitial fluid loses water and becomes hypertonic (more concentration of solutes), water will then diffuse out of the cells

3. if the interstitial fluid gains water and becomes hypotonic (less concentrated), water will diffuse into the cells

Blood plasma Interstitial fluid KIDNEYS 1. aldosterone - controls sodium excretion; steroid hormone released by adrenal cortex; high levels cause sodium retention

2. vasopressin - controls excretion of water; peptide hormone released by posterior pituitary gland; high levels cause water retention

1. supraoptic nucleus

2. paraventricular nucleus

OSMOMETRIC THIRST - stimulated by cellular dehydration RECEPTORS FOR OSMOMETRIC THIRST: 1. salt is absorbed from the digestive tract into the blood

2. this makes the blood hypertonic (high concentration of salt)

3. this draws water into the cell from the interstitial fluid

4. the loss of water from the interstitial fluid makes it hypertonic

5. now water is drawn out of the cells

6. as blood plasma increases in volume, kidneys excrete more water and sodium

7. eventually, excess sodium is excreted, along with the water that was taken from the interstitial fluid and intercellular fluid

8. this results in an overall loss of water from the cells

9. however, blood plasma volume never decreased (it actually was higher temporarily)

VOLUMETRIC THIRST - occurs when the volume of the blood plasma (intravascular volume) decreases

1. direct loss of blood - which causes thirst and salt appetite (lose sodium in blood)


1. Renin-Angiotensin System

1. stimulates adrenal cortex to secrete aldosterone

2. stimulates posterior pituitary gland to secrete vasopressin

3. increases blood pressure by causing muscles in small arteries to contract

4. behaviorally it initiates drinking and produces a salt appetite

2. Baroreceptors FOOD-RELATED DRINKING - most drinking occurs in anticipation of actual need, during meals; appears to involve angiotensin 1. eating causes water to be diverted from the rest of the body into the stomach and small intestine, to be used for digestion

2. once food is absorbed, it increases the solute concentration of the blood plasma and thus induces an osmometric thirst

SALT APPETITE - primary stimulus is presence of aldosterone, whose secretion is under the control of angiotensin


Circumventrical system

1. endocrine - SFO axons project to neurons in the supraoptic and paraventricular nuclei that are responsible for production and secretion of the posterior pituitary hormone vasopressin

2. autonomic - axons project to cells of the paraventricular nucleus and other parts of the hypothalamus, which the send axons to brain stem nuclei which control the sympathetic and parasympathetic nervous system; this system controls angiontensinís effect on blood pressure

3. behavioral - axons sent to median preoptic nucleus, an area which controls drinking and secretion of vasopressin

median preoptic nucleus - receives info from:

1. OVLT regarding osmoreceptors

2. SFO regarding angiotensin

3. baroreceptors via the nucleus of the solitary tract

Lateral Hypothalamus and Zona Incerta