PSYC 211 - Chapter 12


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CHAPTER 12 – PHYSIOLOGICAL REGULATORY MECHANISMS Physiological regulatory mechanism maintains the steadiness of some internal characteristics of the organism in the face of external variability. This mechanism contains 4 essential features: 1. System variable : a characteristic that is regulated by the mechanism 2. Set point : the optimal value of the system variable 3. Detector : a mechanism that monitors the value of the system variable 4. Correctional mechanism : the mechanism that brings back the system variable to the set point An example of a regulatory system is a room whose temperature is regulated by a thermostatically controlled heater: the system variable is the air temperature, the set point is a preset temperature, the detector is a thermostat, and the correctional mechanism is the coils of the heater. The rise in room temperature causes the thermostat to turn the heater off. Since heat production by the correctional mechanism feeds back to the thermostat and causes it to turn the heater off, this process is referred as negative feedback . Negative feedback is essential to all regulatory systems. Ingestive behaviors (eating and drinking) are correctional mechanisms that refill the body’s depleted stores of nutrients or water. Since there is the delay b/w ingestion and replenishment of the depleted stores, ingestive behaviors are regulated by satiety mechanisms as well as by detectors. Satiety mechanisms monitor the activity of the correctional mechanism, not the system variables themselves. When an adequate amount of nutrients or water is supplied, the satiety mechanisms cause termination of the correctional mechanism in anticipation of the replenishment that will occur later. Drinking Some Facts About Fluid Balance Approx. two-thirds of the body’s water is contained in the intracellular fluid , the fluid portion of the cytoplasm of cells. The rest is extracellular fluid , which consists of intravascular fluid (the fluid found within the blood vessels, i.e. the blood plasma), the cerebrospinal fluid, and the interstitial fluid (the fluid that bathes the cells, filling the space b/w the cells of the body). The intracellular fluid and intravascular fluid must be kept within specific limits. The intracellular fluid is controlled by the concentration of solutes in the interstitial fluid. Generally the interstitial fluid is isotonic (equal in osmotic pressure to the contents of a cell), thus no water movement occurs between the cells and the interstitial fluid. If the interstitial fluid loses water ( hypertonic ), water will be pulled out of the cells. On the other hand, if the interstitial fluid obtains water ( hypotonic ) water will move into the cells. Either hypertonic or hypotonic condition can endanger cells. The volume of the intravascular fluid (the blood plasma) must be well regulated due to the mechanics of the operation of the heart. If the blood volume falls too low, the heart can no longer pump the blood efficiently; if the volume is not restores,
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This note was uploaded on 02/13/2011 for the course PSYC 211 taught by Professor Yogitachudasama during the Winter '09 term at McGill.

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