Homoestasis.docx - Ligan, Daphne B. 12-Nickel Homeostasis...

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Ligan, Daphne B.12-NickelHomeostasis and Feedback MechanismsDefinition/DescriptionHomeostasis is a dynamic equilibrium, an interplay between external factors thattend to change the internal environment and internal control mechanisms that opposesuch changes. Note that physiological responses to stimuli are not instantaneous, justas switching on a furnace does not immediately warm a room. As a result, homeostasismoderates but doesn’t eliminate changes in the internal environment. Additionalfluctuation occurs if a variable has a normal range—an upper and lower limit—ratherthan a set point. This is equivalent to a thermostat that turns on a heater when the roomtemperature drops to 19°C (66°F) and turns off the heater when the temperaturereaches 21°C (70°F). Regardless of whether there is a set point or a normal range,homeostasis is enhanced by adaptations that reduce fluctuations, such as insulation inthe case of temperature and physiological buffers in the case of pH. Inbrief, homeostasis is defined as the dynamic constancy of the internal environment. Theterm dynamic is used because conditions are never absolutely constant, but fluctuatecontinuously within narrow limits.Mechanism of HomeostasisTo further understand the mechanism of homeostasis, let us consider theregulation of room temperature. Assuming, you want to keep a room at 20°C (68°F), acomfortable temperature for normal activity. You set a control device—the thermostat—to 20°C. A thermometer in the thermostat monitors the room temperature. If thetemperature falls below 20°C, the thermostat responds by turning on a radiator, furnace,or other heater. Once the room temperature reaches 20°C, the thermostat switches offthe heater. If the temperature then drifts below 20°C, the thermostat activates anotherheating cycle. Like a home heating system, an animal achieves homeostasis bymaintaining a variable, such as body temperature or solute concentration, at or near aparticular value, or set point. A fluctuation in the variable above or below the set pointserves as the stimulus detected by a sensor. Upon receiving a signal from the sensor, acontrol center generates output that triggers a response, a physiological activity thathelps return the variable to the set point. In the home heating example, a drop intemperature below the set point acts as a stimulus, the thermostat serves as the sensorand control center, and the heater produces the response.In a separate article, sensor, control center and effector are further elaborated. Asensor or receptor detects changes in the internal or external environment. An exampleis peripheral chemoreceptors, which detect changes in blood pH. The integrating centeror control center receives information from the sensors and initiates the response tomaintain homeostasis. The most important example is the hypothalamus, a region ofthe brain that controls everything from body temperature to heart rate, blood pressure,satiety (fullness), and circadian rhythms (sleep and wake cycles). An effector is any

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