INTRODUCTION

The activities of physiological systems are controlled—speeded up or slowed down—by actions of the nervous and endocrine systems. But to regulate these systems and maintain homeostasis, information is required. As an analogy, think of the thermostat that controls the furnace and air conditioner to regulate the temperature of a house. The desired temperature is a set point, or reference point on the thermostat. The thermostat acts as a comparator by sensing the current temperature in the house and comparing that value to the set point. Thus, the sensing of the air temperature is feedback information. Any difference between the set point and feedback information results in an error signal. The error signal is converted into commands to the furnace or air conditioner, turning them on or off.

In a similar manner, the human body regulates temperature by balancing heat loss and heat generation. The normal core temperature, set point, of the resting human body is 37 ± 0.5°C. The body’s thermostat is in the hypothalamus. Some hypothalamic neurons are temperature sensitive. When the temperature of the hypothalamus falls below a certain level—a set point—it generates signals that increase metabolic heat production. When the temperature of the hypothalamus rises above a certain level—another set point—it generates signals that increase sweating. There are also set points closer to 37°C that determine blood flow to the skin through vasodilation or vasoconstriction. To explore the body’s response to a sudden drop in hypothalamic temperature (perhaps due to drinking a large bottle of ice water) or to a sudden rise in hypothalamic temperature (perhaps by entering a hot bath), click along the x-axis of the plot below.

CONCLUSION

In general, cells function optimally in a narrow range of temperatures. To stay within those limits in spite of environmental conditions, animals have thermoregulatory adaptations that enable them to tolerate extreme conditions or, as in our simulation, to control their body temperature in spite of environmental conditions. These adaptations may determine the thermal tolerances of species and thereby determine their geographic ranges.

Complex multicellular animals provide a stable internal environment (homeostasis) that provides for the needs (e.g., optimal temperature) of its cells. Organs and organ systems control the composition of the internal environment so as to maintain homeostasis. To do this, the activity level of organs and organ systems is regulated. Regulation requires information about optimal conditions (set points) and existing conditions (feedback) to send commands to effector cells and organs.