Animals regulate their internal environment within relatively narrow limits • The internal environment of vertebrates is called the interstitial fluid and is very different from the external environment • Homeostasis is a balance between external changes and the animal’s internal control mechanisms that oppose the changes
Regulating and Conforming • Regulating and conforming are two extremes in how animals cope with environmental fluctuations • A regulator uses internal control mechanisms to moderate internal change in the face of external, environmental fluctuation • A conformer allows its internal condition to vary with certain external changes
Mechanisms of Homeostasis • Mechanisms of homeostasis moderate changes in the internal environment • A homeostatic control system has three functional components: a receptor, a control center, and an effector • Most homeostatic control systems function by negative feedback, where buildup of the end product shuts the system off • In positive feedback, a change in a variable triggers mechanisms that amplify rather than reverse the change
Feedback Mechanisms in Thermoregulation • Mammals regulate body temperature by negative feedback involving several organ systems • In humans, the hypothalamus (a part of the brain) contains nerve cells that function as a thermostat
