Final answer:
The portion in a feedback loop that brings the variable back to its normal value is negative feedback, which is a control mechanism that reduces an excessive response and keeps a variable within its normal range. It has three components: a sensor, a control center, and an effector. Body temperature regulation is an example of negative feedback.
Step-by-step explanation:
In a feedback loop, the portion that brings the variable back to its normal value is the negative feedback.
A negative feedback loop is a control mechanism that serves to reduce an excessive response and keep a variable within its normal range. It has three basic components: a sensor, a control center, and an effector. The sensor monitors the variable, the control center compares the value to the normal range, and the effector causes a change to reverse the situation and return the value to the normal range.
For example, body temperature regulation is controlled by negative feedback. If body temperature rises above the normal range, sensors in the body send signals to the control center, which activates effectors like sweat glands to cool the body down and bring the temperature back to normal.