Module 2
1. Module 2
1.5. Page 3
Module 2—The Endocrine System
Negative Feedback Loops
In the Student Stress lab you learned that a stressful situation, such as taking a test in a very short period of time, and being bothered while doing so, affects physiological processes. As a result, responses such as blood pressure and heart rate can change. After the stressful situation passes, heart rate and blood pressure return to normal or to “at-rest” values.
set point (set value): ideal or optimum conditions
negative feedback: a mechanism where deviation from the optimum state causes a return to the optimum state; acts to eliminate any deviation from optimal conditions and leads to stability
During every moment of your life, your body regulates carbon dioxide and oxygen levels, maintains water and salt balance, controls blood glucose levels, regulates temperature, maintains a regular heartbeat, and sustains a normal blood pressure. An organism is said to be in homeostasis when the internal environment is maintained at the balanced or best condition for each of these factors, which is called the set point. Homeostasis provides cells with a relatively constant environment, which helps them to work efficiently no matter what is going on outside the body.
Processes that aim to keep a potentially fluctuating value for a system within narrow limits use negative feedback mechanisms. Negative feedback means that when a condition changes, such as a drop in blood sugar level (BSL), the opposite effect is initiated, such as an effort to raise BSL. In a negative feedback system, there needs to be a sensor or receptor that measures the value of the feature to be controlled—for example, chemoreceptors monitor chemical levels in the body, such as the glucose concentration in the blood. If the sensor finds that the value is higher than it should be, it sends the information to an effector, which initiates a response to lower the value back toward the correct level. The effector continues this response until the sensor, which is still measuring the value, finds that the value is now too low. The receptor then sends information to the effector to stop doing whatever it is doing and start doing something to raise the value once more. Information is therefore “fed back” to the sensor from the effector. The feedback is called “negative” because it stops the effector from doing one thing and stimulates it to do the opposite. This is illustrated in the following diagram. Note: Although the term blood sugar level, BSL, is often used to describe the level of sugar in the blood, the term blood glucose level, or BGL, is technically more accurate.
In the case of regulation of blood glucose, an increase in the concentration of glucose in the blood sets into motion the processes that decrease the concentration. Conversely, a decrease in glucose concentration sets into motion the processes that increase the glucose concentration. The result is that the concentration of glucose in the blood automatically returns to its set value. This is shown in the diagram below.
Read
More information about homeostasis and feedback loops is found on page 203 of your textbook.
Watch and Listen
Review the concept of negative feedback by watching the following segment of “The Hypothalamus and Pituitary: The Master Complex.” Select “Bio Fact: Negative Feedback System” from the menu.
You may be required to enter a username and password in order to access the video. Contact your teacher for this information.
Drawing a Feedback Loop
An increase in blood glucose can be represented by a feedback loop where the following conventions and symbols are used.
- An ascending arrow (↑) means an increase in the parameter.
- A descending arrow (↓) means a decrease in the parameter.
- A horizontal arrow (→) means “leads to.”
- A dashed horizontal arrow (→) with the words negative feedback or the negative symbol, sometimes in a circle, means that the opposite effect has been performed.
