Module 2 The Endocrine System

1.2.1 page 3

Read

Negative Feedback Loops

In the Student Stress! lab you learned that a stressful situation such as taking a test in a very short time and being harassed by the experimenter changes physiological processes such as blood pressure and heart rate. After the stressful situation passes, heart rate and blood pressure return to normal or at-rest values.

Every moment of your life your body regulates levels of carbon dioxide and oxygen, maintains water and salt balance, controls blood glucose levels, regulates temperature, maintains a regular heart beat, 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, called the set point. Homeostasis provides cells within the body with a relatively constant environment and this helps them to work efficiently, no matter what is going on outside the body. Processes which aim to keep a potentially fluctuating feature within narrow limits use negative feedback mechanisms. Negative feedback means that when something changes, the opposite effect is instigated. In a negative feedback system there needs to be a sensor/receptor which measures the value of the feature to be controlled—for example chemoreceptors monitor 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 does something to lower the value back towards the correct level. It keeps on doing this until the sensor, which is still measuring the value, finds that the value is too low, and 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. Notice how this is shown in the diagram below.

In the case of blood glucose regulation, an increase in the concentration of glucose sets into motion the processes which decrease it. Conversely a decrease in glucose concentration sets into motion the processes which increase it. The result is that, whatever the direction of the change (i.e. the error) the concentration of blood glucose automatically returns to its set value. This is shown in the diagram below.

Further discussion of homeostasis and feedback loops is found on page 203 of your textbook.

OR

Watch and Listen

Negative Feedback System?

Drawing a Feedback Loop

An increase in blood glucose can be represented by a feedback loop where the following conventions and symbols are used:

• a 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 either the words “negative feedback” and/or the negative symbol, sometimes in a circle, which means that the opposite effect has been performed.