Module 2 Lesson 1 - 3

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Every moment of our lives, our bodies regulate levels of carbon dioxide and oxygen, maintain water and salt balance, control blood glucose levels, regulate temperature, maintain regular heartbeat, and sustain 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 the body cells with a relatively constant environment, and this helps them to function efficiently no matter what is going on outside the body.

Processes that 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 other words, secretion of a hormone is inhibited in response to the adequate amount of that hormone.

In a negative feedback system,  a sensor/receptor is required to measure 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 towards the correct level. It keeps 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.  Therefore, information is "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 above.

Example: Blood Glucose Regulation

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

This is shown in the diagram below.

 

Watch and Listen

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Review the concept of negative feedback by watching the following segment of Biologix-09 on Negative Feedback System (30 seconds).

Positive Feedback Loop

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Sometimes the deviation from the set point or normal value is not corrected. Instead, it leads to a further deviation. The result is a "runaway" situation in which a change triggers more change in the same direction. This is called positive feedback, whereby additional hormone is released in response to the original hormone being released.

At first sight, the positive feedback appears to be damaging and even destructive. It may cause illness or the onset of a disorder.

Returning to the glucose example, if the pancreas were unable to decrease blood glucose after a meal because it could not produce insulin, the glucose level would remain high. Because the glucose is not being absorbed and used for energy, the person will feel hungry and perhaps eat a candy bar. As the glucose from the chocolate bar is absorbed from the stomach and intestines, even higher blood glucose levels result. This is the disorder called diabetes (mellitus).

However, in some cases positive feedback is a good thing. In Unit B, you will study the process of natural childbirth that is dependent on positive feedback involving the hormone oxytocin.