Negative Feedback vs. Positive Feedback

Understanding the mechanisms of these feedback systems is crucial in your MCAT prep.

Maintaining the body’s internal environment in equilibrium, commonly referred to as homeostasis, can at times be a minute-by-minute process. And many aspects of homeostasis are achieved by hormones.

Hormones function in one of the following three ways:

1. Controlling the transport of substances through cell membranes

2. Controlling the activity of specific genes that produce enzymes

3. Controlling cell metabolism

Hormones are the major players in the endocrine system and they function via feedback loops. Those feedback loops may be negative or positive, though the majority are negative.

Negative feedback involves a hormone triggering a cascade of biological activities that ultimately inhibit the further secretion of that hormone. A good example of this process is blood sugar control.

Blood glucose levels rise after you eat a meal. Those elevated levels stimulate beta cells in the pancreas to release the hormone, insulin. Insulin activates cells in the liver, muscles, and fat to absorb and store glucose. As glucose is absorbed, blood glucose levels fall. Once those levels dip to a certain point, beta cells stop releasing insulin.

Some other examples of negative feedback:

• Blood pressure regulation

• Temperature regulation

On the other hand, positive feedback occurs when a hormone’s activity amplifies its release to increase a desired activity or make it occur faster. Childbirth is the perfect example of positive feedback.

During labor in preparation for delivery, the baby repositions itself pushing its head on the mother’s cervix. That increased pressure on the cervix sends signals to the brain which in turn stimulates the release of oxytocin from the posterior pituitary.

Elevated oxytocin levels in the bloodstream induce contractions and those contractions trigger more oxytocin release. Accordingly, contractions become more frequent and powerful until the baby is born.

Some other examples of positive feedback:

• Blood clotting

• Fruit ripening

  
  

Negative and positive feedback loops are essential to maintaining homeostasis and supporting biological function. So, you can expect to see at least one or two questions on the MCAT to test your understanding of these crucial concepts.