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Positive & Negative Feedback in Biological Systems

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  • 0:02 Control & Regulation
  • 1:40 Positive Feedback
  • 2:55 Negative Feedback
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Many biological systems are regulated by feedback mechanisms. In this lesson, we'll investigate the difference between positive and negative feedback as they pertain to living things and explore examples of each feedback type in action.

Control & Regulation

All living things, even the tiniest of bacteria, are able to function only because of the intricate and complex workings of many different systems. You obtain oxygen because of your respiratory system; you can avoid pain because of your nervous system; and you know when your body needs food or rest because of your endocrine system. These are only a few small examples of the myriad of processes that are constantly taking place in your body. Thankfully, you don't have to consciously think about these systems for them to work. Many living things are not capable of thinking at all, yet all of their biological processes function flawlessly. This is because many of these processes are regulated by way of feedback systems, in which the product of a process is used to regulate the production of that product. In other words, the product, also known as the effector, will go back and influence a stimulus within the system, either by reducing it or by further bolstering it.

All feedback systems occur in a series of stages:

  1. The first stage is a stimulus, or a change in the system
  2. The second stage is the sensor, where the change is detected
  3. The third stage is known as control, which is where a response to the change occurs
  4. And finally, the fourth stage is the effector, or the effect of the response

And the cycle then repeats itself ad infinitum.

Feedback systems are classified into two different types: positive feedback and negative feedback. The terms positive and negative aren't meant to denote a good or bad response but rather the type of response the system has to the presence of the effector.

Positive Feedback

In positive feedback systems, the effector of a process bolsters the stimulus, which increases the production of the product. One common example of a positive feedback system in living things is blood clotting. When the skin is cut and a blood vessel experiences damage, platelets in the blood stream collect at the site of the cut and begin releasing several different chemicals that signal more platelet recruitment; more platelets trigger more chemicals to be released, which trigger more platelets which, in turn trigger more chemical signals, and so on, until the platelets and various associated proteins have plugged up the cut with a clot.

Another example is the process of labor and childbirth in mammals. The effector is a hormone called oxytocin, which stimulates uterine contractions, which then cause more oxytocin to be released and, hence, more contractions. This continues until the baby is born.

Positive feedback systems essentially cause a growing cascade reaction in which each new product further amplifies the very process that created it, ensuring a whole lot more product. They are typically not ongoing processes in an individual but rather reactions to specific states of physiological stress, as we see in the blood clotting and oxytocin examples.

Negative Feedback

In negative feedback systems, the effector of a process reduces the effect of the stimulus which, in turn, decreases the production of the product. This is a more common process in living systems as it serves to maintain homeostasis, or the optimal internal environment of organisms. Many different key parameters of living things, such as temperature, pH, and hormone levels, for example, must stay within a particular range for those organisms to thrive. Negative feedback systems maintain homeostasis by ensuring that the product of a process does not reach excessive amounts, as the presence of a product will reduce the further stimulus and production of more product.

One good example of this process is calcium regulation in the human body. The parathyroid glands produce the parathyroid hormone (PTH) in response to low calcium levels in the blood stream. This hormone stimulates the breakdown and release of calcium from bone and other storage sources, until the calcium level in the blood rises to normal levels, at which point the parathyroid glands turn off PTH production. If the blood calcium level rises too high, the thyroid glands release a hormone called calcitonin, which acts to deposit blood calcium into bones and other storage, reducing the blood calcium levels back to homeostasis.

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