trouble understanding the positive feedback loop in reference to homeostasis?

Question

i understand the negative feedback loop, but the positive one is making no sense to me at all.
i can understand the concept but i can't see how it is even remotely relevant to regulating changes in the body.
basically i understand it as something similiar to a domino effect, where one thing begins it triggers multiple reactins from the same thing.
they've given example like child birth, blood clotting, protein digestion.
i don't get it! if there's more of the same reaction then isn't that a bad thing?
can anyone make it clearer to me please?

really great answers. thanks guys i actually get it now!

Answer 1

I can help. It doesn't make much sense until you realize that every positive feedback group must be "nested" inside a negative feedback group. You can have positive feedback, but eventually, it will have to stop by negative feedback so you don't die.

I will use the blood clotting example. So when you cut yourself, you start the clotting process. It takes a lot of proteins and factors to make this happen properly, so when one factor is signaled, it signals another, and then another, and more of that, and more of this, etc. until the cut area is clotted over completely. That is the positive feedback. Your body makes the proteins build up until the clot is of sufficient size and quality. When the clot is just right, then the amount of proteins signal that it is done, and then negative feedback comes in stopping the build up.

Just associate positive feedback with "vicious cycle". The reason there are so few examples is because it is not common in the body. You are right, too much of anything is a bad thing, which is why it is common and why it must be stopped eventually by negative feedback.

Answer 2

Hhhmm... Not quite sure why you're not getting it, because when I first learned about it, the childbirth example made complete and total sense to me... But let's see if this'll help. ;o) Hopefully it will.

Ok, so, for the positive feedback loop in homeostasis... You already seem to grasp that homeostasis is a necessary function of the human body, yes? Ok, good. Well the positive feedback loop of that can be another beneficial aspect of that, but one that can also go potentially wrong - it is all in how delicately the 'program' is handled.
Think of the body in terms of a machine. A machine can be programed to do certain things, once a command has been given, right? Like, if someone pushed a button, then the machine will run the course of its job, until that job is finished. The same goes with the positive feedback loop of homeostasis.
Think of it this way: The body, as a machine, gets a button pushed (a tear in the skin, signaling that blood platelets need to accumulate). The machine continues the process (blood clotting), until the job is complete (a scab has formed). Once that process has been completed, the machine (human body) shuts down the mechanism (the positive feedback loop), until something else happens that triggers it all to start over again.

I hope that makes it a little clearer. What it basically boils down to is this:
- Something occurs that acts as a trigger
- The body responds
- The body continues to respond, in order to finish the "job" that was signalled by the trigger
- Once the "job" (coagulation, childbirth, etc) has been finished, then it is complete.

It is all an aspect of things building on one another, as well. A continuous momentum that raises things above their normal levels, in order to deal with a situation at hand.

Answer 3

All climate change requires positive feedbacks. In natural climate change, the coming and going of ice ages have required feedback to amplify. The Milankovitch Cycle or plate techtronics that initiated past climate change were nowhere near powerful enough on their own to cause the dramatic changes. In past, natural climate change, CO2 has been a primary feedback as the oceans naturally throw off CO2 as they warm. The albedo effects of growing or diminishing ice cover has also been a primary feedback in both the coming and going of ice ages. Among the positive feedbacks that will amplify global warming are the albedo effect (as ice surface diminishes, less energy is reflected by the white ice and more is absorbed by the exposed dark water), the release of vast stores of methane which is stored in the frozen tundra (methane is a stronger greenhouse gas), increased water vapor in the atmosphere due to increased evaporation (water vapor is also more powerful as a greenhouse gas than is CO2), a poleward shift of forested areas reducing the reflective powers of the surface, more fires which put more CO2 into the air, warming of the ocean reduces it's ability to absorb CO2. There are a few expected negative feedbacks. Clouds have been debated and it seems the may be either a positive or negative feedback. It's theorized that more CO2 can help plants, though it is not clear whether other changes will harm overall plant growth; more plants will use more CO2. Increased desertification and land degradation would cause more soil to be blown into the atmosphere which would have a cooling, dimming effect. Of all these, the greatest feedback will likely be the increase of methane in the atmosphere as that has always been a major component of global climate change.

Answer 4

I think the thing you're not considering is that although positive feedback is occuring, cells also have lots of other control mechanisms, so a positive feedback means a fast response and this is definitely a good thing. Lets take blood clotting. Blood clotting ONLY happens at the site of injury, we want that hole in our blood vessel mended as quickly as possible before we bleed to death. This doesn't mean that the blood will continue to clot throughout the body killing us, this is because cells that are intact secrete inhibitory factors that stop the reaction locally. So the wound, selectively, is clotted around, and fast! There are also threshold mechanisms, so it keeps feeding back to the response telling it that it wants more but ONLY until it hits this threshold and then it tells it to back off or the original stimulus is hidden or shut down. Its very beneficial in emergencies or responses that are required to be sustained.

Answer 5

Positive feedback loops are indeed more dangerous than negative feedback loops, and that is why the majority of our feedback loops are negative. Positive feedback promotes change away from the normal conditions in the HOPES that once that change is completed, the body will quickly return to its normal conditions.

So in answer to your question, it does makes things worse and can indeed lead to death, but its a risk worth taking if the body will return to normal after all.

Yes so for example, child birth; when instead of preventing the birth through negative feedback, it is promoted through positive release of hormones, in the hopes that once the baby is delivered, then conditions will most likely return to normal; but of course this process comes with the risk of death and misdelivery

Answer 6

A positive feedback loop is only a bad thing if it lacks a mechanism to stop it; such a loop becomes a "runaway process." Blood clotting, or coagulation, is eventually stopped by inhibitors. Inhibitor failure leads to a disorder called thrombosis, named after the thrombin protein, in which blood clots occur in vessels. But in a healthy person, coagulation is not a runaway process but instead is cut off when it has achieved the desired result.