Feedback Loops: Positive Feedback - Video Tutorials & Practice Problems
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concept
Positive Feedback Loops
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3m
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Video transcript
Now, I'm going to look at the details of 2 positive feedback loops. Now remember, positive feedback loops move the body in the same direction as the stimulus. So if the body is at a set point and something moves off the set point, a positive feedback loop is just going to push it further and further in the same direction, further and further from where it started. Now positive feedback loops, we said, are less common in the body, but there are two main examples that you're likely to see. Now we're gonna go through the details of these now, but don't worry, these details are not going to be as complex as what we did for the negative feedback loop. You are likely to see these though, so it doesn't hurt to remember these at a sort of higher level in case you see them on a test question. So first off, we have birthing. Birthing starts with the head pushing against the cervix. Now, just to be clear, there's more that goes into it and people don't fully understand what starts birthing, but part of it is the baby's head pushing against the cervix. So we have this little picture of an on switch. We're going to say that that turns on the positive feedback loop. Our arrow takes us down. Now we're on the loop and it says pressure on the cervix signals the hypothalamus to release oxytocin. Now oxytocin is a hormone, a chemical signal, and it's gonna take us around the feedback loop. And the next thing it's gonna say is oxytocin causes contractions of the uterus. This increases pressure on the cervix. Well, remember what started the release of oxytocin originally? Well, pressure on the cervix causes the hypothalamus to release oxytocin. Oxytocin causes more contractions, more pressure on the cervix. More pressure, more oxytocin, more oxytocin, more pressure. We're going around and around and around. Labor is getting more and more intense. Contractions are getting stronger and stronger. We need an off switch. We have a little off switch down here. The way we get off this feedback loop, we follow this arrow down and it says that the baby is pushed out of the birth canal. The baby being born is the signal. Shut it all down. Positive feedback loop over. We did the job. Alright. Our other positive feedback loop is gonna be blood, or more specifically, blood clotting. Blood clotting starts when a blood vessel breaks, and that's going to be our signal to turn on the positive feedback loop. So we have this little picture of an on switch here. We're going to follow our arrow down. We're going to get on the loop, and the first thing it's gonna say is that platelets adhere to the wound site, releasing signaling chemicals. Alright. These signaling chemicals attract more platelets to the wound. So this takes us further around our our loop. More platelets to the wound well, more platelets to the wound site starts clotting, and that's gonna release more chemical signals. More chemical signals attract more platelets. More platelets start clotting and release more chemicals, more chemicals, more platelets, more platings platelets, more, clotting and chemicals. You go around and around around, that clot gets bigger and bigger. We need to turn it off or else all our blood will turn into a clot. We have a little off switch. We get to come off this loop, follow this arrow down, when the blood clot form forms and bleeding stops. Okay. So with that, again, these 2 feedback loops at this sort of higher level you should probably want to remember because they do come up often enough. With that, we have an example below and practice problems to follow. I'll see you there.
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example
Feedback Loops: Positive Feedback Example 1
Video duration:
2m
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Video transcript
This example says that hemophilias and thrombophilias are both classes of dangerous blood clotting disorders. If blood does not clot properly, it's referred to as hemophilia. If blood clots too easily, it's referred to as thrombophilia. We previously compared positive feedback mechanisms to a fire. Explain how thrombophilia and hemophilia relate to the analogy of positive feedback as a fire. Alright. So remember our analogy of positive feedback as a fire. You start a fire, the heat from the fire catches more things on fire. As more things catch on fire, it gets hotter. As it gets hotter, it catches more things on fire. As more things catch on fire, it gets hotter, so on and so on. So and then we have down here this picture of the woods that are totally on fire. We have a huge forest fire, completely out of control fire. And over here, we have this wood that looks kind of wet. It's not really burning. Looks like maybe someone started to try to get it going, but they can't get a fire going. So let's compare these to hemophilia and thrombophilia. Let's start with hemophilia. Of the forest out of control forest fire or the wet wood that doesn't really get going, which one to use sounds more similar to hemophilia in terms of our analogy? Well, to me, hemophilia sounds like this wet wood. And that's because hemophilia, the blood doesn't clot properly. That clot never starts. It's sort of like you never have that on switch for the positive feedback loop. You just can't get the fire going. You can't get the blood get the blood clotting properly. And hemophilia is a very dangerous condition, where people who have it can bleed to death very easily, even just from a small cut. Alright. In contrast, well, that leaves the forest fire is probably more like thrombophilia. I'm going to write that out, and as I write it, think why. Well, a forest fire is a positive feedback loop totally out of control. It started maybe just with a little spark. That spark caught some dry tinder going, but everything's so dry that pretty soon the entire forest is going. Well that's, in an analogy, sounds kind of like thrombophilia. Your blood clots too easily. It starts even maybe when there isn't even a cut, and it starts to clot. Those clots can be very dangerous because a blood clot, where it's not supposed to be, can lodge and block blood flow in the wrong place in the body. That can be very dangerous. So a forest fire, in our terms of analogy, sounds a lot more like thrombophilia. Okay. With that, there's some practice problems below. And like always, I'll see you in the next video.
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Problem
Problem
Which of the following is an example of a positive feedback loop?
A
The body increasing heart rate after a drop in blood pressure.
B
The body shivering to increase temperature on a cold day.
C
The action of platelets to form a blood clot when you get a paper cut.
D
Parathyroid hormone signaling bone to release calcium when calcium levels are low.
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Problem
Problem
Ori overhears his study partners discussing a feedback loop where Substance M is released, and the more Substance M is released the more is generated. They note that humans don't release Substance M all the time, only when the pathway is activated. What type of feedback loop is this and why?
A
Negative feedback because it's the most common.
B
Negative feedback because the process is returning the body to the set point.
C
Positive feedback because the amount of the substance is increasing.
D
Positive feedback because the process increasingly moves away from the starting condition.
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Problem
Problem
Labor and delivery is one of the more dangerous normal physiological functions that humans perform. How does this danger relate to the concept of feedback loops?
A
The negative feedback loop of oxytocin release may have the effect of stalling labor.
B
Reducing pressure on the cervix when the baby is born breaks the positive feedback cycle. If the pressure on the cervix cannot be decreased, the positive feedback loop will continue indefinitely.
C
Labor and delivery is an example of a negative feedback loop. Negative feedback loops are inherently more dangerous, because lowering any physiological parameter too far may be irreversible.
D
Labor and delivery can be dangerous because due to physiological variation some people experience it as negative feedback while some experience it as positive feedback.
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