Feedback Loops: Positive Feedback - Online Tutor, Practice Problems & Exam Prep

Positive feedback loops amplify responses in the body, moving further from a set point. Two key examples are labor and blood clotting. In labor, pressure on the cervix triggers the release of oxytocin, leading to stronger contractions and increased pressure until the baby is born. In blood clotting, a broken vessel attracts platelets that release chemicals, drawing more platelets to form a clot until bleeding stops. Understanding these mechanisms is crucial for grasping homeostasis and physiological responses in the body.

Positive Feedback Loops

Now, I'm going to look at the details of two positive feedback loops. 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, away from where it started. Positive feedback loops are less common in the body, but there are two main examples that you're likely to see. We're going to 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. First off, we have birthing. Birthing starts with the baby's head pushing against the cervix. 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 now. We're on the loop and it says pressure on the cervix signals the hypothalamus to release oxytocin. Oxytocin is a hormone, a chemical signal, and it's going to take us around the feedback loop. The next thing it's going to 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. 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 to shut it all down. Positive feedback loop over. We did the job. Alright. Our other positive feedback loop is going to 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 going to say is that platelets adhere to the wound site, releasing signaling chemicals. These signaling chemicals attract more platelets to the wound. So this takes us further around our loop. More platelets at the wound site start clotting, and that's going to release more chemical signals. More chemical signals attract more platelets. More platelets start clotting and release more chemicals, more chemicals, more platelets, more platelets, more clotting and chemicals. You go around and around, and 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 forms and bleeding stops. So with that, again, these two 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.

Feedback Loops: Positive Feedback Example 1

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. 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. And over here, we have this wood that looks kind of wet; it's not really burning. It looks like maybe someone tried to get it going, but they can't get a fire going.

Let's compare these to hemophilia and thrombophilia. Let's start with hemophilia. Of the forest out of control fire or the wet wood that doesn't really get going, which one do you think sounds more similar to hemophilia in terms of our analogy? Well, to me, hemophilia sounds like this wet wood. And that's because, in 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 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.

In contrast, that leaves the forest fire 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 terms of our analogy, sounds a lot more like thrombophilia.

With that, there are some practice problems below. And like always, I'll see you in the next video.

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.

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.

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.

Here’s what students ask on this topic:

What is a positive feedback loop in physiology?

A positive feedback loop in physiology is a process that amplifies a response in the body, moving it further from a set point. Unlike negative feedback loops, which aim to maintain homeostasis by counteracting changes, positive feedback loops enhance the original stimulus. This results in a greater deviation from the initial condition. Two key examples are labor and blood clotting. In labor, pressure on the cervix triggers the release of oxytocin, leading to stronger contractions until the baby is born. In blood clotting, a broken vessel attracts platelets that release chemicals, drawing more platelets to form a clot until bleeding stops.

How does a positive feedback loop work during childbirth?

During childbirth, a positive feedback loop begins when the baby's head pushes against the cervix. This pressure signals the hypothalamus to release oxytocin, a hormone that causes uterine contractions. These contractions increase the pressure on the cervix, leading to the release of more oxytocin. This cycle continues, with contractions becoming stronger and more frequent, until the baby is born. The birth of the baby acts as the 'off switch,' stopping the feedback loop and ending the contractions.

What role does oxytocin play in the positive feedback loop of labor?

Oxytocin plays a crucial role in the positive feedback loop of labor. It is a hormone released by the hypothalamus in response to pressure on the cervix from the baby's head. Oxytocin stimulates uterine contractions, which increase the pressure on the cervix, leading to the release of more oxytocin. This cycle continues, with contractions becoming stronger and more frequent, until the baby is born. The birth of the baby stops the feedback loop, ending the release of oxytocin and the contractions.

How does a positive feedback loop contribute to blood clotting?

A positive feedback loop contributes to blood clotting by amplifying the response to a broken blood vessel. When a vessel breaks, platelets adhere to the wound site and release signaling chemicals. These chemicals attract more platelets to the wound, which release more chemicals, drawing even more platelets. This cycle continues, with the clot growing larger until the bleeding stops. The formation of the blood clot acts as the 'off switch,' stopping the feedback loop and preventing excessive clotting.

Why are positive feedback loops less common in the body compared to negative feedback loops?

Positive feedback loops are less common in the body compared to negative feedback loops because they amplify changes rather than stabilize them. While negative feedback loops maintain homeostasis by counteracting deviations from a set point, positive feedback loops drive the system further from the set point. This can be useful in specific situations, such as childbirth and blood clotting, where a rapid and amplified response is needed. However, in most physiological processes, maintaining stability is crucial, making negative feedback loops more prevalent.