Feedback Loops: Positive Feedback: Videos & Practice Problems

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

Positive feedback loops are mechanisms in the body that amplify a response, moving it further away from a set point in the same direction as the initial stimulus. While less common than negative feedback loops, two significant examples are birthing and blood clotting.

In the birthing process, the loop is initiated when the baby's head exerts pressure on the cervix. This pressure signals the hypothalamus to release the hormone oxytocin. Oxytocin then stimulates uterine contractions, which increase pressure on the cervix even more. This cycle continues, with each contraction leading to more oxytocin release, resulting in progressively stronger contractions. The loop continues until the baby is born, which serves as the signal to terminate the feedback loop.

The second example, blood clotting, begins when a blood vessel breaks. This injury triggers the positive feedback loop. Initially, platelets adhere to the wound site and release signaling chemicals that attract additional platelets. As more platelets gather, they continue to release more chemicals, further attracting even more platelets to the site. This process amplifies until a sufficient clot is formed, at which point the loop is turned off when bleeding stops.

Both of these feedback loops illustrate how the body can respond to specific stimuli in a way that enhances the initial response, ultimately leading to a significant physiological outcome. Understanding these mechanisms is crucial, as they often appear in assessments related to human physiology.

Feedback Loops: Positive Feedback Example 1

Hemophilia and thrombophilia are two significant blood clotting disorders that illustrate the concept of positive feedback mechanisms through the analogy of fire. In this context, hemophilia can be likened to wet wood that fails to ignite. This condition is characterized by the blood's inability to clot properly, akin to a fire that cannot start. Without the necessary clotting factors, the body struggles to initiate the clotting process, leading to severe risks such as excessive bleeding from minor injuries.

Conversely, thrombophilia resembles an uncontrollable forest fire. This disorder occurs when blood clots too easily, similar to how a small spark can ignite a dry forest, leading to a rapid and dangerous spread of fire. In thrombophilia, the clotting process can begin even in the absence of injury, resulting in clots that may obstruct blood flow in critical areas of the body. Such clots can pose serious health threats, including strokes or heart attacks, as they disrupt normal circulation.

Understanding these conditions through the fire analogy highlights the critical balance required in the body's clotting mechanisms. Just as a fire needs the right conditions to either ignite or be contained, the body must maintain proper clotting function to ensure safety and health.

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.

Answer: (C)

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.

Answer: (D)

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.

Answer: (B)

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.