1.10: Positive Feedback Loops
- Page ID
- 90479
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Positive feedback amplifies a change rather than restoring balance.
- Distinguish between negative and positive feedback mechanisms.
- Identify the key components of a positive feedback loop.
- Describe how childbirth illustrates a positive feedback loop and explain how the process is normally terminated.
- Explain how blood clotting functions as a positive feedback mechanism and how the response is shut off.
- Explain why negative feedback predominates in maintaining homeostasis, whereas positive feedback are sparingly used.
Positive Feedback: What It Is and Why It Matters
Positive feedback is a physiological mechanism in which the body amplifies a change rather than reversing it. Unlike negative feedback, which works to restore balance and maintain homeostasis, positive feedback pushes the body further in the direction of the initial change, and the system moves farther away from the normal range.
This might sound like a bad idea, but in certain situations, positive feedback is essential to maintain homeostasis. It allows the body to complete specific processes quickly and efficiently. Therefore, positive feedback in the body is normal only when there is a definite end point.
Examples of Positive Feedback
🤰1. Childbirth (Labor Contractions)
Childbirth is one of the clearest and most commonly used examples of a positive feedback loop in human physiology. It follows a straightforward, self-amplifying process with a clear starting point, increasing intensity, and a definite conclusion. At full term, maintaining the body's current state is no longer beneficial. Major physiological changes are needed to deliver the baby. Once labor begins, it must progress quickly, because delays can put both the mother and baby at risk. The strong muscular contractions of the uterus during labor are controlled by a positive feedback loop, which causes the contractions to become stronger and more frequent until the baby is born.
Here is how it works:
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Trigger: Labor begins when the baby's head pushes against the cervix, causing it to stretch.
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Signal: This stretching activates stretch receptors and sends nerve signals to the brain (specifically the pituitary gland), which responds by releasing the hormone oxytocin.
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Response: Oxytocin increases the strength and frequency of uterine contractions.
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Amplification: These contractions push the baby harder against the cervix, which causes more stretching, more nerve signals, more oxytocin, and therefore stronger contractions.
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Loop continues: This cycle of increasing pressure and increasing hormone release continues until the baby is born.
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Loop ends: This loop continues until the baby is born, at which point the pressure on the cervix stops, oxytocin levels drop, and contractions subside.
🩸 2. Blood Clotting
A second well-known example of a positive feedback loop is the blood clotting process, which helps the body respond to severe injuries like a deep cut or puncture. In this situation, the most immediate threat is excessive blood loss, which can cause a dangerous drop in blood pressure and reduced perfusion — that is, reduced delivery of oxygen and nutrients — to the brain and other vital organs. If perfusion drops too low, vital organs begin to shut down, and the person may die.
To prevent this, the body launches a rapid, localized coagulation (= blood clotting) response.
Here’s how it works:
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Injury occurs: A blood vessel is damaged, and the body detects the break in the vessel wall.
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Platelets rush in: Platelets (small cell fragments in the blood) stick to the injured site and release chemical signals.
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Amplification: These chemicals attract more platelets to the area, which then release even more signals to recruit even more platelets.
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Clot forms: This growing pile of platelets and clotting factors rapidly forms a plug that seals the break and stops the bleeding.
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Loop ends: The process continues until the clot is large enough to seal the injury. Once a clot is formed and the damage is repaired, the positive feedback loop of coagulation is terminated through a combination of mechanisms, including the activation of specific inhibitors and negative feedback pathways.
In short, positive feedback in blood clotting helps the body respond quickly and powerfully to blood vessel injury—but only until the job is done.
💡 Key Takeaways:
✅ Not all feedback is negative — sometimes the body uses positive feedback, too.
Although rare, positive feedback temporarily amplifies a change to reach a specific goal (e.g., childbirth or blood clotting). After that goal is met, the loop stops.
✅ Positive feedback loops are not used for everyday regulation.
They are used for short-term, goal-driven processes that need to be completed efficiently. Once the goal is met (e.g., stopping bleeding or delivering a baby), the loop naturally shuts off.