Do you wonder how mothers lactate while feeding their infants, so there is no shortage of milk? Or how the fruits on the branch of a tree ripen together? These reactions and processes are prompted by specific positive feedback mechanisms present in all organisms for varying functions. Positive feedback means that a reaction can further amplify the output in the same direction.
Let’s explore how positive feedback mechanisms works!
- First, we will define positive feedback.
- Then, we will discuss how positive feedback works and cite several examples.
- Finally, we will enumerate similarities and differences between positive and negative feedback.
What is positive feedback?
Positive feedback is a pathway that, in deviation from homeostasis, amplifies the output and strays away from homeostasis even more.
Let's expand a little on the concepts of homeostasis and feedback loops to fully understand the implications of positive feedback. Our body is constantly trying to maintain equilibrium to ensure it can survive under all conditions. This self-regulating process is called homeostasis. Homeostasis is maintained by feedback loops.
A feedback loop is a mechanism used to bring the body back to the internal steady state of homeostasis. I.e. when the body deviates from homeostasis, there is a signaling processed triggered that will start the mechanisms necessary for the stable state of homeostasis to be reached again.
The feedback mechanism is triggered when there is a change in the system and as a result, triggers an output.
Feedback can be either positive or negative. These systems either increase the output and deviate from the normal interstate of the body or reduce the output to bring the body back to homeostasis.
Negative feedback is a pathway that, contrary to positive feedback, in deviation from homeostasis, triggers a response/output that opposes the change, reducing or reversing the change.
Positive feedback is also known as a self-reinforcing response to external or internal stimuli. Positive feedback enhances a change in the physiological state instead of reversing it. The change in variation is sensed by the receptor, and in response, the effector works to produce the same output and thus, enhances the physiological change. This process keeps occurring in a loop until the original stimulus is removed. Positive feedback processes are less common in the human body and are only required in cases of fast and efficient processes to amplify the output.
Did you already know that homeostasis depends on negative feedback? You can learn more about this by checking out "Homeostasis" and "Negative Feedback"!
Mechanism of Positive Feedback
Positive feedback has 4 steps:
- Stimulation
- Reception
- Processing
- Further activation of stimulus.
Stimulation
When a stimulus is set off that disrupts homeostasis, it initiates a feedback loop, in this case, positive feedback. The stimulus causes the optimum range of homeostasis to be disturbed and moved from the normal range. A stimulus can be anything that disrupts normal physiological processes. Examples of stimuli include physical injuries, infections, childbirth, or other major physiological changes.
Reception
Reception includes receptors, also called sensory units. The sensory units, in response, receive this stimulus and transmit signals to the control unit, which then processes this data. In the case of humans, the sensory units are the nerves and the control unit, as you guessed, is the brain. An example of reception is when the sensory units in the cervix sense the head of the fetus during childbirth and send signals to the brain to process this stimulus.
Processing
After the control unit receives the data, it processes it and if the stimulus is outside the normal range and is disrupting homeostasis, the control unit will display an output. In human bodies, the control centre is the pituitary gland, which is located near the brain. It may not be the main control unit, but it is responsible for the secretion of plenty of hormones like oxytocin, anti-diuretic hormone (ADH), etc. in response to the stimuli.
Further activation of stimuli
The information sent by the control unit via the sensory units to the location triggers an output in response to the stimulus. This output further increases the stimulus in the same direction. An example of this is blood clotting, which we will discuss shortly.
The activation of the stimulus is done by an effector, which can be any organ or cell. For instance, during childbirth, the effector is the uterus which stimulates uterine contraction in order to push the fetus out of the womb in a process called birth.
These 4 processes are also found in a negative feedback loop. The only difference is, while negative feedback seeks to bring the body back to homeostasis by hindering the process, positive feedback amplifies the output to further disrupt homeostasis. That said, positive feedback does not go on forever, as it is usually stopped by negative feedback once the result for positive feedback has been achieved. This way, positive feedback and negative feedback work hand-in-hand.
Positive feedback examples
Now, let's take a look at some important examples of positive feedback: blood clotting, fruit ripening, and childbirth. Let’s explore how positive feedback aids in these processes.
Blood clotting
Have you ever gotten injured and noticed that after a few days, there’s a brown scab covering the wound? This brown scab is the blood that was clotted to stop the bleeding.
In case we get injured and start losing blood, the damaged blood vessels release chemicals that attract clotting factors called blood platelets. These platelets cling onto the injured site and release chemicals that more platelets.
As more platelets are attracted to the injured site and start clotting blood, this process is amplified to increase the speed of blood clotting. Once the blood clot is big enough to cover the injury and stop the blood loss, this chemical stops getting released and positive feedback stops.
Haemophilia is an X-linked recessive disorder characterized by prolonged bleeding in the event of an injury. When we get injured, our bodies have certain proteins called clotting factors that clot the blood to prevent excessive blood loss.
A person with haemophilia does not have enough proteins to clot the blood. As a result, even a minor injury like a cut can prove fatal, as blood loss over a period of time can lead to several complications if proper medical assistance isn’t provided.
The gene responsible for clotting blood has information that produces proteins like factor VIII and factor IX. Hemophilic people have a mutated gene that produces too little of these factors and, as a result, they can't clot blood at the normal rate.
Fruit ripening
Fruits on a plant go through various stages – unripe, to ripe, to overripe – which are triggered by a chemical called ethylene (C2H4) and are further stimulated by positive feedback.
As the fruit will begin to ripen, it will release this gas. The nearby fruits on the branch get exposed to this gas and start to ripen, thereby triggering a chain reaction and releasing this gas. As a result, all the fruits ripen at a rapid rate. The fruit industry is known to utilize this feedback loop to accelerate the ripening of fruits by exposing them to ethylene gas.
Menstrual Cycle
Just before ovulation starts in women, a hormone called estrogen is released by the body which travels to the brain. The hypothalamus, in turn, releases gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) from the hypothalamus. LH stimulates the release of estrogen from the ovaries, which in turn, promotes the release of GnRH and LH. As these hormones increase in concentration, they cause ovulation to occur!
Lactation
When an infant suckles on the breasts of the mother, a chemical called prolactin gets released, which increases the production of milk. More suckling leads to the release of more prolactin which promotes more production of milk. When the child is no longer hungry and stops breastfeeding, prolactin stops getting released and as a result, milk production also stops.
Childbirth
This is perhaps the most common example of positive feedback. Positive feedback, after all, helps in labour and childbirth.
During childbirth, when the fetus’s head pushes against the walls of the cervix, the receptors in the uterus send signals to the hypothalamus. The hypothalamus processes the stimulus and releases oxytocin. The release of oxytocin induces labour and muscle contractions in the uterus. This keeps happening in a loop, with oxytocin being released and the cervix contracting until the original stimulus (fetus) gets delivered from the womb.
Positive feedback vs. negative feedback
You now have an understanding of what positive feedback is. But what about negative feedback? Is it the same as positive feedback? What is the difference between the two? Let's discuss the similarities and differences between the two.
Similarities between positive and negative feedback
There are some similarities between positive and negative feedback. These include:
Both feedback loops are based on a stimulus mechanism and either increase or decrease the effects of the stimulus.
Both of these systems play a crucial role in ensuring survival through homeostasis.
Differences between positive and negative feedback
There are many differences between positive and negative feedback, mainly how they affect the stimulus and whether they move away or towards homeostasis.
Direction of stimulus
Positive feedback maintains the direction of the stimulus and amplifies the output, whereas, negative feedback attempts to reverse the stimulus and decreases the output
Effect on homeostasis
Positive feedback disrupts homeostasis, as increasing the stimulus halts the normal physiological functions of the body. Negative feed, on the other hand, attempts to restore homeostasis by decreasing the stimulus.
Stability
Since positive feedback is known to disrupt the body’s homeostasis and increases the stimulus, it is not stable and may require an external interference to stop its mechanism. Negative feedback, in contrast, is a more stable mechanism as its main purpose is to restore the body’s homeostasis. It is also an independent mechanism and will stop once homeostasis is achieved.
The table below summarizes the key differences between positive and negative feedback.
Characteristics | Positive Feedback | Negative Feedback |
Output | Increases the output | Decreases the output |
Homeostasis Effect | Disrupts homeostasis even more | Attempts to restore homeostasis |
Stability | Less stable | More stable |
Occurrence | Less common | More common |
Physiological changes | Enhances physiological conditions | Resists physiological conditions |
Examples | Childbirth, lactation, fruit ripening, blood clotting | Body temperature regulation, maintenance of blood glucose levels |
Table 1. Summarizing differences between positive and negative feedback | StudySmarter original
Positive Feedback - Key takeaways
- Positive feedback enhances the output of a stimulus in a direction
- The purpose of a feedback loop is to ensure survival by bringing our bodies back to homeostasis
- Positive feedback has 4 steps, namely, stimulation, reception, processing, and further stimulation
- Positive feedback is less stable and disrupts homeostasis further while negative feedback restores homeostasis
- Childbirth, milk lactation, blood clotting, and fruit ripening are just a few examples of positive feedback
References
- Anupama Sapkota, Feedback Mechanism- Definition, Types, Process, Examples, Applications, The Biology Notes, 2021
- Positive Feedback, Biology Online, https://www.biologyonline.com/dictionary/positive-feedback
- Lakna, Difference Between Positive and Negative Feedback Loops in Biology, Podia, 2018
- Positive and Negative Feedback Loops in Biology, Albert.
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