Feedback is defined as information obtained about a reaction to a product, which will allow modification of the product. Feedback loops are thus the process by which a change in the system results in an alarm that will trigger a certain outcome. This result will increase the change in the system or reduce it so that the system returns to normal. Some questions remain: How do these systems work? What is positive feedback? What is negative feedback? Where do we find these systems in nature?
Biological systems operate on a mechanism of inputs and outputs, each one caused by and provoking a certain event. A feedback loop is a biological occurrence in which the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback). Positive and Negative Feedback loops are important because they allow living organisms to maintain homeostasis. Homeostasis is the mechanism that allows us to keep our internal environment relatively constant: neither too hot nor too cold, neither too hungry nor too tired. The energy level an organism needs to maintain homeostasis depends on the type of organism as well as the environment it inhabits.
For example, a cold-blooded fish maintains its temperature at the same level as the water around it, so it does not need to monitor its internal temperature. Compare this to a warm-blooded whale in the same environment: it needs to keep its body temperature higher than that of the surrounding water, so it will spend more energy regulating temperature. This is a difference between ectotherms and endotherms: An ectotherm uses environmental temperature to control its internal temperature (for example, reptiles, amphibians, and fish), while an endotherm uses homeostasis to maintain its internal temperature. Endotherms can keep their metabolism at a constant rate, allowing for constant movement, reaction, and internal processes, while ectotherms cannot keep their metabolism at a constant rate. This means that their movement, reaction, and internal processes depend on adequate external heat, but it also means that they require less energy in the form of food, since their bodies are not constantly burning fuel.
Feedback loops can also occur to a greater extent: at the ecosystem level, a form of homeostasis is maintained. A good example of this is the cycling of predator and prey populations: a boom in the prey population will mean more food for predators, which will increase predator numbers. This will lead to excessive predation and the prey population will decline again. The predator population will decline in response, releasing pressure on the prey population and allowing it to recover.
Another example is what is known as the “evolutionary arms race,” in which a predator and its prey continually try to compete with each other. One such relationship is between nectar-eating birds and the flowers they feed on. Birds develop long beaks to access the nectar inside the flower. In response, the flower develops an increasingly long trumpet shape, in an attempt to prevent the bird from reaching the nectar. The bird responds by developing an even longer break. And so it goes on.
Positive feedback loops
A positive feedback loop occurs in nature when the product of a reaction leads to an increase in that reaction. If we observe a system in homeostasis, a positive feedback loop moves a system further away from the equilibrium goal. It does this by amplifying the effects of a product or event and occurs when something needs to happen quickly.
Example 1: Fruit ripening
There is an amazing effect in nature where a tree or bush suddenly ripens all its fruits or vegetables, without any visible sign. This is our first example of a positive biological feedback loop. If we look at an apple tree, with many apples, seemingly overnight they all go from unripe to ripe to overripe. This will start with the first apple to ripen. Once mature, it emits a gas known as ethylene (C2H4) through its skin. When exposed to this gas, nearby apples also ripen. Once ripe, they also produce ethylene, which continues to ripen the rest of the tree with an effect much like a wave. This feedback loop is often used in fruit production, and apples are exposed to manufactured ethylene gas to make them ripen faster.
Example 2: Delivery
When labour begins, the baby’s head is pushed down and pressure on the cervix increases. This stimulates the receptor cells to send a chemical signal to the brain, allowing the release of oxytocin. This oxytocin diffuses to the cervix through the blood, where it stimulates more contractions. These contractions stimulate the further release of oxytocin until the baby is born.
Example 3: Blood coagulation
When tissue is torn or injured, a chemical is released. This chemical causes platelets in the blood to activate. Once these platelets have been activated, they release a chemical that signals more platelets to be activated, until the wound coagulates.
Negative feedback loops
A negative feedback loop occurs in biology when the product of a reaction leads to a decrease in that reaction. In this way, a negative feedback loop brings the system closer to a goal of stability or homeostasis. Negative feedback loops are responsible for the stabilization of a system and ensure the maintenance of a constant and stable state. The response of the regulatory mechanism is opposite to the output of the event.
Example 1: Temperature regulation
Temperature regulation in humans occurs constantly. The normal temperature of the human body is approximately 98.6°F. When body temperature rises above this, two mechanisms activate the body: it begins to sweat and vasodilation occurs to allow more of the blood surface to be exposed to the cooler external environment. As sweat cools, it causes evaporative cooling, while blood vessels cause convective cooling. Normal temperature is restored. If these cooling mechanisms continue, the body will cool down.
The mechanisms that then come into action are the formation of goosebumps and vasoconstriction. Goosebumps in other mammals raise the hair or fur, allowing more heat to be retained. In humans, they tighten the surrounding skin, reducing (slightly) the surface area through which they lose heat. Vasoconstriction ensures that only a small surface area of the veins is exposed to the cooler outside temperature, retaining heat. Normal temperature is restored.
Example 2: Regulation of Blood Pressure (Baroreflex)
Blood pressure must remain high enough to pump blood to all parts of the body, but not so high that it causes harm in doing so. As the heart pumps, the baroreceptors sense the pressure of the blood passing through the arteries. If the pressure is too high or too low, a chemical signal is sent to the brain via the glossopharyngeal nerve. The brain then sends a chemical signal to the heart to adjust the rate of pumping: if blood pressure is low, the heart rate increases, while if blood pressure is high, the heart rate decreases.
Example 3: Osmoregulation
Osmoregulation refers to the control of the concentration of various fluids within the body, to maintain homeostasis. We will look again at an example of a fish that lives in the ocean. The concentration of salt in the water surrounding the fish is much higher than that of the liquid in the fish. This water enters the fish diffusion through the gills, the consumption of food and drink. Also, because the salt concentration is higher outside than inside the fish, there is a passive diffusion of salt into the fish and water out of the fish. The salt concentration is then too high in the fish and the salt ions must be released through excretion. This occurs through the skin and is highly concentrated urine. Also, high levels of salt in the blood are removed by active transport by chloride-secreting cells in the gills. This maintains the correct salt concentration.
Positive Versus Negative Feedback
The key difference between positive and negative feedback is your response to change: positive feedback amplifies change, while negative feedback reduces change. This means that positive feedback will result in more products: more apples, more contractions, or more clotted platelets. Negative feedback will result in less product: less heat, less pressure, or less salt. Positive feedback moves away from a target point while negative feedback moves toward a target.
Why is feedback important?
Without feedback, homeostasis cannot occur. This means that an organism loses the ability to self-regulate its body. Negative feedback mechanisms are more common in homeostasis, but positive feedback loops are also important. Changes in feedback loops can lead to various problems, including diabetes mellitus.
In type 1 diabetes, the beta cells do not function. This means that when blood glucose levels rise, insulin production does not kick in and therefore blood glucose levels continue to rise. This can result in symptoms such as blurred vision, weight loss, hyperventilation, nausea and vomiting, among others. In type 2 diabetes, chronically high blood glucose levels have occurred as a result of poor diet and lack of exercise. This results in the cells no longer recognizing insulin, so blood glucose levels continue to rise.
Closure of positive and negative feedback loops
Feedback loops are biological mechanisms by which homeostasis is maintained. This occurs when the product or output of an event or reaction changes the body’s response to that reaction. Positive feedback occurs to increase change or output: the result of a reaction is amplified so that it occurs more quickly. Negative feedback occurs to reduce change or output: the result of a reaction is reduced to bring the system back to a stable state. Some examples of positive feedback are labour contractions and fruit ripening; examples of negative feedback include regulation of blood glucose levels and osmoregulation.