Negative feedback is a crucial feature of most homeostatic regulatory systems within the body. While some systems utilise positive feedback, these are generally the exception rather than the rule. These feedback loops are essential mechanisms in homeostasis to maintain the body's internal environment.
Characteristics of Negative Feedback
Negative feedback occurs when there is a deviation from a variable or system's basal level in either direction. In response, the feedback loop returns the factor within the body to its baseline state. A departure from the baseline value results in the activation of a system to restore the baseline state. As the system moves back toward the baseline, the system is less activated, enabling stabilisation once again.
The baseline state or basal level refers to a system's 'normal' value. For example, the baseline blood glucose concentration for non-diabetic individuals is 72-140 mg/dl.
Negative Feedback Examples
Negative feedback is a crucial component in the regulation of several systems, including:
- Temperature regulation
- Blood Pressure Regulation
- Blood Glucose Regulation
- Osmolarity Regulation
- Hormone Release
Positive Feedback Examples
On the other hand, positive feedback is the opposite of negative feedback. Instead of the system's output causing the system to be down-regulated, it causes the system's output to be increased. This effectively amplifies the response to a stimulus. Positive feedback enforces a departure from a baseline instead of restoring the baseline.
Some examples of systems that use positive feedback loops include:
- Nerve Signals
- Ovulation
- Birthing
- Blood Clotting
- Genetic Regulation
The Biology Of Negative Feedback
Negative feedback systems generally contain four essential parts:
- Stimulus
- Sensor
- Controller
- Effector
The stimulus is the trigger for the activation of the system. The sensor then identifies changes, which reports these changes back to the controller. The controller compares this to a set point and, if the difference is sufficient, activates an effector, which brings about changes in the stimulus.
Fig. 1 - The different components in a negative feedback loop
Negative Feedback Loops and Blood Glucose Concentration
Blood glucose is regulated by the production of the hormones insulin and glucagon. Insulin lowers blood glucose levels while glucagon raises it. These are both negative feedback loops that work in concert to maintain a baseline blood glucose concentration.
When an individual consumes a meal and their blood glucose concentration increases, the stimulus, in this case, is the increase in blood glucose above the baseline level. The sensor in the system is the beta cells within the pancreas, thereby enabling glucose to enter the beta cells and triggering a host of signalling cascades. At sufficient glucose levels, this makes the controller, also the beta cells, release insulin, the effector, into the blood. Insulin secretion lowers blood glucose concentration, thereby down-regulating the insulin release system.
Glucose enters beta cells through GLUT 2 membrane transporters by facilitated diffusion!
The glucagon system works similarly to the insulin negative feedback loop, except to raise blood glucose levels. When there is a decrease in blood glucose concentration, the alpha cells of the pancreas, which are the sensors and controllers, will secrete glucagon into the blood, effectively raising the blood glucose concentration. Glucagon does this by promoting the breakdown of glycogen, which is an insoluble form of glucose, back into soluble glucose.
Glycogen refers to insoluble polymers of glucose molecules. When glucose is in excess, insulin helps create glycogen, but glucagon breaks down glycogen when glucose is scarce.
Fig. 2 - The negative feedback loop in the control of blood glucose levels
Negative Feedback Loops And Thermoregulation
Temperature control within the body, otherwise referred to as thermoregulation, is another classic example of a negative feedback loop. When the stimulus, temperature, increases above the ideal baseline of around 37°C, this is detected by the temperature receptors, the sensors, located throughout the body.
The hypothalamus in the brain acts as the controller and responds to this elevated temperature by activating the effectors, which are, in this case, sweat glands and blood vessels. A series of nerve impulses sent to the sweat glands trigger the release of sweat which, when evaporated, takes heat energy from the body. The nerve impulses also trigger vasodilation in peripheral blood vessels, increasing blood flow to the surface of the body. These cooling mechanisms help to return the body's internal temperature back to baseline.
When the body's temperature drops, a similar negative feedback system is used to raise the temperature back to the ideal baseline of 37°C. The hypothalamus responds to the lowered body temperature, and sends out nerve impulses to trigger shivering. Skeletal muscle act as the effectors and this shivering generates more body heat, aiding to restore the ideal baseline. This is aided by the vasoconstriction of peripheral blood vessels, limiting surface heat loss.
Vasodilation describes the increase in blood vessel diameter. Vasoconstriction refers to the narrowing of the blood vessel diameter.
Fig. 3 - The negative feedback loop in thermoregulation
Negative Feedback Loops and Blood Pressure Control
Blood pressure is another factor variable that is maintained by negative feedback loops. This control system is only responsible for short-term changes in blood pressure, with long-term variations being controlled by other systems.
Changes in blood pressure act as the stimulus and the sensors are pressure receptors located within blood vessel walls, mainly of the aorta and carotid. These receptors send signals to the nervous system which act as the controller. The effectors include the heart and blood vessels.
Increases in blood pressure stretch the walls of the aorta and carotid. This activates the pressure receptors, which then send signals to the effector organs. In response, the heart rate decreases and blood vessels undergo vasodilation. Combined, this lowers blood pressure.
On the flip side, decreases in blood pressure have the opposite effect. The decrease is still detected by pressure receptors but instead of the blood vessels being stretched further than normal, they are less stretched than normal. This triggers an increase in heart rate and vasoconstriction, which work to increase the blood pressure back to baseline.
The pressure receptors found in the aorta and carotid are commonly referred to as baroreceptors. This feedback system is known as the baroreceptor reflex, and it is a prime example of the unconscious regulation of the autonomic nervous system.
Negative Feedback - Key takeaways
- Negative feedback occurs when there is a deviation in a system's baseline and in response, the body acts to reverse these changes.
- Positive feedback is a different homeostatic mechanism which acts to amplify changes of a system.
- In the negative feedback loop of blood glucose concentration, the hormones insulin and glucagon are key components of regulation.
- In thermoregulation, negative feedback enables regulation via mechanisms such as vasodilation, vasoconstriction and shivering.
- In blood pressure control, negative feedback changes the heart rate and triggers vasodilation/vasoconstriction for regulation.
Explanations 
Exams 
Magazine 
