Negative Feedback

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.

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.

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.

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.