Positive and Negative Feedback Loops

Homeostatic control systems, such as temperature regulation, operate through feedback response loops. These loops start with a stimulus that changes a variable and end with an effector that adjusts the variable. If the change in the variable brings it back toward a set point, this is known as a negative feedback loop. The term “negative” is used because the resulting change in the variable is opposite to the initial change. For example, if a stimulus raises body temperature to 99°F, sweating acts to lower it back to 98.6°F. Since the initial stimulus caused an increase and the response resulted in a decrease, this is a negative feedback loop. This process helps maintain the constancy of the internal environment. Other examples of negative feedback loops include the replenishment of oxygen by the lungs, the regulation of blood pH at 7.4, and blood glucose control by insulin. These mechanisms are essential to maintaining homeostasis.

Positive feedback loops, on the other hand, occur when the response to a stimulus increases the deviation from the set point. This often happens when a negative feedback system fails to adequately address a problem because one or more effectors is broken or unable to function effectively. For example, in response to a significant loss of blood, the body's negative feedback response would increase heart rate to restore blood pressure. However, if the blood loss is too severe, the increased heart rate may not be enough to raise blood pressure. As a result, less blood reaches the heart, depriving it of oxygen and nutrients. This weakens the heart's ability to pump blood, creating a downward spiral where the heart receives even less blood, leading to worsening conditions. In this case, the body has shifted into a positive feedback loop, where the deviation from normal blood pressure continues to increase. Without medical intervention, this can lead to critical failure.

Positive feedback can also have beneficial roles in specific situations. For example, during childbirth, labor contractions are enhanced through positive feedback. The hormone oxytocin, released from the brain, travels through the bloodstream to the uterus, causing stronger contractions. These contractions push the baby’s head downward, stretching the cervix. Stretch receptors in the cervix send signals to the brain to release more oxytocin, intensifying contractions until the baby is born. A synthetic form of oxytocin, known as Pitocin, can be administered to induce or assist labor when the natural system is insufficient.

In addition to feedback loops, some systems utilize feedforward control mechanisms that anticipate changes. For instance, humans in warm environments with low sweat rates can be induced to sweat almost immediately after drinking water. Similarly, the sight, smell, or even thought of food can trigger salivation and stomach acid secretion before food is consumed, a common example of feedforward control. Another interesting example involves the consumption of diet soda. Although diet soda does not contain "sugar" the sweetness comes from chemicals that mimic the sugar taste. Thus, in response, the digestive system "thinks" it is getting a big load of sugar so it adapts by adding more structures that help to absorb sugar. The result of this feed forward mechanism is that any sugar consumed after a diet soda is absorbed much quicker than it would have been with out the diet soda priming.

Feedback loops rarely operate in isolation; instead, they are part of complex networks that interact with each other. Some loops may compete, making treatment more challenging. A hierarchy also exists among feedback systems, with brain function being prioritized over other systems. For example, the body will sacrifice bone calcium to maintain proper brain function.

In medicine, the goal is to help individuals return to homeostasis when their own systems become inadequate. Medical interventions use physiological parameters as reference points and aim to restore balance when deviations occur.

Above is an image representation of a Feedback Response Loop. Notice that feedback loops can result in Negative or Positive Feedback. The red arrows in the top left graph show what would happen if the effector(s) caused the variable to come back to set point (Negative Feedback). The red arrow in the right-hand graph (inside the cycle) shows what would happen if the effector(s) caused the variable to go further and further from the set point (Positive Feedback).