The Body's Feedback Loops

Your blood pH, a measure of acidity, is normally kept within a narrow range, roughly 7.35 to 7.45. Chemical buffers that resist sudden changes in acidity, together with breathing and kidney function, help keep it there. Feedback loops use the same basic idea: detect a change, coordinate a response and adjust what happens next.

Many feedback systems can be simplified into three jobs: detecting a change, coordinating what to do and carrying out the response. These jobs are often described as the sensor , controller and effector . This model is a useful way to organise many physiological processes. However, roles may overlap, several loops may interact and some responses begin before a change occurs. The sensor A sensor detects a change in a regulated variable. The cell or structure doing the detecting is often called a receptor . Temperature sensitive receptors are found in the skin and brain, while specialised receptors in artery walls respond to stretching associated with blood pressure. Some hormone producing cells, called endocrine cells, also act as sensors. Pancreatic beta cells respond to rising blood glucose and release insulin, so sensing and responding partly occur within the same cells. The control centre A controller or integrating centre uses incoming information to coordinate a response. In some loops, this role lies in the brain. In others, endocrine cells or several connected tissues share the task. The controller does not consciously decide. It changes nerve or hormone signals according to the information it receives. The effector An effector carries out the response. It may be a sweat gland, blood vessel, muscle, target cell or hormone producing tissue. In negative feedback, the response opposes the original change. In positive feedback, it increases the change until another event or control mechanism limits the process. Negative feedback Negative feedback is the main pattern supporting homeostasis. The response opposes the original change. If body temperature rises, heat loss responses help lower it. If blood glucose rises after a meal, insulin helps muscle and fat cells use glucose and helps the liver store some of it. As the variable moves towards its working range, the signal driving the response usually becomes weaker. This reduces the chance of the response continuing beyond what is needed. Positive feedback Positive feedback amplifies the original change rather than opposing it. It is less common and usually supports a process that needs to progress rather than remain steady. During labour, stretching of the cervix promotes oxytocin release. Oxytocin strengthens uterine contractions, producing more cervical stretch. The cycle ends when birth removes the stretching stimulus. Blood clotting also includes amplifying steps. Activated platelets and clotting proteins promote further activation near the injury. Positive feedback is not self limiting by definition. Normal clotting is kept local by natural anti clotting processes and by mechanisms that break down fibrin, the protein mesh of a clot. These are examples of limiting mechanisms that restrain amplification.

Many feedback systems involve sensing, coordinating and responding. Negative feedback opposes change, while positive feedback amplifies change and therefore needs a stopping event or limiting control.