The Body's Feedback Loops

Homeostasis depends on systems that detect change and produce a response. Many of these systems can be understood through three functions: sensing, coordinating and responding. These roles form a useful first model, although they are not always carried out by separate structures.

Many feedback systems can be simplified into three functions: a sensor detects a change, a controller coordinates the response and an effector carries it out. The functions may overlap. The same cell, tissue or organ can sometimes perform more than one role. The sensor A sensor, sometimes called a receptor, detects a change in a regulated variable. Temperature receptors are found in the skin and brain. Specialised receptors in artery walls respond to stretching associated with blood pressure. Some 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 and balanced by anticoagulant and clot breakdown mechanisms. Why the pattern matters The sensor, controller and effector model is a useful way to organise many physiological processes. It helps you ask what was detected, what signal was produced and what response followed. It is not a complete map of every regulatory system. Roles may overlap, several loops may interact and some responses begin before a change occurs.

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.