Feedback

In negative feedback, a system's output reduces its own input, holding a variable near a set point. This is how thermostats work and how almost every homeostatic mechanism in the body works.

Classic examples: blood glucose (high glucose → insulin → uptake by cells → glucose drops; low glucose → glucagon → glucose released from liver), body temperature, blood pH, ATP inhibiting phosphofructokinase in glycolysis when ATP is plentiful.

Positive feedback amplifies the original signal, pushing a system away from its starting state toward a definite endpoint. These loops are rare because they're inherently unstable, but they're essential when a process needs to be driven all the way to completion.

Examples: oxytocin in childbirth (contractions stretch the cervix → more oxytocin → stronger contractions, until birth); blood clotting (each activated clotting factor activates more, until the clot forms); the action potential in neurons (Na⁺ entry depolarizes the membrane → more Na⁺ channels open → faster depolarization).

Disrupted feedback causes disease. In type 2 diabetes, cells stop responding to insulin (insulin resistance), so the negative feedback loop that lowers blood glucose fails and glucose stays dangerously high. Autoimmune disorders, hormone-secreting tumors, and many cancers can all be understood as broken feedback control.