Positive feedback

Positive feedback is the opposite of the stabilizing negative feedback that maintains homeostasis. Instead of reversing a change, the response strengthens it, so each turn of the loop pushes the system further from where it started. Because such a loop would otherwise run away, it is useful in the body only when something outside the loop, usually the completion of an event, brings it to a stop.

Childbirth is the textbook example. As the baby's head presses on the cervix, stretch receptors send signals to the brain, and the posterior pituitary releases oxytocin. Oxytocin makes the uterus contract harder, which stretches the cervix further and releases still more oxytocin, until delivery ends the stretching. Blood clotting works in a similar way: each activated clotting factor activates more factors, and platelets stuck to a damaged vessel release chemicals that draw in more platelets, so a plug forms quickly.

Another example is the surge of luteinizing hormone before ovulation, set off when high estrogen from a maturing follicle switches from inhibiting the pituitary to stimulating it. Positive feedback can also be harmful, as when falling blood pressure in severe shock reduces blood flow to the heart muscle and weakens the heart further.