A mechanism that affects the physiology of an organism by modifying a process or variable in the body of that organism and which increases its effect each time it detects a further change in the intensity of that process or variable. Positive feeback loops do have their place in a healthy organism, but are rare. They must also, as a necessity, be halted by some event outside the cause and effect of the loop itself.

Two examples will be given as positive feedback loops in the human organism, one as an example of a "good" effect, and one as an example of a "bad" effect.

The first is the "good" effect. Near the end of childbirth in a healthy female individual, the uterus is stretched to accomodate the baby's size. This stretching stimulates the uterine muscles to contract forcefully, pushing the baby further out of the uterus. Because of the stretching, the uterus is stimulated to contract even more forcefully, stretching the uterus even further, and so on. The halting mechanism is the complete exit of the baby from the uterus. Once the baby exits the mother, the uterus is no longer stretched, and stimulus for continuation of the feedback loop is lost.

The example of the "bad" effect considers blood loss, say from a gunshot wound. Once a large quantity of blood leaves the body, less blood is available for the heart to pump through the body. Thus, the heart gradually begins to lose its own blood supply. Because the heart does not have blood, the oxygen level in cardiac tissue decreases to a critical level, and the heart is no longer able to create the required force to pump blood to itself. So the heart receives even less blood. This results in further degeneration of its ability to pump blood due to oxygen depletion, and so even less blood is available, and so on until the patient dies, or experiences a rapid increase in blood pressure through a transfusion or other means.