Positive feedback mechanism is a vicious cycle. In rare instances, the body has learned to use positive feedback to its advantage as follows:
1. Blood clotting: When a blood vessel is ruptured a clot begins to form. Multiple enzymes called clotting factors are activated with the clot itself. Some of these factors activates the other inactivated enzyme and causing still more clot.
2. child-birth : When uterine contractions become strong enough for the baby’s head to begin pushing through the cervix, stretch of the cervix sends signals back to the body of the uterus, causing more powerful contractions and parturition occurs.
3. Generation of nerve signals : When the membrane of a nerve fibre is stimulated, this causes slight leakage of sodium ions through sodium channels in the nerve membrane to the interior of the fibre. The sodium ions entering the fibre then change the membrane potential, which in turn causes mort opening of the channels, more change of potential, still more opening of channels and so forth. Thus creates the nerve action potential which excites the nerve fibre still further along its length and ultimately signals goes all the way to all ends of the nerve fiber.