Explain the role of oxytocin in childbirth. What sort of feedback system is operating in this example and why?

Oxytocin is the hormone that increases the force of contraction of the uterine muscles. Oxytocin is acting as the efferent pathway in this homeostatic mechanism. The controlled variable is the diameter of the cervix. As labour begins the head of the baby is pushed into the cervix and begins to force it open (it becomes wider or dilated). This triggers the release of oxytocin, and the oxytocin increases the force and frequency of the contractions. As a result the baby's head is pushed further into the cervix and the cervix becomes more dilated. The initial change that the homeostatic mechanism is responding to was dilation of the cervix. In other words the controlled variable is being moved away from its set point. The homeostatic mechanism is implementing changes that result in the controlled variable moving even further away from its set point, so this is an example of positive feedback.