3.5.1 Feedback Mechanism

Many endocrine glands are controlled by feedback mechanisms. The two types of feedback mechanisms are negative feedback and positive feedback. Most endocrine glands are under the control of negative feedback mechanisms.
An example of negative feedback is the regulation of the blood calcium level. The parathyroid glands secrete parathyroid hormone, which regulates the blood calcium amount. If calcium decreases, the parathyroid glands sense the decrease and secrete more parathyroid hormone. The parathyroid hormone stimulates more blood calcium. Conversely, if blood calcium increases too much, the parathyroid glands reduce parathyroid hormone production. Both responses are examples of negative feedback because in both cases the effects are negative (opposite) to the stimulus.
An example of positive feedback can be found in childbirth. The hormone oxytocin stimulates and enhances labor contractions. As the baby moves toward the vagina (birth canal), pressure receptors within the cervix (muscular outlet of uterus) send messages to the brain to produce more oxytocin. Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract even more. The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin production stops and labor contractions cease.