Answer
In the human body, a change in blood pressure or body temperature can initiate a negative feedback response. Some changes that can start a positive feedback response are stretching of the uterus at the beginning of labor, and suckling of mother's breast by a baby
If a person loses a significant quantity of blood from a wound, the blood pressure ( controlled condition) against the blood vessel walls will decrease, and the baroreceptors (pressure sensors) in the blood vessel walls will decrease their rate of firing. This will generate a neural signal which will go to the medulla oblongata of the brain ( control center) by an afferent pathway to alert the brain that a change in blood pressure has taken place. The control center ( brain) will send out an efferent signal to the heart and blood vessels (effectors) by an efferent nerve pathway. This signal will cause the pacemaker to increase the heart rate, and cause the muscles in the blood vessels to contract and constrict the blood vessels. The end result of these two effector responses is to raise the blood pressure to levels within the normal range.
An example of positive feedback mechanism is encountered in the milk ejection response of a mother's mammary glands when a baby starts to suckle. The initiating stimulus in this response is the suckling action of the baby. This sends a neural signal to the hypothalamus by an afferent pathway. The hypothalamus sends a stimulus to the pituitary to release oxytocin into the bloodstream. The hormone oxytocin travels by blood vessels to the mammary glands where it causes milk to be let down, or ejected, as the baby suckles. As the baby continues to suckle, the hypothalamus and pituitary increase the combined neuro-hormonal positive feedback stimulus and the mammary glands (effector) continue to increase milk release. The positive feedback will only be terminated by an external event, in this case, the cessation of suckling by the baby.
Work Step by Step
In the human body changes in body temperature or changes in blood pressure can initiate a negative feedback response.
In the beginning of labor in humans the head of the baby presses against the walls of the cervix and stretches it . Stretch receptors in the walls of the cervix send impulses to the brain to alert it that the controlled condition has changed. The brain sends impulses to the posterior pituitary which releases oxytocin which causes the cervix to stetch and uterine muscles to contract . These response of the cervix sends furter messeages to the hypothalamus which stimulatesthe pituitary to rlease moire oxytocin which induces more stretching of the uterine cervix and stronger contraction of uterine muscles. This strengthening or increasing of a change in, or departure from homeostasis is an example of a positive feed back response which can only teterminated by an external condition , in this case the birth of the baby and the cessation of stretching of the cervix.
