Calcium levels in the body are maintained by hormones. if the blood calcium level rises above a set point, calcitonin is secreted by the thyroid gland and sets into motion responses to lower the blood calcium back to the set point. if the blood calcium level falls below a set point, the parathyroid gland secretes parathyroid hormone that sets into motion responses to raise the blood calcium. these homeostatic responses are examples of
a. positive feedback
b. effectors
c. sensors
d. negative feedback