Final answer:
PTH is secreted in response to low blood calcium to increase levels, while calcitonin is secreted in response to high blood calcium to reduce levels, together maintaining calcium balance.
Step-by-step explanation:
Low blood calcium levels stimulate the secretion of parathyroid hormone (PTH); high blood calcium levels stimulate the secretion of calcitonin. Calcitonin, on the other hand, decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and promoting calcium excretion by the kidneys. It works in opposition to parathyroid hormone to maintain homeostatic calcium levels in the body.
When blood calcium levels are low, the parathyroid glands secrete PTH to increase blood calcium. PTH stimulates osteoclasts to release calcium from bones, increases renal reabsorption of calcium, and promotes the production of calcitriol which aids in calcium absorption from the diet. Conversely, high levels of blood calcium trigger the thyroid gland's parafollicular cells to release calcitonin. This hormone lowers blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and encouraging the excretion of calcium by the kidneys. Together, PTH and calcitonin maintain calcium homeostasis in the body.