Question

blood levels of calcium
blood ca²⁺ deficiency stimulates ________ secretion, which leads to ________
calcitriol; more urinary phosphate excretion
thyroid hormone; less urinary calcium excretion
calcitonin; more urinary phosphate reabsorption
growth hormone; increased osteoblast activity
parathyroid hormone; increased osteoclast activity

Explanation:

When blood calcium ($Ca^{2+}$) levels are low, parathyroid hormone (PTH) is secreted by the parathyroid glands. PTH acts on bones, kidneys, and the intestine to raise blood - calcium levels. One of its actions on bones is to increase osteoclast activity, which breaks down bone tissue and releases calcium into the bloodstream.

Answer:

E. parathyroid hormone; increased osteoclast activity