Question

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

Explanation:

When blood calcium ($Ca^{2+}$) is low, the parathyroid gland secretes parathyroid hormone. Parathyroid hormone stimulates osteoclasts to break down bone tissue, releasing calcium into the blood. It also promotes reabsorption of calcium in the kidneys and activation of vitamin D (calcitriol) which aids in calcium absorption from the intestines. Osteoclasts break down bone (resorption), while osteoblasts build bone. Calcitonin lowers blood - calcium levels. Growth hormone is mainly involved in growth and metabolism regulation, not immediate calcium - homeostasis in response to low calcium. Thyroid hormone is involved in metabolism and not the primary response to low calcium. Calcitriol does not directly cause more urinary phosphate excretion as its main role is calcium regulation related to absorption and reabsorption.

Answer:

C. parathyroid hormone; increased osteoclast activity