Which of the following best describes the overall effect of PTH on calcium and phosphate balance?

A. Promote calcium excretion by the kidneys: While calcitonin can have a mild phosphaturic effect, its primary physiological target for blood calcium reduction is the bone. Renal excretion of calcium is more significantly regulated by parathyroid hormone levels. Calcitonin primarily functions to inhibit osteoclast-mediated bone breakdown.

B. Enhance calcium absorption from the gut: Increased intestinal absorption would elevate serum calcium levels, contradicting the goal of calcitonin during hypercalcemia. Calcitriol is the hormone responsible for upregulating calcium-binding proteins in the enterocytes. Calcitonin does not stimulate the synthesis of active Vitamin D.

C. Encourage calcium deposition into bone: This hormone, secreted by parafollicular C cells of the thyroid, inhibits osteoclast activity and stimulates osteoblastic mineral deposition. It effectively shifts ionized calcium from the plasma into the hydroxyapatite matrix. This decreases the circulating concentration of calcium.

D. Stimulate bone resorption: Bone resorption releases calcium and phosphate into the bloodstream, which would worsen a state of hypercalcemia. Parathyroid hormone is the agent that triggers this process to raise blood calcium. Calcitonin acts as a functional antagonist by suppressing bone mineral release.

A. equation shifts left, respiratory acidosis, hypoventilation: Respiratory acidosis is caused by the retention of carbon dioxide, not the loss of gastric hydrochloric acid. A leftward shift would occur to consume excess hydrogen ions, which contradicts the loss of acid. Hypoventilation is a response to alkalosis.

B. equation shifts right, metabolic alkalosis, hypoventilation: Loss of gastric protons causes a rise in systemic pH, characterizing metabolic alkalosis. To replace the lost hydrogen ions, the carbonic acid-bicarbonate equation shifts right. The lungs then slow the breathing rate to retain carbon dioxide and lower pH.

C. equation shifts right, metabolic acidosis, hyperventilation: Metabolic acidosis involves an excess of hydrogen ions, which is the opposite of what occurs during vomiting. Hyperventilation is the compensation for acidemia, intended to blow off carbon dioxide. Vomiting specifically depletes acids, leading to an alkaline state.

D. equation shifts left, metabolic alkalosis, hyperventilation: While metabolic alkalosis is the correct diagnosis, a leftward shift would only occur if there were an excess of hydrogen and bicarbonate. Furthermore, hyperventilation would worsen alkalosis by removing more acid. The body compensates by slowing respiration to retain acid.