How does ADH contribute to homeostasis during dehydration?

A. Promotes calcium excretion and phosphate retention: Parathyroid hormone is designed to elevate serum calcium by increasing renal reabsorption. It simultaneously inhibits phosphate reabsorption in the proximal convoluted tubule. This choice describes the opposite of the hormone's actual physiological function in the kidneys.

B. Increases both calcium and phosphate in the blood: While PTH increases both ions during bone resorption, its renal effect is specifically phosphaturic. This ensures that the calcium released from bone does not immediately precipitate with phosphate in the tissues. The net result is a rise in calcium and a drop in phosphate.

C. Decreases calcium and increases phosphate levels: This description matches the state of hypoparathyroidism or the effects of calcitonin. PTH is the primary regulator for increasing calcium levels during hypocalcemia. Decreasing calcium would fail to maintain the necessary ionized calcium for neuromuscular function.

D. Increases calcium while promoting phosphate excretion: PTH enhances calcium reabsorption in the distal tubules and stimulates osteoclast activity. In the kidneys, it decreases the transport of phosphate into the blood. This dual action prevents the formation of calcium-phosphate salts while raising the free calcium concentration.

A. It is metabolized to produce ammonium and new bicarbonate ions: In the proximal tubule cells, glutamine deamination yields ammonium ions and alpha-ketoglutarate, which is further metabolized to bicarbonate. The ammonium is excreted to safely eliminate protons, while the "new" bicarbonate is reabsorbed. This renal response provides a long-term correction for systemic acidosis.

B. It is converted to glucose for energy: While the kidneys can perform gluconeogenesis during prolonged fasting, the specific metabolic pathway for glutamine during acidosis is prioritized for pH regulation. The carbon skeleton is used to generate bicarbonate rather than just providing cellular fuel. It serves a primary homeostatic function in acidemia.

C. It is excreted unchanged: Glutamine is a valuable nitrogen source and is generally reabsorbed by the renal tubules rather than wasted in the urine. During acid-base disturbances, its active metabolism is required to generate the buffer needed to neutralize excess acidity. Passive excretion would fail to support pH recovery.

D. It is used to produce sodium and chloride: Sodium and chloride are inorganic electrolytes that cannot be synthesized from organic amino acids like glutamine. While the metabolism of glutamine may involve sodium-coupled transporters, the amino acid itself is not a precursor for these ions. Its primary products are ammonium and bicarbonate.