When blood calcium levels rise, calcitonin is released to:

A. Protein concentration increases in the blood plasma: Hydrostatic pressure drives the filtration of water and small solutes, leaving large plasma proteins behind. While the relative concentration of proteins may rise slightly as fluid leaves, this is a result of the filtration process. The primary effect is volume movement.

B. Fluid and nutrients are filtered out into surrounding tissues: High hydrostatic pressure at the arterial end of a capillary overcomes the inward pull of oncotic pressure. This net filtration pressure forces water and dissolved nutrients into the interstitial space. This process is essential for delivering oxygen and glucose to cells.

C. Fluid is reabsorbed into the capillaries: Reabsorption occurs when the colloid osmotic pressure, exerted by plasma proteins, is greater than the capillary hydrostatic pressure. This typically happens at the venous end of the capillary bed. It allows metabolic wastes to enter the bloodstream for excretion.

D. Fluid movement halts between compartments: Fluid movement only halts when hydrostatic and osmotic pressures reach an equilibrium point where net filtration is zero. In a functioning circulatory system, these pressures are dynamic to ensure continuous exchange. Constant movement is required for systemic nutrient delivery.

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.