Questions: How does PTH increase blood calcium levels? stimulates osteoblasts to release RANK, which stimulates osteoclasts to resorb bone stimulates osteocytes to release calcium from bone at the kidney, stimulates the production of vitamin D3 which allows calcium to be absorbed at the GI tract from the food that has been consumed at the kidney, stimulates the reabsorption of calcium

The answer is the fourth one: at the kidney, stimulates the reabsorption of calcium.

Explanation for each option:

1. Stimulates osteoblasts to release RANK, which stimulates osteoclasts to resorb bone:

   • This statement is partially correct. Parathyroid hormone (PTH) does indeed stimulate osteoblasts to express RANKL (Receptor Activator of Nuclear factor Kappa-Β Ligand), which in turn stimulates osteoclasts to resorb bone, releasing calcium into the bloodstream. However, this is not the primary mechanism by which PTH increases blood calcium levels.

2. Stimulates osteocytes to release calcium from bone:

   • This statement is incorrect. Osteocytes are involved in the maintenance of bone tissue, but they do not directly release calcium in response to PTH. The primary cells involved in bone resorption are osteoclasts.

3. At the kidney, stimulates the production of vitamin D3 which allows calcium to be absorbed at the GI tract from the food that has been consumed:

   • This statement is correct but incomplete. PTH does stimulate the conversion of vitamin D to its active form (calcitriol) in the kidneys, which enhances calcium absorption from the gastrointestinal tract. However, this is not the direct mechanism by which PTH increases blood calcium levels.

4. At the kidney, stimulates the reabsorption of calcium:

   • This statement is correct. PTH increases blood calcium levels by stimulating the reabsorption of calcium in the kidneys, reducing the amount of calcium excreted in the urine. This is a direct and significant mechanism by which PTH increases blood calcium levels.

Summary: PTH increases blood calcium levels primarily by stimulating the reabsorption of calcium in the kidneys, thereby reducing calcium excretion. This is a direct and effective mechanism for increasing blood calcium levels.