Question

discuss how parathyroid hormone (PTH) maintains normal blood calcium ion concentration?

Answer 1

Parathyroid hormone regulates calcium levels in the blood, largely by increasing the levels when they are too low, Low calcium levels in the blood stimulate parathyroid hormone secretion, whereas high calcium levels in the blood prevent the release of parathyroid hormone. Parathyroid hormone maintains normal blood calcium ion concentration by its actions on the kidneys, bones and intestine:

When blood levels of calcium are low, PTH is released at the same time calcitonin secretion is inhibited, in the skeleton PTH stimulates osteoclast, which are the bone cells that cause bone to be reabsorbed, releasing calcium from bone into the blood, simaltenously it inhibits osteoblast cell production, these are cells which deposit bone, reducing calcium deposition in bone.

In kidneys, parathyroid hormone reduces loss of calcium in urine. In kidneys PTH blocks reabsorption of phosphate in the proximal tubule, while promoting calcium reabsorption in the asscending loop of henle, distal tubule and collecting duct.  Parathyroid hormone also stimulates the production of active vitamin D in the kidneys, which in turn increases calcium levels in blood.

In intestine parathyroid hormone indirectly increases calcium absorption from food in the intestine, PTH triggers the formation of calcitrol, an active form of Vitamin D, which acts on the intestine to increase absorption of dietary calcium.

If the blood concentration of calcium is high PTH release is inhibited, when calcium concentration is high parafollicular cells of thyroid increases calcitonin secretion in blood, the resulting high level of calcitonin in blood stimulats the bone to remove calcium from the blood plasma and deposit it as bone.