Final answer:
The correct response to the relationship between blood calcium levels and parathyroid hormone (PTH) secretion is 'Low calcium = high PTH; High calcium = low PTH.' This is based on the role of PTH in regulating calcium homeostasis by increasing calcium levels when they are low and decreasing PTH secretion when calcium levels are high.
Step-by-step explanation:
The correct answer to the question Low calcium = ______ PTH; High calcium = ______ PTH is b) Low calcium = high PTH; High calcium = low PTH. This is because the parathyroid hormone (PTH) plays a critical role in managing calcium levels in the blood. When calcium levels are low, PTH is secreted by the parathyroid glands to increase blood calcium levels. It does so by stimulating the release of calcium from the bones, increasing reabsorption of calcium in the kidneys, and upregulating the production of calcitriol, which promotes calcium absorption in the gut. Conversely, when calcium levels are high, the secretion of PTH is decreased, which results in maintaining calcium homeostasis.
Furthermore, PTH has an inverse relationship with phosphate levels as it increases the loss of phosphate through the kidneys. This is to prevent the formation of calcium phosphate which would lower circulating calcium levels. Therefore, when there are low calcium levels, there is high PTH, and when there are high calcium levels, there is low PTH.