Final answer:
Option (2), Parathyroid hormone (PTH) increases blood calcium levels by enhancing calcium absorption in the intestines and kidneys, and mobilizing calcium from bones, while it decreases blood phosphate levels by increasing phosphate excretion in the kidneys.
Step-by-step explanation:
How Parathyroid Hormone Regulates Blood Calcium and Phosphate Levels
Parathyroid hormone (PTH) acts to increase serum calcium (Ca2+) and decrease serum phosphate by targeting the skeleton, the kidneys, and the intestine. The correct answer to the question is: PTH increases Ca2+ absorption in the kidneys, thereby lowering the excretion of calcium, and it also increases the excretion of phosphate in the kidneys, lowering its serum concentration. This is reflected in Option 1 which states that PTH increases Ca absorption in the intestines and decreases phosphate excretion in the kidneys.
PTH performs several functions, including:
- Mobilization of Ca2+ from the bones, by stimulating osteoclasts that cause bone resorption.
- Reduction in the excretion of calcium by the kidneys, promoting renal reabsorption of Ca2+.
- Increases the gastrointestinal absorption of dietary calcium by facilitating the conversion of vitamin D to calcitriol, which then increases calcium absorption in the intestines.
- Increases loss of phosphate through the kidneys, which prevents the formation of calcium phosphate that could reduce circulating Ca2+ levels.
The release of PTH is a response to low blood levels of Ca2+, and its secretion is inhibited when blood calcium levels rise.