Passage IV
In human blood, calcium (Ca2+) concentration is regulated by vitamin D3. First, vitamin D3 is converted to calcidiol (CD) in the liver. An elevated concentration of CD inhibits further conversion of vitamin D3. Next, CD is converted to calcitriol (CT) in the kidneys in a reaction that requires parathyroid hormone (PTH). An elevated concentration of CT increases Ca2+ concentration, which in turn inhibits further release of PTH. See Figure 1.
Figures 2 and 3, respectively, show how dietary consumption of vitamin D3 affects relative blood CD concentration and how blood Ca2+ concentration affects relative blood CT concentration.
Figures adapted from Hall, John E., Guyton and Hall Textbook of Medical Physiology, 12th ed. ©2011 by Saunders Elsevier.
23. According to Figure 1, what activates the conversion of CD to CT?
Your Answer is
Correct Answer is D
Explanation
Look at Figure 1, the dark black arrow represents activation. In the process from CD to CT, PTH plays the role of activation.