Final answer:
The process that will NOT occur when blood calcium levels are low is calcitriol increasing calcium uptake from urine, as calcitriol facilitates calcium absorption from the intestines, not the urine.
Step-by-step explanation:
When blood calcium levels are low, calcitriol increases absorption of calcium from the intestines will occur. Therefore, the correct answer to which of the following will NOT occur is that calcitriol increases calcium uptake from urine. This statement is inaccurate because calcitriol's role is to aid in the absorption of calcium in the intestines but it does not lead to calcium uptake from urine. Instead, parathyroid hormone (PTH) increases reabsorption of calcium in the kidneys, preventing it from being excreted in the urine.
PTH is secreted by the parathyroid glands and plays a crucial role in regulating blood calcium levels. When there is a drop in blood calcium levels, PTH acts on bones to increase osteoclast activity, which releases calcium into the bloodstream. PTH also promotes the conversion of vitamin D into its active form, calcitriol, in the kidneys, which then enhances calcium absorption from the digestive tract.