When blood calcium levels are low, PTH stimulates:

excretion of calcium from the kidneys.

excretion of calcium from the intestines.

osteoblasts.

osteoclasts.

osteoblasts.

Explanation:

PTH is produced to prevent hypocalcemia, ie low blood calcium concentrations, which can lead to seizures and heart failure in more severe cases and when untreated. In bone, PTH stimulates and determines the activation and recruitment of osteoclasts (cells that participate in the process of absorption and remodeling of the bone matrix), which results in bone resorption. However, mature osteoclasts do not have PTH receptors, unlike their bone marrow progenitors, known as monoblasts, which undergo the action of PTH leading to stimulation and differentiation in mature osteoclasts.