In humans, dietary calcium helps maintain strong bones and teeth. Calcium is absorbed into the blood stream by the cells that line the small intestine. Bone cells called osteoblasts absorb calcium from the blood and use it to form the mineralized part of bone called bone matrix. Osteoporosis is a condition in which bones become thin and brittle because of a decrease in the density of bone matrix. Explain how radioisotopes could be used to determine whether intestinal cells or osteoblasts lack the ability to process calcium in a person with osteoporosis.