Exchange of calcium between bone and extracellular fluid takes place under the influence of certain hormones.
$(a)$ What will happen if there is an excess of $Ca^{++}$ in the extracellular fluid?
$(b)$ What will happen if there is a very low amount of $Ca^{++}$ in the extracellular fluid?

(N/A) An excess level of $Ca^{++}$ in the extracellular fluid is often associated with hyperparathyroidism. This leads to excessive demineralization of bones. The bones become soft and brittle,a condition that can lead to osteoporosis.
$(b)$ $A$ low level of $Ca^{++}$ in the extracellular fluid is associated with hypoparathyroidism. This condition increases the excitability of muscles and nerves. As a result,it causes tetany,characterized by sustained spasms or contractions of the muscles in the larynx,face,and limbs.