The Endocrine System - Chapter questions - Problem 16CP

PTH regulates blood levels of Caa++, Mg++ , a HPOR--. If serum levels of Ca++ falls this triggers increases activity of the principal cells of the parathyroids and thi increases the serum level of paratharmone. An increase in the amount of parathormone results in an increase in bone extracellular matrix resorption by the increase in number of osteoclasts produced by PTH stimulation. In crease in serum level of parathormone has the following effects: Increase in serum levels of Ca++, Mg++, an HPOR--. Decrease in rate of loss of Ca++, and Mg++ lost from urine Increase in the rate of HPO4-- lost from urine These combined actions of PTH result in a increase in levels of Ca++, and Mg++ but a los in plasma HPO$--. Parathormone also stimulates the kidneys to secrete and release calcitriol. The effect of calcitriol is to increase the absorption of Ca++, Mg++ and HPO4-- from the GI tract Calcitonin and parathormone are antagonists in the control of serum levels of Ca++. The release of parathormone is regulated directly by plasma levels of Calcium through negative feedback loops that exclude the anterior pituitary. If blood calcium levels get too high , parafollicular cells of the thyroid release Calcitonin which act to decrease the number of osteoclasts , the rate of bone matrix resorption and lower the blood level of calcium. If , on the other hand blood calcium levels get too low, Parathormone secretion rises ; this increases osteoclast activity , bone resorption and the level of blood level of Mg++ and Ca++ increase. Urinary loses of Ca++ and Mg++ are also lowered by parathormone action.