Answer
The target tissues of parathormone (PTH) are bones and kidneys. The target tissue of calcitonin is bone. The target of calcitriol is the gastrointestinal tract (GI tract).
Work Step by Step
PTH action on the extracellular matrix of bones results in bone matrix resorption; this gives rise to the release Ca++ HPO4-- into the blood. PTH also targets the kidneys; the effects of PTH action on the kidneys include slowing of urinary excretion of MG++ and Ca++ as well as the increase of loss of HPO4-- from blood to urine. In addition, PTH action on kidneys also promotes the formation of calciferol , the active form of Vitamin D.
Calcitonin is the hormone formed by the parafollicular cells of the thyroid. It acts on bones to inhibit the resorption of extracellular matrix by osteoclasts. The result of this activity is to increase the uptake of Ca++ and phosphates into the bone extracellular matrix.
Calcitriol is a hormone which is the active form of Vitamin D. It is produced by kidneys under the stimulation of PTH. The main target of calcitriol is the gastrointestinal tract . Its effect is to increase the absorption of Ca++, HPO4--, and Mg++ from food by the digestive system.
