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→Calcium Homeostasis
#*The kidneys activate Vitamin D3 to create '''Active Vitamin D3''' also known as '''Calcitriol''' to '''INCREASE''' calcium levels in the blood.
#*The kidneys activate Vitamin D3 to create '''Active Vitamin D3''' also known as '''Calcitriol''' to '''INCREASE''' calcium levels in the blood.
#*C-Cells of the thyroid gland secrete '''Calcitonin''' which '''DECREASES''' calcium levels in the blood.
#*C-Cells of the thyroid gland secrete '''Calcitonin''' which '''DECREASES''' calcium levels in the blood.
===Parathyroid Hormone (PTH)===
===Parathyroid Hormone (PTH)===
Synthesis of PTH is from a preprohormone of 115 amino acids into a [[Prohormones - Anatomy & Physiology|prohormone]] of 90 amino acids. This prohormone is then packaged into vesicles as the 84 amino acid PTH molecule. It is secreted by the chief cells of the parathyroid gland continuously with a basal secretory rate of around 25% of the maximum possible rate. Secretion rate increases with a '''decrease in serum ionised calcium''' (hypocalcemia). This is a direct [[Negative Feedback - Anatomy & Physiology|negative feedback]] mechanism.
** This is a highly sensitive system due to membrane receptors on Chief Cells coupled to G-Proteins. Receptor stimulated decreases secretion.
** This is a highly sensitive system due to membrane receptors on Chief Cells coupled to G-Proteins. Receptor stimulated decreases secretion.
* The half-life of PTH in circulation is short - less than 10 minutes which allows tight regulation of calcium levels.
* The half-life of PTH in circulation is short - less than 10 minutes which allows tight regulation of calcium levels.