Changes

==Serum Calcium Abnormalities==

==Serum Calcium Abnormalities==

'''Elevated''' blood calcium levels (hypercalcaemia) can be attributed to increased parathyroid hormone (PTH) concentration and increased active vitamin D3. '''Reduced''' blood calcium levels (hypocalcaemia) can occur with decreased PTH, reduced Vitamin D activation and calcitonin inhibition of calcium mobilisation from bone.

'''Elevated''' blood calcium levels ([[Hypercalcaemia|hypercalcaemia]]) can be attributed to increased parathyroid hormone (PTH) concentration and increased active vitamin D3. '''Reduced''' blood calcium levels ([[Hypocalcaemia|hypocalcaemia]]) can occur with decreased PTH, reduced Vitamin D activation and calcitonin inhibition of calcium mobilisation from bone.

==Calcium Homeostasis==

==Calcium Homeostasis==

===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). Regulation of PTH is highly sensitive due to membrane receptors on chief cells coupled to G-proteins. Receptor stimulation decreases secretion; this is therefore a direct [[Negative Feedback - Anatomy & Physiology|negative feedback]] mechanism. The half-life of PTH in circulation is short - less than 10 minutes which also allows tight regulation of calcium levels. PTH is metabolised in the liver and kidneys.

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''' ([[Hypocalcaemia|hypocalcemia]]). Regulation of PTH is highly sensitive due to membrane receptors on chief cells coupled to G-proteins. Receptor stimulation decreases secretion; this is therefore a direct [[Negative Feedback - Anatomy & Physiology|negative feedback]] mechanism. The half-life of PTH in circulation is short - less than 10 minutes which also allows tight regulation of calcium levels. PTH is metabolised in the liver and kidneys.

PTH leads to increased calcium levels in the blood by actions on bone. There are two phases;  

PTH leads to increased calcium levels in the blood by actions on bone. There are two phases;  

===Calcitonin===

===Calcitonin===

Calcitonin acts to decrease calcium levels in the plasma. It is overall a weaker regulatory mechanism than PTH. Secreted by the parafollicular cells of the thyroid gland, calcitonin is stimulated by hypercalcemia, and has the opposite effects of PTH on the bone:

Calcitonin acts to decrease calcium levels in the plasma. It is overall a weaker regulatory mechanism than PTH. Secreted by the parafollicular cells of the thyroid gland, calcitonin is stimulated by [[Hypercalcaemia|hypercalcemia]], and has the opposite effects of PTH on the bone:

1. Fast Phase - puts calcium into bone fluid by inhibiting osteoclasts' absorptive abilities.

1. Fast Phase - puts calcium into bone fluid by inhibiting osteoclasts' absorptive abilities.