( Source, Release & Function )

1.structure of bone & teeth 6. hormone secretion 5. hormonal actions 2.neurotransmission 4. muscle contraction 3. blood clotting

Total Body Calcium 1100 g 99% Bone and teeth (skeleton) 1% Body fluids

mg% 40% Non-Diffusible 60% Diffusible 50% Ionized active form 10% Non-Ionized Bound to protein as albumin

Un-exchangable Ca+2 pool Exchangable Ca+2 pool

There are 3 types of bone cells: 1.Osteoblasts are the differentiated bone forming cells and secrete bone matrix on which Ca ++ and PO precipitate. 2.Osteocytes, the mature bone cells are enclosed in bone matrix. 3.Osteoclasts is a large multinucleated cell derived from monocytes whose function is to resorb bone. These cells are responsible for Bone remodeling which is a process which continues throughout life, long after epiphyseal fusion and cessation of linear growth of bone. Remodeling consists of bone formation and bone resorption.

They are 4 in number and each gland is 3-5 mm in diameter. Polypeptide hormone 84 aa.

calcium PTH controls calcium within the blood in a very tight range between 9.0 and 11 mg/dl. It also maintains a constant ratio ( ) Ca and inorganic phosphate PO4-, so that; Solubility product = Ca x PO4 = K (constant). Ca and PO4 are, thus, inversely related. PTH tends to raise the lowered Ca level by acting on;.

 Serum calcium  Parathormone  Ca ++ absorption  Ca ++ reabsorption  bone resorption  Ca Chief cells

a) Kidney:a) Kidney: i) It ↑es the reabsorption of Ca and Mg from the renal tubules. ii) It ↑es PO4 excretion. iii) It activates Vit. D by 1 α hydroxylase enzyme. b) Bone:b) Bone: It ↑es the number and level of activity of osteoclasts (bone destroying cells) in the skeleton → results in bone resorption → release of Ca into the blood stream and hypercalcaemia. a) Small intestine :a) Small intestine : a) It ↑es the absorption of Ca (mediated by active vitamin D (1, 25 DOH cholecalciferol)→ activated in the kidney by PTH. b) It ↑es the absorption of PO4 and Mg.

a) Plasma Ca level: It the main regulator of PTH secretion. ↓ Plasma Ca level → ↑ PTH secretion b) Plasma Mg level: as Ca c) Plasma PO4 level: opposite to Ca d) Nervous factors: β-adrenergic receptors agonists as isoproterenol→↑ PTH secretion

Protein hormone (32 a.a)

parafollicular cells secreted by parafollicular cells or c-cells of the thyroid gland

1- The major stimulus to its secretion is a rise in serum Ca +2 due to increase intracellular cyclic AMP. 2- Ingestion of food stimulates it and this effect is mediated by GIT hormone of which gastrin is the most potent.

1-The major effect of calcitonin administration is a rapid fall in plasma Ca +2. Young growing animals are most affected, whereas in adults, who have more stable skeleton, only minimal response is seen. 2-It produce inhibition of osteolysis by osteocytes and reduce bone resorption by osteoclasts Ca +2 mobilization from bone to blood is also decreased, as calcitonin inhibit Ca +2 permeability of bone cells.

3-It is a physiological antagonist to parathormone with respect to Ca +2, But has the same effect of parathormone on PO4 i.e. decrease plasma PO4 level, and has no effect on plasma magnesium level. 4-It increase urinary excretion of Ca +2 and PO4.

5-It also decrease Ca +2 absorption from the intestine. 6-can be used therapeutically for the treatment of hypercalcemia or osteoporosis.

Vit. D has dietary and endogenous precursors: D2 D3 D2 (ergocalciferol) formed in plants D3(cholecalciferol) formed in skin

1-stimulate Ca +2 and phosphate reabsorption in intestine and kidney. 2-provide Ca +2 and phosphate needed for bone formation. 3-promote differentiation of monocyte precursor to monocytes and macrophage.