( Source, Release & Function )
1.structure of bone & teeth
2.neurotransmission
3. blood clotting
4. muscle contraction
5. hormonal actions
6. hormone secretion
Total Body Calcium
1100 g
1%
Body fluids
99%
Bone and teeth
(skeleton)
9 - 11 mg%
60%
Diffusible
50%
Ionized active
form
10%
Non-Ionized
40%
Non-Diffusible
Bound to protein as
albumin
Exchangable
Ca+2 pool
Un-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.
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;.
Chief cells
 Serum calcium
-
 Parathormone
 Ca++ absorption
 Ca++ reabsorption
 Ca++
 bone resorption
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:
• 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) 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)
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 (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.