( Source, Release & Function ) 1.structure of bone & teeth 6. hormone secretion 5. hormonal actions 2.neurotransmission 4. muscle contraction 3. blood.
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1.structure of bone & teeth
6 .hormone secretion
5 .hormonal actions
2.neurotransmission
4 .muscle contraction
3 .blood clotting
9 - 11 mg%
40%Non-Diffusible
60%Diffusible
50%Ionized
active form
10%Non-
Ionized
Bound to protein as albumin
• 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.
PTH controls calcium 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
Serum calcium
Parathormone
Ca++ absorption Ca++ absorption Ca++ reabsorption
Ca++ reabsorption
bone resorption bone
resorption
Ca++ Ca++
--
Chief cellsChief 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
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 D2 (ergocalciferol) formed in plantsD3D3(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.
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