(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

9 - 11 mg%

40%Non-Diffusible

60%Diffusible

50%Ionized

active form

10%Non-

Ionized

Bound to protein as albumin

Un-exchangable

Ca+2 pool

Un-exchangable

Ca+2 poolExchangable 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.

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

Protein hormone (32 a.a)

secreted by parafollicular cellsparafollicular 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 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.