1. Ca homeostasis: Role of PTH & vit.
D3
Nermeen Bastawy
Physiology Department
Faculty of Medicine
Cairo University

2. Calcium
• Functions?
• Distribution: 1 kg
• Bony skeleton: 99%
• IC/ EC (plasma): 1%

3. Bony skeleton
• Large stable pool (99%): calcium phosphate
• Small labile pool (1%): rapidly exchange
with ECF
ECF
Non exchangeable
Exchangeable Calcium
Calcium

4. ++ Ca
• Functions?
• Distribution: 1000 g
• Bony skeleton: 99%
• IC/ EC (plasma): 1%

5. Plasma Calcium
• Total : 9.4 -11 mg %:
• Diffusible (mostly ionized as Ca++)
• Non diffusible (plasma ptn-bound)
• Ionised Ca is necessary for:
Coagulation
Muscles contraction
Nerve function

6. ABSORPTION OF CALCIUM
Upper small intestine
Passive diffusion & active transport.
AFFECTED BY:
Active vit. D3
Local GIT factors:
Acidity of food products & GIT secretion.
Amino acids (Protein meals)
Phosphates & oxalates.

7. PHOSPHOROUS
 500 - 800 g (90% in the skeleton).
 12 mg/dl
 ATP, cAMP, 2,3DPG,
Phosphoproteins, Phospholipid

8. Solubility product
If electrolytes concentration in a
solution below a certain value
(the solubility product) not ppt.
Ca X P = 10 Ca X P > 10
Calcium and phosphorous in ECF >
solubility product, but will not
precipitate due to presence of
Pyrophosphate (inhibitor).

9. BONE HISTOLOGY
 ECM:
 Ptn: type I collagen
 Crystals: hydroxyapatite
 Cells:
 Osteocytes
 Osteoblasts
 Osteoclasts
(Remodling unit)

10. RANK

11. Regulation of Ca Homeostasis
1 line
st 2nd line
Liver, intestine mitochondria
• PTH
• Vit D
• Calcitonin

13. Parathyroid gland
Mg++ is necessary for optimal function

14. ACTIONS OF PTH ON BONE
 Rapid Phase: (min-hrs.)
 Slow phase: (days- weeks).

16. Slow action of PTH
PTH

17. Osteoprotegrin & Denosumab
• OPG Prevent bone resorption & osteoprosis
by prevent excess stimulation of osteoclast

18. Calbindin
DCT ++H+, Mg
PCT

19. Growth factor
C N N
CPTH-R

20. Mechanism of PTH action

21. Low Ca or high Pi

22. DISTURBANCES OF PARATHYROID GLAND
• Hypoparathyroidism
• Primary hyperparathyroidism:
due to a tumor of the parathyroid.
• Secondary hyperparathyroidism:
CRF phosphate retention

23. TETANY
+Ca Excitability
+
+Ca
+ Excitability

26. Conditions producing tetany

28. TYPES OF TETANY
• Latent (hidden) tetany:
plasma calcium below 9.4 but above 7 mg/dl. Its manifestations don’t appear during rest.
It can be provoked to appear by the following tests
Tapping the area over the facial nerve:
-Normal: there is only feeling of tap.
-hyperexcitable facial nerve: chovestek sign.
Ischemia of the muscles of the upper limb:
-normally : feeling of ischemia pain in the upper limb.
-hyperexciatble upper limb nerves: carpal spasm or accoucheur hand or trousseau
sign.
Galvanic stimulation of the upper limb nerves.
• Manifest tetany: it occur when the plasma calcium drops below 7 mg/dl, the patient
is presented with carpal spasm or spasm of laryngeal or spasm of respiratory
muscles.