This document discusses calcium homeostasis and the roles of parathyroid hormone (PTH) and vitamin D3. It covers calcium distribution in the body, absorption in the small intestine, and regulation of calcium levels. PTH acts on bone to increase calcium levels by promoting bone resorption. Vitamin D3 aids in calcium absorption from the gut. Disturbances in PTH can cause hyperparathyroidism or hypoparathyroidism and the resulting condition of tetany.

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.