Calcium homeostasis

hormones that control ca level in blood

1. Calcium homeostasis Edited by Dr Dina Merzeban physiology lecturer Fayoum university www.facebook.com/merzeban physiology

2. Calcium homeostasis The ability to maintain internal equilibrium of blood calcium and phosphate ion levels. It is maintaining blood calcium level within normal range. Blood calcium level: 9-11 mg/100 ml   Hypercalcemia: An abnormally high concentration of calcium ions in the blood; at worst, cardiac arrest may occur. Hypocalcemia: An abnormally low concentration of calcium ions in the blood; at worst, cardiac arrest may occur.

3. The adult human body contains about 1 kg of calcium; 99% is present in bones and 1% in blood and other tissues.   Plasma calcium level: 9 -11 mg/100 ml 1. 50% ionized calcium. 2. 40% protein-bound calcium (bound to albumin). 3. 10% complex diffusible calcium bound to phosphates, bicarbonates and citrates.  

4. Functions of Ca 2+ 1- Necessary for activation of clotting factors in plasma. 2- It controls membrane excitation in nerve and muscles. 3- It is needed for muscular contraction (SK. & cardiac), also it is essential for cardiac rhythmicity. 4- It is needed for release of hormones by endocrine cells and release of neurotransmitters by neuron. 5- It is necessary for the production of milk and formation of bone and teeth. 6- It acts as a second messenger for certain hormones.

5. Absorption of Ca2+ by GIT Factors that increase the solubility of calcium salts increase its rates of absorption: 1- PH: acid medium ⇈ Ca++ absorption, most Ca++ absorption occurs in the duodenum due to acid chyme. alkalies form insoluble Ca soaps ⇊ Ca++ absorption. CaCl 2 ingested as acidifying salt and lactose due to production of lactic acid ⇈ Ca++ absorption. 2-   Citrates in drugs aid Ca++ absorption due to formation of soluble calcium citrate, while phosphates and oxalates ⇊ Ca++ absorption due to formation of insoluble Ca salts.  

6. Absorption of Ca2+ by GIT    4- Protein diet ⇈ Ca++ absorption due to formation of soluble complexes with amino acids. 5- Vitamin D promotes Ca++ absorption from the GIT.   6- Feed back mechanism (PTH effect): ⇊ concentration in ECF will ⇈ Ca++ absorption from the gut through stimulation of PTH.  

7. Calcium regulation Involves: (1) Three hormones Parathormone(PTH) Calcitonin active vitamin D3   (2) Three tissues Bone Intestine Kidney (3) Three bone cells   Osteoblast Osteocytes Osteocalst

8. Bone cells Osteoblasts: These are bone-forming cells. They secrete collagen and other bone matrix proteins, as well as alkaline phosphatase, which promotes deposition of calcium phosphate crystals .   Osteocytes: When osteoblasts become entrapped in collagen matrix, they differentiate into osteocytes.   Osteoclasts: These cells erode and resorb bones. They achieve this by acidifying the area of bone surrounding them, which will dissolve calcium phosphate crystals. They also secrete acid proteases, which dissolve collagen.    

9. Parathormone(PTH)

10. Parathormone action (PTH)   Secreted from parathyroid gland. (A) On bone 1. Rapid phase: Osteolysis transport of Ca from bone fluid to ECF by activating a pump in osteocytic membrane (Osteoblast and osteocytes) 2. Slow phase:  Activity of osteoclasts  Bone decalcification  

11. Parathormone action (PTH)    (B) on Kidney 1- It decreases PO4 reabsorption in the proximal convoluted tubules (PCT). 2- It increases Ca2+ reabsorption in distal convoluted tubules (DCT). 3- ↑↑ 1, 25 dihydroxycholecalciferol (1,25 DHCC) in PCV.   (C) on Intestine PTH increases Ca2+ absorption in intestine by (1,25 DHCC).

12. Regulation of PTH secretion:   PTH is increased by: (1) Low serum Ca2+ : (2) High serum PO4:   PTH is decreased by: opposite changes.  

13. Vitamin D 3 Photosynthesis of vitamin D3 ( 1,25 DHCC)

14. Actions of 1,25 DHCC :   1) On intestine: It stimulates Ca2+ and phosphate absorption.   2) On bone: ♠ Low Ca2+ & PO4 → It stimulates osteoclastic activity → bone resorption   3) On kidney: ++ Ca2+ reabsorption from distal tubules. ++ phosphate reabsorption from proximal tubules.  

15. Calcitonin   It is a polypeptide hormone that is secreted by the parafollicular cells of thyroid (C cells or clear cells).   Hypocalcemic hormone Actions (1) On bones: - It inhibits directly bone resorption and stimulate bone deposition by: a- Inhibiting activity & formation of osteoclasts. b- Activation of osteoblasts (2) On kidney: It increases Ca & phosphate excretion in urine.              

16.   Parathormone Calcitonin Vitamin D 3 Source: Parathyroid gland Parafollicular cells of thyroid gland 7-dehydro-cholesterol -> Cholecalciferol -> 25-hydroxy-cholecalciferol -> 1,25-(OH) 2 -cholecalciferol Actions:     1- On bones : a) Rapid phase: “Osteolysis” b) Slow phase:  Activity of osteoclasts. Inhibits activity of osteoclasts. It both deposits and mobilizes Ca and phosphates resulting in remodeling of bone. 2- On kidneys  Reabsorption of Ca 2+ from DCT  Phosphate reabsorption by PCT.  Excretion of both Ca and PO 4 3- in urine  Reabsorption of Ca from DCT.  Reabsorption of PO 4 3- from PCT. 3- On GIT:  Reabsorption of Ca ++ indirectly by activating vit. D3    Intestinal absorption of Ca and phosphate

17. Tetany   Definition: It is a condition of increase neuromuscular excitability due to decrease ionized Ca2+ . It resulted in extensive spasm of the skeletal muscle.   Causes: 1- Hypoparathyroidism following thyroidectomy. 2- Alkalosis: by persistent vomiting, hyperventilation. 3- Renal failure: results in retention of phosphates → low Ca2+ & failure of α hydroxylation. 4- Rickets (vitamin D deficiency in children).  

18.   Types of tetany Manifest tetany: - Occurs when serum Ca2+ is below 7 mg% - Carpopedal spasms occur in fingers & toes (accoucheur hand). - Laryngeal spasms and respiratory spasms leading to fatal asphyxia.   Latent tetany (Hidden): - Occurs when serum Ca2+ is above 7 mg% but < 9.4 mg% (7-9.4 mg%). - It appears by tests to show increased excitability.    

19. Latent tetany tests to show increased excitability. 1- Chovstek's sign -Tapping the skin in front of the ear → Twitching of facial muscles (hyperexcitable facial nerve).   - A tap on the peroneal nerve at lateral aspect of fibula below its head → dorsiflexion and abduction of the foot.   2- Trousseau sign Occluding the circulation of the arm with the blood pressure cuff for few minutes → carpal spasm (accoucheur hand).  

20. Accoucheur hand: flexion of wrist & metacarpo -pharyngeal joints with extension of interphalangeal joints and adduction of the thumbs in the palm.

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