True about calcium
|B||Binds to proteins|
- The body of a young adult human contain about 1100 gm calcium (=27.5 mol. of calcium)
i. 99% of calcium is present in bones.
ii. 0.1 % of calcium present in ECF
iii. 0.5 to 1% of skeletal calcium is freely exchangeable with that in the ECF.
iv. Plasma calcium level- 9-11 mg/dL : of which
• 50% ionized (diffusible)
• Calcium complexed to anions ( with phosphate, citrate, HC03 - Diffusible) Q
• Protein bound calcium - 41 % (non diffusible) with - Albumin (major) - Globulin
It is free, ionized calcium (Ca++, divalent cation) in the body fluid that is vital second messenger and is necessary for blood coagulation, muscle contraction (all types) and nerve function.
- Absorption and Excretion:
i. Intestine: the feedback controlled hormonal regulation of intestinal absorptive efficiency result in a relatively constant daily net calcium absorption of approximately 200 to 400 mg/day despite large changes in daily dietary calcium intake.
• Daily intake → 400 - 1500 mg/day (Av. = 1000 mg/day)
• On average 350 mg - absorbed
- 250 mg/day calcium enters the intestine via secretions and sloughed mucosal cell → lost in faces
• Total ingestion = 1000 mg/day
• Total loss = 650 + 250 = 900 mg/day
- Therefore 90% of daily intake of calcium is excreted in faces. Q
- Site → calcium is absorbed in upper small intestine (Duodenum and proximal jejunum) Q
Mechanism of intestinal absorption:
o Active (Transcellular) Mechanism → Main mechanism → controlled principally by Vit D3
o Passive (paracellular) mechanism: accounts for 5%
o Optimal rates of calcium absorption require gastric acid.
o High calcium intake → Serum Ca++ ↑ →↑ synthesis of Vit D3→↑ intestinal absorption and vice versa.
ii. Renal regulation of calcium metabolism
• Approximately 10% (100 mg/day) of ingested calcium is excreted in urine Q
• 99% of filtered calcium is reabsorbed
• In PCT → 65% reabsorption → via passive, paracellular route, coupled to con comitant Nacl reabsorption, not specifically regulated.
- In the cortical thick ascending limb of Henle's Loop (cTAL) → 20% reabsorption → via passive paracellular mechanism → it requires paracellin-l for reabsorption of calcium and is inhibited by ↑ plasma Ca++ and Mg++, acting via CaSR.
- In DCT → 10% reabsorbed -7 regulated by PTH (Transcellular active mechanism)