Extracellular calcium - Métabolism
Metabolism
Intestinal absorption
Calcium intake ranges from 200 to 2500 mg/day. From a daily intake of 1g, approximately 300 mg are absorbed; but since there is a secretion of approximately 150 mg in the intestinal lumen, the quantity kept by the body is approximately 150 mg. The mechanisms of absorption are complex.:
- a paracellular absorption through intercellular spaces from the small intestine, not directly controlled but which is predominant.
- a controlled transcellular absorption especially from the duodenum in 3 steps:
- an entry of Ca 2+ into the epithelial cell through its apical pole, by nonvoltage-dependant channels
- a transfer of Ca 2+ from the apical pole to the basal pole of the cell via calbindine which also takes part in the transfer of protons
- an extrusion of Ca 2+ from the basal pole of the epithelial cell into the interstitial fluid. This extrusion is achieved by a Ca 2+ -ATP-dependant pump, a Na + / Ca 2+ exchanger and exocytosis. These mechanisms require an intake of energy.
The bioavailability of calcium, i.e. the absorbed percentage, increases when the intake decreases but this increase can remain insufficient to compensate for a too important reduction of the intake. Twice 500 mg has a better bioavailability than once 1000 mg.
Vitamin D, parathyroid hormone, sugars like lactose, perhaps while supplying energy, increase the digestive absorption of calcium.
The bioavailability of calcium is decreased by glucocorticoids, phosphate, phytate and oxalate.
Tissue distribution
The calcium concentration in plasma or calcemia is 100 mg/, that is to say 2.5 mm/L or 5.0 meq/L distributed as follows:
- 40% bound to proteins among which albumin.
- 10% diffusible but bound to anions such as citrate and phosphate.
- 50% free, acting as a regulator of the secretion of the hormones implicated in calcium and phosphorus metabolism.
When calcemia is lower than 88 mg/L, there is a hypocalcemia and when it is higher than 105 mg/L, there is a hypercalcemia, each having several causes.
The skeleton contains more than 90% of the calcium of the body, in the form of hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ]. In adults, uptake and loss of calcium in bone are about equivalent, but during ageing, the loss prevails.
Renal elimination
Free calcium is filtered by the glomerulus and mainly reabsorbed by the tubule. The percentage of reabsorption reaches nearly 70% at the proximal tubule, 20% at the Henle's loop, and 10% at the distal tubule. It is estimated that approximately 150 mg of calcium are eliminated daily in urine.
Calcium reabsorption is increased by parathyroid hormone and decreased by calcitonin. Diuretics of Henle's loop decrease it, whereas thiazide diuretics increase it.
In patients with renal impairment, the glomerular filtration of calcium decreases.
Effects
- Extracellular c alcium is involved in bone metabolism. Bone is in perpetual renewal, approximately 10% of bone mass are renewed each year. Bone is synthesized by osteoblasts and degraded by osteoclasts. Osteoblasts secrete an organic matrix called osteoid tissue, made of non rigid collagen. This matrix is rigidified by calcium and phosphate deposis to give hydroxyapatite. The osteoclasts erode osseous tissue by acid secretion which solubilizes the mineral part, and secretion of hydrolases which digest the organic matrix.
Certain hormones and vitamin D are involved directly and indirectly in this process.
- Calcium plays a determining role in coagulation: its chelation by EDTA-Na 2 has an in vitro anticoagulant effect (See “Drugs and coagulation”).
- Calcium is involved in the phenomena of adherence of between cells and to the extracellular matrix.
- Calcium has a stabilizing membrane effect: a diet rich in calcium tends to decrease the arterial pressure by decreasing peripheral resistances, probably secondary to the stabilization of the membranes of vascular smooth muscles.
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