Calcium is the most prevalent divalent cation in the body, occurring mainly in the bone but also playing key roles in the function of nerve and muscle cells.
Of dietary calcium, 25-30% is absorbed in the gut, mainly in the duodenum and proximal jejunum. Absorption occurs transcellularly and is increased by the hormonal action of 1,25(OH2)D3, the active form of vitamin D.
Bioavailability is important for calcium absorption. Foods rich in calcium include:
Supposedly, calcium carbonate is better absorbed if taken with acidic meals, and you can only absorb 500 mg of elemental calcium at one time.
An adult has about 1000 g of calcium in the body - 99% of it in bones and the rest in the extracellular fluid and soft tissue.
Although there is some controversy surrounding the lower limits of what is required, estimates include:
Normal blood calcium levels are 8.0-10.4 mg/dl, or 2.00-2.60 mmol/L, while normal intracellular calcium levels are 10-4 mmol/L (check this).
In the plasma, 45% is protein bound, 5% is complexed to other ions such as citrate and phosphate, and 50% is free ionized Ca2+.
Plasma calcium and phosphate concentrations are close to the saturation point at which calcium phosphate precipitates out of solution onto the bone matrix. Accordingly, plasma values of the two ions are inversely related, as a rise in one will cause a fall in the other.
Serum calcium levels depend on albumin, requiring a correction of increased sCa2+ by 0.2 mmol/L for a decrease in 10 g/L (below 40 g/L) of albumin.
For example, a sCa2+ of 1.9 mmol/L and albumin of 20 g/L requires an increase in sCa2+ of 0.4 mmol/L, resulting in a corrected value of 2.30 mmol/L.
Another formula states (Albnor - Albpt)0.8 + Ca2+pt
Ionized calcium levels are 4.25-5.25 mg/dl, or 1.05-1.30 mmol/L. This is an important indicator of clotting capacity.
Most of the calcium in the body is present in the bone, where it combines with phosphate to form hydroxyapetite, the building block of bones.
Calcium is a key mediator of muscle cell (myocyte) contraction. Calcium enters the cytoplasm from outside, and from the sarcoplasmic reticulum of these cells, doing something with myosin and leading to contraction.
Calcium is invloved in neutrotransmitter release.
Along with the functions in the specialized cells already listed, calcium is also a key component of signaling patwhays in all cells. Intracellular levels are kept very low, with most calcium bound to proteins or sequestered in the endoplasmic reticulum or mitochondria.
DAG, PI3K, ER, etc
Calcium also plays a role in coagulation. Citrate acts by binding calcium, preventing clotting.
It is critical to maintain levels of ionized calcium in the extracellular fluid in maintain function of nerves and muscle.
The parathyroid gland monitors levels of ionized calcium in the plasma. PTH and vitamin D increases calcium levels, while calcitonin decreases them.
Following filtration in the kidney, 70% is reabsorbed in the proximal tubule and a further 20% in the thick ascending loop of Henle. The latter is controlled by the voltage differential generated by active Na/Cl/K absorption and is mediated by the Ca2+-selective paracellular pathway.
As calcium reabsorption depends on sodium movement in the loop of Henle, diuretics such as furosemide can lead to hypocalcemia. In contrast, thiazides can actually enhance calcium reabsorption.
Calcium transport in the distal tubule , accounting for 5-10%, is active, transcellular, and the major target for hormonal control by PTH and vitamin D. In order to keep intracellular levels low, calcium-binding proteins are used. Lumenal uptake proceeds down an electrochemical gradient through Ca2+ channels regulated by PTH, while Ca2+ export is active and depends on the Na+/Ca2+ exchanger or a Ca2+ ATPase. Both Ca2+-binding proteins and Ca2+ ATPase are under control of vitamin D.