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The plasma total calcium concentration is in the range of 2.2–2.6 mmol/L (9–10.5 mg/dL), and the normal ionized calcium is 1.3–1.5 mmol/L (4.5–5.6 mg/dL). [4] The amount of total calcium in the blood varies with the level of plasma albumin, the most abundant protein in plasma, and therefore the main carrier of protein-bound calcium in the blood.
It has been hypothesized that three mechanisms contribute to IH: increased calcium absorption in the intestines, faulty renal tubule calcium reabsorption in the kidneys, and an increased rate of bone resorption. [8] Others estimate IH may arise due to excessive expression of vitamin D receptors.
In the bone marrow, estrogen downregulates the proliferation of hematopoietic stem cells through an IL-7 dependent mechanism. [17] In the kidney, around 250 mmol of calcium ions are filtered into the glomerular filtrate per day. Most of this (245 mmol/d) is reabsorbed from the tubular fluid, leaving about 5 mmol/d to be excreted in the urine.
The transport of calcium across the enterocyte cytoplasm appears to be rate-limiting for calcium absorption in the intestine; the presence of calbindin increases the amount of calcium crossing the cell without raising the free concentration. [8] S100G may also stimulate the basolateral calcium-pumping ATPases.
The US Institute of Medicine (IOM) established Recommended Dietary Allowances (RDAs) for calcium in 1997 and updated those values in 2011. [6] See table. The European Food Safety Authority (EFSA) uses the term Population Reference Intake (PRIs) instead of RDAs and sets slightly different numbers: ages 4–10 800 mg, ages 11–17 1150 mg, ages 18–24 1000 mg, and >25 years 950 mg. [10]
Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [8] [9] [10] It is also known as 1,25-dihydroxycholecalciferol.It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [11]
Vitamin D increases absorption of calcium and phosphate in the intestinal tract, leading to elevated levels of plasma calcium, [4] and thus lower bone resorption. Calcitriol (1,25-dihydroxycholecalciferol) is the active form of vitamin D 3. [10] It has numerous functions involved in blood calcium levels.
The sodium-calcium exchanger (often denoted Na + /Ca 2+ exchanger, exchange protein, or NCX) is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium (Na +) by allowing Na + to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions (Ca 2+).