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Calcium regulation in the human body. [6]The plasma ionized calcium concentration is regulated within narrow limits (1.3–1.5 mmol/L). This is achieved by both the parafollicular cells of the thyroid gland, and the parathyroid glands constantly sensing (i.e. measuring) the concentration of calcium ions in the blood flowing through them.
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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]
Calcium homeostasis. The plasma ionized calcium (Ca 2+) concentration is very tightly controlled by a pair of homeostatic mechanisms. [60] The sensor for the first one is situated in the parathyroid glands, where the chief cells sense the Ca 2+ level by means of specialized calcium receptors in their membranes.
The bones act as a (metaphorical) "bank of calcium" from which the body can make "withdrawals" as needed to keep the amount of calcium in the blood at appropriate levels despite the ever-present challenges of metabolism, stress, and nutritional variations. PTH is "a key that unlocks the bank vault" to remove the calcium.
Calcitonin lowers blood levels of calcium by inhibiting the resorption of bone by osteoclasts, and its secretion is increased proportionally with the concentration of calcium. [ 7 ] Parafollicular cells are also known to secrete in smaller quantities several neuroendocrine peptides such as serotonin , somatostatin or CGRP .
Though calcium is the most plentiful electrolyte in the body, a large percentage of it is used to form the bones. [14] It is mainly absorbed and excreted through the GI system. [ 14 ] The majority of calcium resides extracellularly, and it is crucial for the function of neurons , muscle cells , function of enzymes , and coagulation . [ 14 ]
Calcium-binding proteins have specific domains that bind to calcium and are known to be heterogeneous. One of the functions of calcium binding proteins is to regulate the amount of free (unbound) Ca 2+ in the cytosol of the cell. [1] The cellular regulation of calcium is known as calcium homeostasis.