endocrine and metabolic disorders
Overview of Disorders of Calcium Concentration
Introduction
Calcium (Ca) is required for the proper functioning of muscle contraction, nerve conduction, hormone release, and blood coagulation. In addition, proper calcium concentration is required for various other metabolic processes. Maintenance of body calcium stores (to avoid Hypocalcemia or Dietary calcium intake Absorption of calcium from the gastrointestinal (GI) tract Renal calcium excretion ), roughly 200 to 400 mg of this calcium is absorbed from the intestine each day (see also Vitamin D Deficiency and Dependency ). The remaining 800 to 1000 mg appears in the stool. Calcium balance is maintained through renal calcium excretion averaging 200 mg/day, which also depends on circulating PTH and Both extracellular and intracellular calcium concentrations are tightly regulated by bidirectional calcium transport across the plasma membrane of cells and intracellular organelles, such as the endoplasmic reticulum, the sarcoplasmic reticulum of muscle cells, and the mitochondria. Ionized calciumis the physiologically active form. Cytosolic ionized calcium is maintained within the micromolar range (<1/1000 of the serum concentration). Ionized calcium acts as an intracellular 2nd messenger; it is involved in skeletal muscle contraction, excitation-contraction coupling in cardiac and smooth muscle, and activation of protein kinases and enzyme phosphorylation. Calcium is also involved in the action of other intracellular messengers, such as cAMP (cyclicadenosinemonophosphate) and inositol 1,4,5-triphosphate, and thus mediates the cellular response to numerous hormones, includingepinephrine,glucagon,vasopressin(antidiuretic hormone),secretin, and cholecystokinin. Despite its important intracellular roles, about 99% of body calcium is in bone, mainly as hydroxyapatite crystals. About 1% of bone calcium is freely exchangeable with the extracellular fluid and, therefore, is available for buffering changes in calcium balance. Normaltotal serum calciumconcentration ranges from 8.8 to 10.4 mg/dL (2.20 to 2.60 mmol/L). About 40% of the total blood calcium is bound to plasma proteins, primarilyalbumin. The remaining 60% includes ionized calcium plus calcium complexed with phosphate and citrate. Total calcium (ie, protein-bound, complexed, and ionized calcium) is usually what is determined by clinical laboratory measurement. However, ideally,ionized (or free) calciumshould be estimated or measured because it is the physiologically active form of calcium in plasma and because its blood level does not always correlate with total serum calcium. Ionized calcium is generally assumed to be about 50% of the total serum calcium. levels (see Estimation of ionized calcium concentration ). Direct determination of ionized calcium, because of its technical difficulty, is usually restricted to patients in whom significant alteration of protein binding of serum calcium is suspected. Normal ionized serum calcium concentration range varies somewhat between laboratories, but is typically 4.7 to 5.2 mg/dL (1.17 to 1.30 mmol/L). 