The bulk of Ca in the adult body (about 99%) is found in bone tissue as part of hydroxyapatite crystals. In the blood serum of most mammals, the calcium concentration is about 3 mmol / L (10-12 mg / 100 ml) and is constant. Daily fluctuations of this constant do not exceed 3-5%. Ca is necessary for the normal functioning of the nervous system. In neuromuscular synapses, calcium ions contribute to the release of acetylcholine and its connection with the choline receptor, and with an excess of acetylcholine, cholinesterase is activated, an enzyme that hydrolyzes acetylcholine.
Ca ions contained in the sarcoplasmic reticulum contribute to the interaction of actin and myosin, that is, a reduction in muscle fiber. This effect is carried out in the presence of magnesium ions.
Calcium ions activate enzymatic processes in the initial stages of blood coagulation - with the formation of thromboplastin and thrombin. The binding of Ca in plasma oxalates or citrates prevents blood coagulation.
The unique role of calcium in the vital activity of cells has recently been proved, in the cytoplasm of which its concentration does not exceed 10-6 mol. At the cellular level, six main physiological processes are distinguished, which depend on the concentration of Ca ions:
a) motor activity of cells (muscle contraction);
b) the excitability of cells that are capable of generating an electrostatic action potential (muscle tone, nerve impulse, photoreception, cardiac contraction);
c) the release of substances that are synthesized in the cell (secretion of hormones and neurotransmitters);
d) incorporation of soluble substances into the cell by vesiculation (phagocytosis);
e) intracellular metabolism (glucose synthesis, lipolysis, etc.);
f) reproduction of cells (fertilization and mitosis of eggs, sperm motility).
Ca ions together with specific proteins provide elasticity and firmness of bone tissue. Modern medicine uses many drugs containing calcium ions (glucanate, citrate, chloride, lactate, carbonate, phosphate, etc.). Calcium citrate is of particular importance. The main stage of calcium release from its complexes (phosphates, sulfates, chlorides) with organic and mineral components of food occurs in the stomach under the influence of perchloric acid and proteolytic enzymes. Calcium carbonate and phosphate salts dissociate in the acidic environment of the stomach, and calcium in the ionized form or in combination with soluble chelates enters the small intestine. Therefore, in conditions of decreased secretory function of the stomach, the availability of Ca for absorption is insufficient, which may become one of the reasons for the deficiency of this element in the body. Calcium citrate is best soluble in the acidic environment of the stomach. Ionized calcium easily penetrates biological membranes, even across the placental barrier. The main part (about 80%) of calcium is excreted from the body with feces, about 20% is excreted in the urine. Due to the fact that calcium citrate, or rather, citrate salt helps to reduce oxalates in the urine, the use of the drug does not lead to the formation of kidney stones.
It has been established that at night the release of mineral salts from the body is activated. Therefore, doctors recommend taking calcium citrate after dinner or in the evening. A negative correlative relationship between the dose of the drug and the pharmaceutical activity of calcium is proved. What does it mean? Calcium citrate is better absorbed by the body in small doses. Therefore, it is best to take the drug several times a day. For each age group, physiological norms for calcium intake are established. Calcium preparations are prescribed for both prophylactic and therapeutic purposes (rickets, osteoporosis, osteomalacia, to strengthen tooth enamel). Potassium citrate is prescribed for heart failure, hypertension, to normalize the acid-base and water-salt balances in the body.