Mineral Metabolism

the consumption of inorganic substances; their assimilation (usually in the gastrointestinal tract), distribution in the body, participation in physical and chemical phenomena and biochemical reactions, and excretion. The principal significance of mineral metabolism is the maintenance of specific physicochemical conditions in the internal environment of the body, the formation and preservation of structures of dense tissues (the skeleton), and specific regulation of enzyme reactions.

The unequal distribution of ions between the cell and its environment is the basis of bioelectric phenomena. The predominant cation in the blood plasma and intercellular and cerebrospinal fluids of humans is Na⁺, and the predominant anions are Cl^- and HCO₃^- (see Table 1). The ion composition of fluids secreted by the pancreas and the lacteal and other glands differs substantially from that of the blood plasma and is determined by the specific secretory activity of the cells of the glandular epithelium.

Variations in ion concentration may be especially great in fluids produced by excretory organs, which intensify excretion of ions when there is an excess in in the body and reduce it when there is a deficiency. In most cells the predominant cation is K⁺; the content of Mg²⁺ in cells is higher than in the blood plasma. The erythrocytes of humans, rabbits, and chickens contain more K⁺ than Na⁺, but in some animals (for example, dogs and sheep of a number of genetic lines), Na⁺ predominates in the erythrocytes as well as in the plasma. The ions are also unevenly distributed among individual cell organelles (there is more Na⁺ in the nucleus than in the cytoplasm).

The human daily requirement for certain chemical elements varies and depends on age and sex, the climate, the type of activity, and the diet. On the average a human being must ingest daily with food and drink 800–1,500 mg of calcium, 1,200–2,000 mg of phosphorus, 2,000–3,000 mg of potassium, 4,000–6,000 mg each of sodium and chlorine, 500–600 mg of magnesium, and about 15 mg of iron. Some elements are completely assimilated (potassium and sodium); others (calcium and iron) are partially assimilated. Ions absorbed in the gastrointestinal tract enter the blood and lymph; some ions become bound to specific plasma proteins and thus are carried in the bloodstream. A number of elements are deposited in the liver and other tissues (for example, there is a great deal of calcium, magnesium, strontium, and fluorine in the bones). Among humans and other mammals, excess salts are excreted through the intestines (mainly calcium, iron, copper, and strontium) and kidneys (mainly sodium, potassium, chlorine, boron, and iodine). Concentrations of certain ions in the body are maintained with great precision by special regulatory systems: Na⁺ and K⁺, by hormones of the adrenal cortex, and Ca²⁺, by hormones of the thyroid and parathyroid glands. The body of a man who weighs 70 kg contains about 100 g of sodium (of which 40–45 percent is in bone tissue, 50 percent is in extracellular fluids, and less than 10 percent is in the cells) and about 120 g of potassium (of which 2 percent is in extracellular fluids). Upon an increase in the potassium concentration in the blood plasma, cardiac activity is disrupted; upon a decrease, there is muscle weakness, periodic paralysis, and disruption of the function of the kidneys and gastrointestinal tract. More than 90 percent of the calcium (about 900 g) is concentrated in the bones. Calcium carbonate and calcium phosphate are used in the majority of animals not only for constructing the skeleton but also for maintaining a certain calcium level in the plasma, irrespective of its intake with food. Some organisms are capable of accumulating large quantities of certain elements. Thus, the concentration of vanadium in some ascidians is 5 x 10⁵ times higher than in seawater; other tunicates actively accumulate niobium.