The control factors of mineral metabolism is a very complicated phenomenon. In the unicellular animals the mechanism is relatively simple and is probably carried out by adjusting the permeability of the cell membrane. But in the higher animals and human beings, although this primitive system persists, yet a lot of other factors have stepped in, as a result of which, a very intricate system of control has evolved.
The supply, metabolism, balance and excretion are controlled by the following factors:
Factor # 1. Salt Hunger:
It has been mentioned that under natural conditions, salt deficiency does not take place. In any condition where partial deficiency occurs, it is at once replenished by the intake of more salts. This salt hunger is felt in such conditions of deficiency. In cattle establishments huge blocks of rock salts are kept and the animals are often found to lick those blocks eagerly.
Factor # 2. Absorption:
It is known that absorption of inorganic salts is controlled by a number of factors, but above all, the need of the body for a particular salt, chiefly determines the degree of its absorption. As for instance, anaemic subjects absorb more iron than normal people.
Factor # 3. Storage:
There is ample evidence that almost all the essential inorganic elements can be stored in the body to some extent. As for instance, iron is stored in the liver, spleen, bone-marrow, etc., Ca, P, Mg in the bones, NaCl in the skin and subcutaneous tissue, etc. In times of need body can derive these elements from the store. It is found that, with regard to the inorganic salts, the body follows a principle of utmost economy.
Factor # 4. Specific Characteristic of the Cells:
On comparing the distribution of the various inorganic elements in the body it is seen that there is a remarkable difference between their distribution inside and outside the cells—For instance, Ca is almost solely present in the plasma and is practically absent from the corpuscles. Na also follows the same principle. But the distribution of Mg and K is just the reverse.
Moreover, the cell membrane of certain cells exhibits certain peculiarities of permeability. They allow certain types of ions to pass, not the others. This is best seen in the case of red blood corpuscles, the membrane of which allows the anions to pass, not the cations. With the help of this characteristic quality of the membrane, the cells control the distribution and metabolism of the inorganic elements in them.
Factor # 5. Endocrines:
The hormones are probably the chief controllers of mineral metabolism in human beings. Although the informations collected uptil now are fragmentary, yet there is ample evidence to show that number of hormones exert enormous influence upon mineral metabolism.
A brief summary is given below:
a. Adrenal cortex secretes mineralocorticoids, e.g., deoxycorticosterone, aldosterone, etc., which increase reabsorption of Na, chloride and bicarbonates and depress that of potassium and phosphates by renal tubules. Thus by controlling the excretion of these salts, the mineralocorticoids keep an adequate balance of inorganic ions in blood, other body fluids and tissue cells. Aldosterone is most effective in controlling the electrolytic balance.
b. Parathyroid controls Ca and P metabolism.
c. Thyroid controls metabolism of iodine. It has also got some effect on Ca metabolism through Thyrocalcitonin.
d. Posterior pituitary, with the help of antidiuretic hormone, controls the excretion of water and therefore the excretion of salts which remain dissolved in it.
e. Anterior pituitary, by its thyroid stimulating hormone or thyrotropic hormone (TSH) and adrenocorticotropic hormone (ACTH), regulates the secretion of thyroid and adrenal cortex respectively, and through these glands controls the mineral metabolism.
Factor # 6. Kidneys:
One of the normal functions of kidney is believed to be to maintain adequate concentration of salts and ions in the blood and tissue fluids. If excess salt is taken, the extra amount is readily excreted by the kidneys. In conditions of salt deprivation, the kidney tubules reabsorb these salts to the maximum degree so that almost nothing is excreted. In this way, kidneys act as one of the most important organs in maintaining the salt balance of the body.
Factor # 7. Nervous System:
Since nervous system controls the activity of all systems including the endocrine glands, it is reasonable to believe that it takes an important role in the control of mineral metabolism. A number of inorganic elements, for instance Na, K, Ca, Mg, etc., take an enormous role in maintaining the functions of nervous system. After stimulation of the vagus, potassium is found to increase in the perfusion fluid.
The hypothalamus sends a bundle of fibre-the supra-opticohypophyseal tract to the posterior pituitary, and controls the secretion of antidiuretic hormone. From similar facts it is reasonable to believe that the nervous system, particularly the vegetative nervous system, exerts some influence in regulating mineral metabolism.