Water-Salt Exchange

a complex of processes of absorption, distribution, consumption, and excretion of water and salts in animals and man. Water-salt exchange maintains constant osmotic pressure, ionic composition, and acid-base equilibrium of the body’s internal environment (homeostasis).

A man weighing 70 kg requires about 2.5 l of water per day: 1.2 l as drinking water, 1 l with food, and 0.3 l is formed in the body (1.07 g, 0.556 g, and 0.396 g of water are formed during the oxidation of 1 g of fat, carbohydrates, and proteins respectively). The total water content of the human body is over 60 percent, including 40 percent within the cells in the form of water of hydration and poorly exchangeable water, 4.5 percent in the blood vessels, and 16 percent in intercellular fluid. The body contains sulfate, phosphate, bicarbonate, Na⁺, K⁺, Ca^+-, Mg⁺⁺, and Cl^- ions, which determine the nature of the physicochemical reactions in the tissues. The body also needs trace elements—iron, zinc, cobalt, copper, and so on—which participate in oxidation-reduction reactions, activate enzymes, and are part of vitamins and other biologically active substances. Electrolytes are absorbed in the intestine with the participation of enzymes and systems of active transport for ions. Absorbed ions enter the blood or lymph and are transported to all the cells. The extracellular and intracellular fluids differ substantially from each other in their salt composition. The K⁺ and Mg⁺⁺ ions and the phosphates predominate in the cells, and the Na⁺, Ca⁺⁺, and Cl^- ions predominate outside the cells. This difference is maintained by the activity of biological membranes and by the binding of ions by the chemical components of the cell—for example, brain, muscle, and liver phospholipids bind more sodium than potassium. The body also has salt reserves. Bone tissue contains a great deal of calcium; various minerals, including trace elements, are deposited in the liver.

Freshwater animals excrete water, which is resorbed by the skin and intestine, through the kidneys or their analogues (in invertebrate animals). They get salts with food or extract them from the environment by means of special cells located in the gills (in fish), skin (in reptiles), and so on. The blood of some marine animals—for example, mollusks—has the same osmotic pressure as seawater. Other animals have a capacity for osmoregulation (marine teleosts, reptiles, and so on). The blood of these animals contains a lesser quantity of salts than does seawater; they drink seawater with a high salt content and desalinize (dilute) it by excreting concentrated sodium chloride solutions through the salt glands (the nasal gland of reptiles and birds and the gills of teleosts). Magnesium and calcium salts are excreted by the intestine and kidneys. Sharks, skates, and several other marine animals have a high urea concentration in their blood and body fluids; their organism receives water chiefly through the skin according to the osmotic gradient. The principal organs regulating the water balance in mammals are the kidneys. In case of an excess of water, the kidneys excrete dilute urine; in case of dehydration, the urine is concentrated.

The water-salt exchange is regulated neurohormonally. A change in the osmotic pressure of the blood stimulates special sensory structures (osmoreceptors); information from them is transmitted to the central nervous system and from it to the posterior lobe of the pituitary. An increase in osmotic pressure stimulates the secretion of antidiuretic hormone, which reduces the excretion of water in the urine. An excess of water inhibits the secretion of this hormone and stimulates the excretion of water through the kidneys. The volume of body fluids is maintained by a special regulatory system, whose receptors react to changes of the blood volume of the major vessels, heart chambers, and so on. The result is a reflex stimulation of the secretion of hormones that influence the kidneys to alter the excretion of water and sodium salts. Vasopressin and glucocorticoids are the most important hormones in the regulation of water metabolism; aldosterone and angiotensin, in sodium metabolism; and parathyroid hormone and calcitonin, in calcium metabolism. The central nervous system coordinates the activity of the various organs and systems to maintain water-salt homeostasis. The regulation of the ionic and osmotic stability of the internal environment becomes increasingly efficient during phylogenesis.