WATER-ELECTROLYTE OBMEN: MODERN PRESENTATIONS AND MECHANISMS (Review of scientific publications)

Abstract. Water and salt balance is a key component of maintaining homeostasis of the organism. Water and electrolytes ensure normal functioning of cells, regulate osmotic pressure, acid-base balance, as well as the work of cardiovascular and neuromuscular systems. The physiological basis of water and electrolyte distribution, regulation mechanisms involving kidneys, respiratory system, skin, gastrointestinal tract and hormonal factors (ADH, aldosterone, renin-angiotensin system, natriuretic peptides) are considered in the article. Special attention is paid to the clinical significance of disorders of water-salt metabolism, including dehydration, hyperhydration and electrolyte imbalances. Diagnostic methods and principles of correction based on restoration of fluid volume and normalization of electrolyte composition are described. A promising direction is the introduction of individualized approaches to therapy and the development of technologies for monitoring hydration status. 

Keywords: water-salt balance, water, homeostasis of the organism, electrolytes, dehydration, aquaporin, hyponatremia, hypernatremia, hypokalemia, hyperkalemia. 

Introduction. Water and salt balance is one of the key conditions for maintaining homeostasis of the organism. Optimal content of water and electrolytes is necessary for normal functioning of cells, organs and systems. Disturbances of water-salt metabolism accompany a wide range of pathological conditions - from dehydration in infections to edema in cardiac or renal failure. This determines the relevance of further study of the mechanisms of regulation and development of new methods of diagnosis and correction.  

Purpose: To analyze modern ideas about the physiological basis and mechanisms of regulation of water-electrolyte metabolism, as well as to consider the clinical significance of its disorders. 

Materials and Methods. Analysis of scientific publications included in scientific bases such as CyberLeninka, PubMed, National Library of Medicine, Medscape. 

Results and  Discussion: Total water accounts for about 60% of the body weight of an adult (up to 75% in newborns and less than 50% in the elderly). Intracellular fluid contains approximately 2/3 of the total volume of water and extracellular fluid contains about 1/3, including blood plasma and interstitial space. The body's water balance is maintained through a variety of interrelated mechanisms that ensure that the volume of water intake (from drinking fluids, food, and as a result of metabolic processes) is coordinated with its losses through the kidneys, skin, gastrointestinal tract, and respiratory organs. The degree of hydration of the body is directly determined by the state of water balance. Hydration deficit develops when a decrease in the available volume of water due to its excessive losses or insufficient intake activates adaptation mechanisms aimed at limiting further fluid losses and increasing its intake into the body [6]. The distribution of electrolytes is strictly ordered: Sodium (Na⁺), chloride (Cl-), and bicarbonate (HCO₃-) dominate the extracellular sector; potassium (K⁺), magnesium (Mg²⁺), and phosphate dominate the intracellular sector. This distribution ensures maintenance of osmotic pressure, transmembrane potential and normal operation of cardiovascular and neuromuscular systems. The central role in the regulation of water-electrolyte metabolism belongs to the kidneys, which provide filtration, reabsorption and secretion processes. Through the regulation of glomerular filtration, selective reabsorption of sodium, potassium, chlorine and water, as well as through the participation of aquaporins, the kidneys maintain the stability of the internal environment [9]. The maintenance of acid-base equilibrium through the secretion of hydrogen ions and reabsorption of bicarbonate is also of key importance. The endocrine system also plays an important role. Antidiuretic hormone regulates the expression of aquaporins in the collecting tubes and enhances water reabsorption. The reninangiotensin-aldosterone system controls sodium and water reabsorption and vascular tone [11]. At the same time, natriuretic peptides promote sodium and fluid excretion, acting antagonistically to aldosterone and vasopressin [2]. Calcium metabolism is regulated by parathormone and calcitonin. The regulation of thirst and secretion of antidiuretic hormone is controlled by the hypothalamus, where osmoreceptors sensitive to changes in plasma osmolarity are located [1]. When osmolarity increases, they activate the thirst center and increase vasopressin secretion, which leads to fluid retention and restoration of balance. At the molecular level, aquaporins - channel proteins that provide rapid water transport through cell membranes - are of key importance. ACPs contribute to various cellular processes including cell migration, adhesion and polarity, and act upstream from several intracellular and intercellular signaling pathways to regulate processes such as cell proliferation, apoptosis and cell invasiveness [10]. In the regulation of ionic transport, the main function is performed by sodium-potassium ATPase, which maintains the transmembrane gradient, as well as various channels and transporters in the kidneys, which are targets for the action of aldosterone and a number of pharmacological agents. The clinical significance of disorders of water-electrolyte metabolism is extremely high. Fluid deficiency is manifested by various forms of dehydration and electrolyte disorders - hyponatremia, hypernatremia, hypo- and hyperkalemia [11]. Hyponatremia is the most common electrolyte balance disorder, diagnosed in more than 15% of hospitalized patients. The most frequent cause of hypotonic hyponatremia is considered to be the syndrome of inadequate antidiuresis (SNSAD). The pathogenesis of this condition is related to the nonosmotic release of arginine vasopressin (AVP, formerly antidiuretic hormone), which, interacting with renal V2 receptors, increases water reabsorption and promotes its retention in the body. As a result, there is a dilution of plasma and a decrease in sodium concentration. The etiologic spectrum of SNSAD is quite broad. The main causes include malignant tumors, lung diseases and pathologies of the central nervous system. However, in clinical practice, hyponatremia often has a multifactorial origin, which complicates diagnosis and requires dynamic monitoring. Regular reassessment of the patient's clinical condition and monitoring of biochemical parameters allow timely detection of additional pathogenetic mechanisms that may manifest themselves during therapy. According to modern clinical recommendations, in the absence of life-threatening manifestations (e.g., seizures, cerebral edema), the first line of therapy is fluid restriction (FR). Despite its widespread use, this approach is ineffective in about half of patients. In such cases, there is a need for second-line methods, which are currently under active investigation. Therapeutic strategies such as the use of tolvaptan, urea, and sodium-glucose cotransporter type 2 (SGLT2) inhibitors are of particular interest. SNSAD represents a major cause of hyponatremia, which has important clinical significance in the inpatient setting [12]. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for renal fluid retention in hyponatremia. However, there have been no studies demonstrating improvement in severe outcomes or long-term prognosis. A possible reason is that vasopressin receptors exert many secondary effects beyond AQP2 function. It is expected that the development of drugs that directly target AQP2 may lead to improved specificity and efficacy in the treatment of water balance disorders [13]. Excessive fluid accumulation accompanies edematous syndromes in cardiac, renal, or hepatic insufficiency and is also observed in the syndrome of inadequate ADH secretion [5]. Disturbances at the molecular level include hereditary nephropathies caused by mutations in aquaporin and ion channel genes. In clinical practice, pharmacological agents affecting water-electrolyte balance, such as diuretics and vasopressin antagonists, are actively used [7]. Thus, waterelectrolyte metabolism is a complex multilevel system that ensures the maintenance of homeostasis. Modern research has led to a deeper understanding of the role of the kidneys, endocrine system and molecular mechanisms in the regulation of fluid and electrolytes. Particular attention has been paid to the study of aquaporins and ion channels, which are becoming important targets for new therapeutic approaches [8]. 

Conclusion. Water-electrolyte metabolism plays a key role in maintaining homeostasis of the organism. It is regulated at the systemic, organ and molecular levels with the participation of kidneys, endocrine system, aquaporins and ion channels. Disturbances in this balance underlie many pathological conditions, including dehydration, edema and electrolyte disorders. Modern research opens new possibilities for diagnosis and therapy, which makes the study of this topic relevant in both basic and clinical medicine.