Peculiarities of the urinary system functional state in neonates and its role in maintenance of the body homeostasis (literary review)

Abstract

The urinary system activity is of great importance for homeostasis maintenance, which is determined by its participation in elimination of final metabolites, foreign and toxic substances from the body; providing stable blood volume and extracellular fluid; constant concentration of osmotic active substances and ions; regulation of acid-alkali balance, arterial pressure, erythropoiesis, blood clotting, modulation of hormone action; participation in the processes of protein, lipid and carbohydrate metabolism. Clinicians should know peculiarities of formation of the main kidney functions in neonates depending on gestation term and day of life, which can enable to diagnose renal damage in time. Antenatal development of the urinary system occurs during embryonic and fetal periods. During the embryonic period all the nephron structures are formed, and during the fetal period the system is maturing. In case of disorders of normal embryogenesis of the urinary system during the first three months of gestation severe embryopathy is formed. Its severity depends on the damage of an appropriate gene. During later periods fetopathy develops under the effect of various factors promoting disorders of the processes of renal differentiation, and renal pathology is characterized by dys- and hypoplasia. A leading factor of the kidney growth during postnatal ontogenesis is enlargement of the amount of cells but not their sizes. After birth the systemic circulation is reorganized, systemic blood pressure and hydrostatic pressure in glomeruli increase; a relative area of glomerular capillaries, the size of afferent and efferent arterioles increase; and arteriole resistance decreases. Glomerular-tubular imbalance is typical for neonates. It consists of morphological and functional prevalence of the glomeruli over proximal tubules in the immature nephron. The structure of the glomerular membrane changes during the postnatal period: endothelial cells become flat, fenestrations appear, the common basal membrane is formed between the endothelium and podocytes, podocyte legs are formed in the cells of the visceral layer of the glomerular capsule resulting in 40% enlargement of the filtration surface. The processes of tubular maturation in neonates consist of the formation of a dense location and structure of specific transport proteins, electrochemical gradient of the cellular membrane, sensitivity to hormones, phospholipid content, the volume of intracellular fluid and cellular metabolic activity associated with anatomical and functional nephron heterogeneity. Anatomical immaturity of the urinary system in neonates determines peculiarities of its functional state, that is, insufficiently adequate glomerular filtration rate, decreased ability to reabsorb filtered amino acids and minor peptides, phosphates, bicarbonates, glucose and non-organic ions, decreased secretion of organic acids, alkali and medicines, as well as reduced concentration ability of the kidneys.Awareness of peculiarities of the renal function development in healthy neonates is essential for correct interpretation of the results of laboratory and instrumental examinations, understanding the mechanisms of development of pathologic deviations.