INTRODUCTION: When staying in hyperbaric conditions there is an increase in diuresis, developing a negative water balance, loss of electrolytes and tissue dehydration after diving different in depth, duration, intensity and other parameters, which can be conditionally combined into specific physiological symptom complex – “pressure diuresis” phenomenon (or “diver diuresis”). OBJECTIVE: To investigate the mechanisms of “pressure diuresis” under the action of high gaseous medium pressure and determine the physiological nature of this phenomenon in divers. MATERIALS AND METHODS: 44 men aged 19–23 were examined. In the first study all the subjects had initial sustainability to hyperbaric adverse factors (decompression gas formation – DG, hypoxic hypoxia – HH, nitrogen toxic effect – NTE and oxygen toxic effect – OTE). Hormonal status and body’s osmotic homeostasis rates were defined in all the subjects. The second study evaluated changes of the subjects’ water-electrolyte metabolism, kidney function and osmotic homeostasis during the oral loading renal test with the water load in the baseline and conditions of hyperbaric adverse factor exposure on divers. Statistics: To perform statistical analysis, application packages of Statistica for Windows 10.0 were used. RESULTS: 17 (38,6 %), 29 (65,9 %), 20 (45,5 %) and 35 (79,6 %) subjects had medium and low resistance to DG, HH, OTE and NTE, respectively; 27 (61,4 %), 15 (34,1 %), 24 (54,5 %) and 9 (20,4 %) subjects had high resistance to DG, HH, OTE and NTE. Increase in the concentration of antidiuretic hormone was identified in the diving group with low and medium resistance to NTE, HH, and OTE (by 146,8, 141,1 and 93,2 %, respectively, compared to the results before hyperbaric adverse factor exposure). The highest concentration increase of aldosterone was found among the divers with low and medium resistance to OTE, DG and HH (by 41,5, 39,1 and 36,2 %, respectively). The increase in blood plasma osmolality was observed in the subjects with low and medium resistance to OTE (an increase by 6,6 % compared to the divers with high resistance). A significant reduction in blood plasma osmolality was recorded in the subject group with low and medium resistance to NTE and HH (reduction by 5,2 and 4,2 %, compared to the group with high resistance). The most significant decrease in urine osmolality was identified in the diving group with low and medium resistance to DG and OTE (reduction by 14,5 and 17,7 %, respectively). A significant increase in urine osmolality under the action of hyperbaric factors was defined in the diving group with low and medium resistance to NTE and HH (by 19,8 and 19,3 %). DISCUSSION: The data obtained indicate the emergence of a new body hydration status during person’s staying in hyperbaric conditions. Body hydration status will be connected with its individual resistance to hyperbaric adverse factor effect. Thus, with the development of marked DG iso-osmotic (isotonic) hyperhydration is formed, with OTE hyperosmotic (hypertonic) hyperhydration occurs, with NTE and HH hypo-osmolar (hypotonic) hyperhydration develops. CONCLUSION: The study showed a certain connection (correlation) between types of hyperhydration in the tissues of a diver’s body, mechanisms of occurrence, low and medium initial resistance to a particular hyperbaric factor. “Pressure diuresis” phenomenon (“diver diuresis”), that occurs in divers with high resistance to hyperbaric adverse factors, will be a normal body’s physiological response, aimed at eliminating hypervolemia and reduction in the volume of circulating plasma.
