Polyuria

Polyuria is diagnosed when more than 3 liters of urine are excreted per day and its osmotic pressure is less than 300 mmol/kg. When water reabsorption is disturbed in the nephrons, a large amount of water is excreted – polyuria. The V₂ type vasopressin-selective receptors are present in the membrane of the distal tubular cells of the kidneys. When vasopressin stimulates these receptors, adenylate cyclase is activated, synthesizing cAMP, which activates protein kinase A, phosphorylating cellular proteins, including aquaporins, which form water channels. Through this channel, water returns from urine to the tubular cells of the kidneys, then to the parenchyma and into the blood. Therefore, vasopressin is the only hormone that activates water, but not electrolyte (sodium ions) reabsorption. Due to mutations in the V₂ receptors gene, the signal of vasopressin is not transmitted to the target cell, and due to mutations in the aquaporin gene, the water channel does not form, or its formation is incomplete. In both cases, water reabsorption in the kidney tubules is disrupted, and a large amount of urine is excreted – polyuria. Daily diuresis is 3.15 liters, sometimes – 20-40 liters. The excreted urine is unconcentrated – relative density – 1000-1005 g/l. Due to polyuria, generalized tissue dehydration occurs. Plasma osmolality increases (more than 290 mosmol/kg), the volume of intracellular and plasma fluid decreases, and the resulting thirst indicates disrupted water homeostasis. When blood volume decreases, aldosterone synthesis is stimulated through the renin-angiotensin system, which increases sodium ion reabsorption in the kidney nephrons. Therefore, the concentration of these ions in the blood and interstitial fluid increases even more (hypernatremia). If there is no possibility to drink and replenish the excreted water, life-threatening dehydration of the body occurs.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė