What are the common kidney causes of hypovolemia?
• Diuretics impair kidney sodium reabsorption. Overuse can cause volume contraction.
• Osmotic diuresis can cause volume contraction. The presence of a nonreabsorbable solute in the tubular fluid inhibits sodium and water reabsorption. In uncontrolled diabetes mellitus, the rate at which glucose is filtered by the glomeruli exceeds the rate at which glucose can be reabsorbed by the renal tubules. Glucose in the tubular fluid acts as an osmotic agent and can produce large losses of sodium and water.
• Adrenal insufficiency can cause volume depletion and hypotension. In this disorder, aldosterone deficiency leads to kidney sodium losses.
• Diabetes insipidus is due to impaired ADH secretion (central diabetes insipidus) or impaired kidney response to ADH (nephrogenic diabetes insipidus). These disorders are characterized by decreased water reabsorption by the collecting tubules and by the excretion of large volumes of dilute urine. In well patients, this water loss will be matched by water intake. In patients with altered mental status or without easy access to water, however, ongoing water losses may produce hypernatremia and volume contraction.
• Bilateral urinary tract obstruction can produce kidney failure and volume overload. Urine output often increases after relief of bilateral urinary tract obstruction. In almost all cases, however, this post-obstructive diuresis represents physiologic correction of volume overload. Attempts to match urine output with intravenous fluid intake will lead to persistently high urine output.