Figure 131

Physiologic approach to polyuric disorders. Among euvolemic hyper-natremic patients, those affected by polyuric disorders are an important subcategory. Polyuria is arbitrarily defined as urine output of more than 3 L/d. Urine volume can be conceived of as having two components: the volume needed to excrete solutes at the concentration of solutes in plasma (called the osmolar clearance) and the other being the free water clearance, which is the volume of solute-free water that has been added to (positive free water clearance [CH2O]) or subtracted (negative CH2O) from the isotonic portion of the urine osmolar clearance (Cosm) to create either a hypotonic or hypertonic urine.

Consumption of an average American diet requires the kidneys to excrete 600 to 800 mOsm of solute each day. The urine volume in which this solute is excreted is determined by fluid intake. If the urine is maximally diluted to 60 mOsm/kg of water, the 600 mOsm will need 10 L of urine for effective osmotic clearance. If the concentrating mechanism is maximally stimulated to 1200 mOsm/kg of water, osmotic clearance will occur in a minimum of 500 mL of urine. This flexibility is affected when drugs or diseases alter the renal concentrating mechanism.

Polyuric disorders can be secondary to an increase in solute clearance, free water clearance, or a combination of both. ADH—antidi-uretic hormone.

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