UREA RECYCLING - AN AID TO THE EXCRETION OF POTASSIUM DURING ANTIDIURESIS

Urea absorption in the inner medullary collecting duct provides a mech anism to elevate the concentration of urea in the papillary interstiti al fluid and thereby permit the excretion of urea with as little water as possible. Urea reabsorption may have another important effect - to aid in the excretion of potassium (K). K excretion depends on two pro cesses: first, factors such as aldosterone which cause the concentrati on of K in the luminal fluid of the cortical distal nephron to be high and, second, factors which augment the flow rate through those nephro n segments. Since the osmolality of the luminal fluid in the cortical collecting duct (CCD) and plasma are equal when antidiuretic hormone a cts, the flow rate in the CCD is dependent on solute delivery. Urea is a major solute in the lumen of the CCD and thereby plays an important role in maintaining the CCD flow rate. Since urea and K are often fou nd in the same foods, having urea help the excretion of K is potential ly advantageous. If the excretion of urea was low, the flow rate in th e terminal CCD would decline. In this circumstance, the luminal K conc entration would have to rise in proportion to the fall in flow rate or there would be a diminished rate of excretion of K and, possibly, hyp erkalemia.