Why does urine contain more urea than blood plasma?

Urine contains a significantly higher concentration of urea than blood plasma primarily because of the way the kidney processes glomerular filtrate.12

As the glomerulus filters blood, it produces a large volume of liquid that initially contains urea at the same concentration as plasma. However, as this filtrate travels through the renal tubules, the body reabsorbs a vast majority of the water—approximately 99 percent—to maintain fluid balance.1324

While some urea is reabsorbed into the blood, the tubular epithelium is less permeable to urea than it is to water. Consequently, as water is removed from the filtrate, the concentration of the remaining urea in the tubular lumen rises steadily. Because the kidneys are designed to eliminate urea—a nitrogenous waste product of protein metabolism—rather than retain it, this process results in a final urine concentration of urea that is roughly 60 times higher than that found in blood plasma.315