Why does the concentration of urea increase in the proximal tubule?

The concentration of urea increases in the proximal tubule primarily due to the reabsorption of water and solutes. As water is reabsorbed from the filtrate, the remaining urea becomes more concentrated. Additionally, urea is passively reabsorbed to some extent, contributing to its higher concentration in the proximal tubule compared to the initial filtrate. This process helps maintain osmotic balance and facilitates the kidney's ability to concentrate urine.