Passive tubular reabsorption involves water moving from the tubules into the blood. This process causes the solutes left in the tubular fluid to:

• A. Decrease in concentration
• B. Becoming diluted
• C. Increase in concentration
• D. Exit into urine

Answer: (C)

Water reabsorption by osmosis pulls solvent out of the tubular lumen, reducing the tubular fluid volume while the total amount of solutes in the fluid stays roughly the same. With less liquid but about the same solute, the concentration of solutes increases. In other words, as water leaves, the remaining solutes become more concentrated. They aren’t diluted by added water or fully dumped into urine at this stage, so the right idea is an increase in concentration.