During filtration in the kidneys, red blood cells and plasma proteins do not pass into the urine because

• A. They are actively reabsorbed
• B. They are too large to pass through the filtration barrier
• C. They are destroyed by enzymes in the filtrate
• D. They are attracted to the urine

Answer: (B)

The filtration barrier in the kidneys acts as a size- and charge-selective sieve. Red blood cells are whole cells and are far too large to pass through the tiny pores of the glomerular capillary walls. Most plasma proteins are also relatively large and carry charge that the barrier helps repel, so they stay in the bloodstream. Because of this selective barrier, these components do not appear in the filtrate that becomes urine. Other options describe processes that don’t govern entry into the filtrate: destruction in the filtrate would not be how filtration works, attraction to urine isn’t a mechanism of filtration, and active reabsorption happens after filtration, not to prevent entry of these large components.