During ultrafiltration, which component is not filtered into the filtrate?

• A. Water
• B. Plasma protein
• C. Urea
• D. Glucose

Answer: (B)

The main idea is that the glomerular filtration barrier is selective for size (and charge): small molecules and water pass into the filtrate, while large proteins stay in the blood. Water and small solutes like urea and glucose are able to cross the barrier and become part of the filtrate. Glucose and other small molecules are subsequently reabsorbed in the tubules, but that reabsorption happens after filtration. Plasma proteins are large enough that they cannot pass through the barrier under normal conditions, so they remain in the bloodstream and do not appear in the filtrate. This is why plasma proteins are not filtered into the filtrate.