During filtration in the kidney, red blood cells and plasma proteins do not appear in the filtrate because which reason?

• A. The capillary membrane blocks all substances from passing.
• B. The capillary membrane completely prevents passage of small molecules.
• C. Large molecules, such as blood and plasma proteins, do not fit through the membrane, so they cannot enter the tubules.
• D. Urine contains only urea and water, so other substances cannot pass through.

Answer: (C)

The filtration process in the kidney is selective by size and structure. The glomerular filtration barrier lets water and small solutes pass into the tubules to form filtrate, but large particles like red blood cells and most plasma proteins are too big (and often carry a charge) to pass through the barrier formed by the capillary endothelium, the basement membrane, and the podocyte slit diaphragms. Because of this size restriction, red blood cells and plasma proteins stay in the blood and do not appear in the filtrate. The other statements aren’t accurate: small molecules do pass, the barrier isn’t completely impermeable to all substances, and urine contains more than just urea and water.