Filtration in the glomerulus occurs primarily because there is more pressure inside than outside.

• A. There is more pressure outside than inside
• B. Active pumping of filtrate into tubule
• C. There is more pressure inside than outside
• D. The presence of valves in capillaries

Answer: (C)

Filtration in the glomerulus is driven by a hydrostatic pressure gradient, a passive force that pushes fluid from the inside of the glomerular capillaries into Bowman's space. The pressure inside the capillaries is higher than the pressure in Bowman's space, so fluid moves out of the blood into the filtrate. In numbers, glomerular capillary hydrostatic pressure is high, Bowman's space pressure is lower, and opposing forces like plasma oncotic pressure reduce filtration but do not negate the primary inward push. That higher inside pressure relative to outside is what makes filtration occur.

The remaining ideas aren’t the driving force: filtration isn’t due to active pumping into the tubule, external higher pressure outside would not promote filtration, and valves in capillaries don’t drive this process.