Understanding Filtration: A Passive Process Explained

Filtration is a passive process — it occurs without cellular energy (ATP) expenditure, driven solely by physical forces across a semipermeable membrane.

Governing Forces (Starling Forces):

• Hydrostatic pressure gradient (ΔP): Pushes fluid out of capillary (blood pressure).
• Oncotic (colloid osmotic) pressure gradient (Δπ): Pulls fluid in (plasma proteins, mainly albumin).

Net Filtration Pressure (NFP) = (P_c – P_if) – (π_c – π_if)

• P_c = capillary hydrostatic pressure (~35 mmHg arterial end, ~15 mmHg venous end)
• P_if = interstitial fluid hydrostatic pressure (~0 to -3 mmHg)
• π_c = capillary oncotic pressure (~25–28 mmHg)
• π_if = interstitial oncotic pressure (~5–10 mmHg)

Result:

• Arterial end: NFP > 0 → filtration out (nutrient delivery).
• Venous end: NFP < 0 → reabsorption in (waste removal).
• Net ~10% filtered volume → returned via lymphatics.

Key Features of Passive Transport:

• No metabolic energy required.
• Direction determined by gradient (high → low pressure/concentration).
• Rate proportional to gradient, surface area, permeability (hydraulic conductivity).
• Not under voluntary/neural control (unlike active transport, vesicular transport).

Physiological Examples:

• Glomerular filtration (kidney) — driven by glomerular hydrostatic pressure.
• Capillary exchange (systemic).
• Filtration in spleen, liver sinusoids.