Colloid Osmotic Pressure & Capillary Exchange

Capillary Bulk Flow

• Exchange site for water & small solutes (O_2, glucose, ions, hormones)
• Driven by capillary blood (hydrostatic) pressure
• Plasma proteins are too large → remain intravascular

Arterial End (Filtration)

• Higher hydrostatic pressure pushes fluid out to interstitial space
• Loss of water increases intravascular protein concentration

Venous End (Reabsorption)

• \uparrow plasma protein concentration ⇒ \uparrow solute concentration
• Water moves back into capillary via osmosis (colloid osmotic pressure)

Colloid Osmotic Pressure (COP)

• Generated by plasma proteins (mainly albumin)
• Pulls fluid into bloodstream opposing hydrostatic pressure

Physiological Role

• Reclaims most filtered fluid across millions of capillary beds
• Maintains blood volume → sustains arterial blood pressure
• Loss of COP (e.g., low plasma proteins) ⇒ edema & hypotension