Learning Objective: By the end of this lesson, the learner will be able to explain the determinants of fluid flux across capillary membranes using hydrostatic and osmotic forces, differentiate between filtration and absorption, and predict changes under physiological and pathological conditions.
Fundamental Forces Governing Capillary Fluid Exchange
- Hydrostatic pressure (P): Pressure exerted by fluid, pushes water out of capillaries into the interstitium
- Osmotic (oncotic) pressure (π): Pressure exerted by solutes (mainly proteins) that cannot cross the capillary membrane, pulls water into capillaries
Fluid Movements
- Filtration: Fluid moves from plasma → interstitium
- Absorption: Fluid moves from the interstitium → plasma
Activity
Starling Forces
| Force | Symbol | Direction | Determinants |
|---|---|---|---|
| Capillary hydrostatic pressure | PCP_C | Out of the capillary | Blood flow, arteriole resistance, venous pressure, and blood volume |
| Interstitial oncotic pressure | πIF\pi_{IF} | Out of the capillary | Protein concentration in interstitial fluid (normally minor) |
| Plasma oncotic pressure | πC\pi_C | Into capillary | Plasma proteins, mainly albumin |
| Interstitial hydrostatic pressure | PIFP_{IF} | Into capillary | Usually close to zero or negative; rises in edema |
Net Filtration/Absorption:
- Positive → net filtration
- Negative → net absorption
Activity
Clinical Relevance
Edema:
Hypoalbuminemia:
↓ plasma oncotic pressure → reduced absorption → edema
Glomerular filtration:
Bowman’s space hydrostatic pressure (PBSP_{BS}) is analogous to PIFP_{IF} in determining net filtration
