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M04.03.10 Filtration and Absorption
Filtration and absorption are critical processes governing fluid movement across capillary membranes. These processes are driven by two fundamental forces: Hydrostatic pressure and osmotic (oncotic) pressure.
Fundamental Forces Governing Fluid Movement
- Hydrostatic Force
- The pressure exerted by the fluid within the capillary.
- Influenced by blood pressure, blood flow, and blood volume.
- Osmotic (Oncotic) Force
- The force generated by solutes (primarily proteins) that cannot cross the membrane.
- Influences the movement of water based on solute concentration differences across the membrane.
Definitions
- Filtration: The movement of fluid from plasma into the interstitium.
- Absorption: The movement of fluid from the interstitium into plasma.
Starling Forces
Filtration Forces
- PCP_CPC = Hydrostatic pressure in the capillary
- Related to blood flow and regulated at the arterioles.
- πIF = Oncotic pressure in the interstitium
- Determined by protein concentration in the interstitial fluid.
Absorption Forces
- πC= Oncotic pressure of plasma
- Mainly due to plasma proteins, with albumin being the most significant contributor.
- PIFP = Hydrostatic pressure in the interstitium
- Typically close to zero or negative, thus often negligible in normal conditions but may become significant in pathological states (e.g., edema).
Fluid Flux Summary
| Type of Movement | Direction | Main Force |
|---|---|---|
| Filtration | Plasma → Interstitium | Hydrostatic Pressure PCP – Oncotic Pressure πIF |
| Absorption | Interstitium → Plasma | Oncotic Pressure (πC) -Hydrostatic Pressure PIFP |
Points to Remember
- Hydrostatic pressure drives fluid out of the capillary, while oncotic pressure draws fluid back in.
- Albumin is the most critical plasma protein influencing oncotic pressure.
- In normal physiological conditions, the balance between these forces maintains fluid homeostasis; however, conditions such as edema can disrupt this balance.
Bibliography
- Guyton, A.C., & Hall, J.E. (2016). Textbook of Medical Physiology. Elsevier.
- Moore, K.L., Persaud, T.V.N., & Torchia, M.G. (2013). Before We Are Born: Essentials of Embryology and Birth Defects. Elsevier.
- Johnson, L.R. (2013). Gastrointestinal Physiology. Elsevier.