Substances are transported across the capillary wall in two main methods
- By diffusion : across receptors, channels
- By filtration : as a result of Starlings forces
Direction and amount of fluid transported by Starlings forces is determined by the hydro static and oncotic pressures on both sides of the capillary walls.
- Hydrostatic pressure : the pushing force exerted by a fluid when it is at rest
- Oncotic/colloid osmotic pressure : the pulling force exerted by small, insoluble particles in a fluid
These pressures act separately on both the capillary fluid and the interstitial fluid.
- Capillary hydrostatic pressure (Pc) : Blood flowing inside the capillaries exerts a hydrostatic pressure on the walls, trying to move out of the capillaries
- Capillary oncotic pressure (πc) : Dissolved proteins (plasma proteins) in blood try to draw water from the ECF, resulting in an inward colloid pressure.
- Interstitial fluid hydrostatic pressure (Pi) : Blood in the ISF exerts a pressure on capillary walls, trying to move inside.
- Interstitial fluid oncotic pressure (πi) : Comparatively lesser amount of proteins in the ECF try to draw water out of the blood vessels
The net movement of fluid across the capillary wall is decided by the Starling Equation
Fluid movement = k (Pc – Pi – πc +πi )
Here ‘k’ is known as the capillary permeability, decided by the size and number of pores on the capillary wall