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Fluid Filtration Across Capillaries: Starling Forces

  • Filtration = Capillary Filtration Coefficient (Kf) × Net Filtration Pressure

Startling Forces

  • are driving forces that move the fluid across the capillary wall.

  • Hydrostatic pressure is exerted by fluid. It pushes fluid away.

  • Colloid osmotic pressure is exerted by proteins. It pulls fluid close.

Hydrostatic pressure in capillary (Pc)
  • Pushes fluid out of the capillary

  • At arterial end: 30 mmHg

  • At venous end: 10 mmHg

Hydrostatic pressure in interstitium (Pif)
  • Positive hydrostatic pressure in interstitium → pushes fluid into the capillary.

  • However, due to fluid removal by lymphatics → it's negative → pulls fluid out of capillary.

  • About -3 mmHg.

Colloid osmotic pressure in capillary (πp)
  • Exerted by plasma proteins.

  • Pulls fluid into capillary.

  • about 28 mmHg.

Colloid osmotic pressure in interstitium (πif)

Although capillary wall is highly impermeable to proteins, some proteins do leak through pores and by transcytosis


These proteins and proteoglycans produce colloid osmotic pressure in interstitium


Pulls fluid into interstitium.

  • About 8 mmHg

Net Filtration Pressure

  • Net Filtration Pressure = Pc - Pif - πp + πif

  • Where it is positive → there is net fluid movement out of capillary i.e. filtration

  • Where it is negative → there is net fluid movement into the capillary i.e. reabsorption

At Arteriolar End
  • Net filtration pressure = Pc - Pif - πp + πif = 30 - (-3) - 28 + 8 = 13 mmHg

  • Its positive → favors filtration of fluid.

At Venous End
  • Net filtration pressure = Pc - Pif - πp + πif = 10 - (-3) - 28 + 8 = -7 mmHg

  • Its negative → favors reabsorption of fluid.

  • At venous end, capillaries are more numerous and more permeable → this pressure is sufficient for reabsorption of most of the fluid. Some fluid that is left, is taken up by lymphatics.

Capillary Filtration Coefficient (Kf)

  • Its permeability of capillary.

  • Depends on

    • number and size of the pores in the capillary

    • number of capillaries.

  • Expressed as the net fluid filtration rate for each mmHg of the net driving force.

  • The average capillary filtration coefficient for the whole body is 6.67 ml/min.

  • However, capillaries in different tissues have different permeability → exact filtration coefficient is different in different tissues.


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