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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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