Starling's Law of Fluid Exchange

Definition: 

        Fluid exchange across the capillary wall obeys Starling's Law,
        which is stated in the equation below.  

Importance and Relevance:

	The problem of keeping the alveoli free of fluid is critical
because only a very thin tissue lining separates the capillary blood from
the air in the lung.  If the alveoli fill with fluid, they cannot
receive adequate ventilation, so the arterial pressure of oxygen would
decrease.  

	The Starling equation describes the net force out from the lung 
that keeps fluid out.  The force pushing fluid out of the capillary
is the hydrostatic pressure difference, Pc - Pi. The force pulling fluid
in is the colloid osmotic pressure difference, pi_c - pi_i, multiplied by
the reflection coefficient, which is a measure of effectiveness of the
capillary wall in keeping proteins out.

	This equation cannot be used for much because we cannot measure
some of the values.  We know pi_c.  We assume that Pc is halfway between
arterial and venous pressure, but it changes depending on location in the
lung.  Pi_i is  not known, so we must guess a value for it.  Pi is also
unknown, but it is known to be much lower than atmospheric pressure.  The
result of the Starling equation is that the net flow is probably out of
the alveoli, creating a small lymph flow.  

The Starling Equation:

	Net Flow Out = K[(Pc - Pi) - sigma(pi_c - pi_i)]

	Pi = hydrostatic pressure of interstitial fluid around
	     capillary
	Pc = hydrostatic pressure in capillary
	pi_c = colloid osmotic pressure in capillary
	pi_i = colloid osmotic pressure in interstitial fluid
	sigma = reflection coefficient
	K = filtration coefficient

A sample problem:

Calculate the lymph flow in mL/hr given that the colloid osmotic pressure of the interstitial fluid is 20 mm Hg, the colloid osmotic pressure of the capillary is 28 mm Hg, the hydrostatic pressure difference is 26 mm Hg, K is 2 mL/hr*mm Hg, and sigma is 2.

Answers:

20 mL/hr

For more information on this topic, please refer to West , pages 45, 170.

Also, check out the following links that may be helpful:

• Control of GFR
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by Valerie Leupp , a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.