Starling Equation - The Equation

The Equation

The Starling equation reads as follows:

where:

• is the net fluid movement between compartments.
• is the net driving force,
  • P_c is the capillary hydrostatic pressure
  • P_i is the interstitial hydrostatic pressure
  • π_c is the capillary oncotic pressure
  • π_i is the interstitial oncotic pressure
  • K_f is the filtration coefficient – a proportionality constant
  • σ is the reflection coefficient

By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (J_v). If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables. Note that previously it was believed that at steady state the arterial capillaries filter fluid and the venous capillaries reabsorb it, as shown by the diagram. Though many physiology textbooks still use this misconception, modern evidence shows that in most cases venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force. This indicates that capillaries are normally in a state of filtration along their entire length.

Pressure are often measured in millimetres of mercury (mmHg), and the filtration coefficient in millilitres per minute per millimetre of mercury (ml·min−1·mmHg−1).

In essence the equation says that the net filtration (J_v) is proportional to the net driving force. The first four variables in the list above are the forces that contribute to the net driving force.