Starling equation

Starling equation is a mathematical formula that illustrates the physiological principles of fluid balance and exchange across capillary walls. It was formulated by the British physiologist Ernest Starling in 1896. The equation is fundamental to understanding how fluid balance is maintained within the body.

Overview

The Starling equation is expressed as follows:

Jv = Kf [(Pc - Pi) - σ(πc - πi)]

where:

• Jv is the net fluid movement due to filtration
• Kf is the filtration coefficient
• Pc is the capillary hydrostatic pressure
• Pi is the interstitial hydrostatic pressure
• σ is the reflection coefficient
• πc is the capillary oncotic pressure
• πi is the interstitial oncotic pressure

Principles

The Starling equation is based on two main principles: the hydrostatic pressure gradient and the oncotic pressure gradient.

Hydrostatic Pressure Gradient

The hydrostatic pressure gradient is the force exerted by the fluid against the capillary wall. This pressure tends to push fluid out of the capillary into the interstitial space. The hydrostatic pressure is higher at the arterial end of the capillary, causing fluid to move out of the capillary.

Oncotic Pressure Gradient

The oncotic pressure gradient is the osmotic pressure exerted by proteins, mainly albumin, in the blood plasma and the interstitial fluid. This pressure tends to draw fluid back into the capillary. The oncotic pressure is higher in the capillary than in the interstitial fluid, causing fluid to move into the capillary.

Clinical Significance

The Starling equation is used to understand the pathophysiology of various medical conditions such as edema, heart failure, and kidney disease. It helps in predicting the movement of fluids and solutes in different disease states.

See Also

• Capillary
• Fluid balance
• Hydrostatic pressure
• Oncotic pressure
• Edema
• Heart failure
• Kidney disease