Starling’s equation quantifies the movement of fluid into and out of a capillary as a result of filtration. It is based on two important variables: hydrostatic and oncotic pressure.
But before getting too scientific, let’s get back to the basics… Capillaries are the tinniest blood vessels in the body. They are the main site of gas and nutrient exchange for the tissues that they serve.
The fluid (ie: blood) inside the capillaries has a particular pressure associated with it. This is known as the “hydrostatic” pressure. In addition, there are numerous proteins floating around in the blood. These proteins determine the “oncotic” pressure.
Fluid, and to some degree proteins, are able to seep into and out of the capillary. However, the amount of “seepage” is based not only on the capillaries’ hydrostatic and oncotic pressure, but also on the hydrostatic and oncotic pressure of the surrounding tissues.
The tissue surrounding capillaries is composed of cells and their supporting structures. Depending on the particular tissue type, the supporting structures are generally proteins like collagen and long chain carbohydrate molecules known as proteoglycans. All of these molecules are collectively referred to as the “cellular interstitium”. The interstitium is where fluid seeps into and out of the capillary network.
The Equation
Taking the hydrostatic and oncotic pressures of the blood and interstitium into account we can predict which way fluid will move: in to or out of the capillary. This can be done numerically as shown in the following equation:
Driving pressure across capillary wall ≡
(PHS capillary – PHS interstitum)
– (PO capillary – PO interstitium)
HS = hydrostatic pressure
O = oncotic pressure
If this number is positive, it means that fluid wants to leave the capillary and enter the interstitium (ie: there is a large driving pressure trying to push fluid out of the capillary); if it is negative, it means that fluid wants to leave the interstitium and re-enter the capillary.
Now to throw a monkey wrench into the equation… Although the hydrostatic and oncotic pressures are the main driving forces there are two additional factors that must be taken into account.
The first of these factors is the filtration co-efficient. It is based on how large and “leaky” the capillary wall is. Simply stated, if the capillary wall is large and leaky then more fluid can be filtered across it, duh! Increased leakiness can be caused by many different things such as histamine release (ie: allergies), mechanical damage to the capillary, etc.
The second factor that can alter the above equation is known as the reflection co-efficient. It is based on the fact that some proteins from the blood are able to cross the vessel wall into the interstitium. This effectively reduces the oncotic pressure within the capillary, and increases the oncotic pressure within the interstitium. Suffice it to say that different capillary beds have different reflection co-efficients depending on which organ system is being studied.
Taking everything into account we get the following equation:
Driving pressure across capillary wall ≡
Kf(PHS capillary – PHS interstitum)
– σ(PO capillary – PO interstitium)
HS = hydrostatic pressure
O = oncotic pressure
Kf = filtration co-efficient
σ = reflection co-efficient
Overview
Starling’s equation predicts how much fluid will be filtered into, or out of, a capillary. It is based on four things: oncotic pressure, hydrostatic pressure, the reflection co-efficient, and the filtration co-efficient.
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References and Resources
- Manthous CA. Starling’s equation and bedside critical care. J Crit Care. 2008 Sep;23(3):354-6. Epub 2007 Dec 11. Review. No abstract available. Erratum in: J Crit Care. 2008 Dec;23(4):607.
- Fishel RS, Are C, Barbul A. Vessel injury and capillary leak. Crit Care Med. 2003 Aug;31(8 Suppl):S502-11. Review.
- Edelman DA, Jiang Y, Tyburski J, et al. Pericytes and their role in microvasculature homeostasis. J Surg Res. 2006 Oct;135(2):305-11. Epub 2006 Aug 23.
- Civetta, JM. A new look at the Starling equation Crit Care Med. 1979 Mar;7(3):84-91.