Physical Factors that Determine Capillary Fluid Exchange
There is a free exchange of water, electrolytes, and small molecules between the intravascular and extravascular compartments of the body. The primary site of this exchange is capillaries and small post-capillary venules (sometimes grouped together and called "exchange vessels"). Several mechanisms are involved in this exchange; however, the most important are bulk flow and diffusion. The rate of exchange, in either direction, is determined by physical factors: hydrostatic pressure, oncotic pressure, and the physical nature of the barrier separating the blood and the interstitium of the tissue (i.e., the permeability of the capillary wall).
There are two important and opposing hydrostatic forces: capillary hydrostatic pressure (PC) and tissue interstitial pressure (Pi). Because PC is normally much greater than Pi, the net hydrostatic pressure gradient across the capillary is positive, meaning that hydrostatic forces are driving fluid out of the capillary and into the interstitium. There are also two opposing oncotic pressures influencing fluid exchange: capillary plasma oncotic pressure (πC) and tissue (interstitial) oncotic pressure (πi). πC is much greater than πi, therefore the oncotic pressure gradient across the capillary, if unopposed by hydrostatic forces, would reabsorb fluid from the interstitium into the capillary. The oncotic pressure difference (πC - πi) should be multiplied by the reflection coefficient, which is a measure of the permeability of the capillary barrier to the proteins responsible for generating the oncotic pressure. The net driving force (NDF) for fluid movement is the net pressure gradient determined by the sum of the individual hydrostatic and oncotic pressures, and is expressed in the units of mmHg. When the NDF is positive, there is net fluid filtration, and when it is negative, there is net fluid reabsorption.
For a given NDF, the amount of fluid filtered or reabsorbed per unit time (fluid flux, or JV) will be determined by the permeability of the capillary barrier and by the surface area available for exchange. The permeability is usually referred to as the filtration constant (KF), and is determined by the physical properties of the barrier (i.e., size and number of "pores" and the thickness of the barrier). For example, fenestrated capillaries have a higher KF than continuous capillaries. Furthermore, substances such as histamine, which are released in response to tissue injury or inflammation, increase KF. The surface area (A) is related to the length, diameter, and number of capillaries available for exchange. The surface area is dynamic in vascular beds such as skeletal muscle where the number of perfused capillaries increase several-fold during exercise.
To summarize: JV = KF A [(PC − Pi) − σ(πC - πi)]
The expression in brackets represents the NDF. If this is positive, filtration occurs, and if negative, reabsorption occurs. At a given NDF, the JV is determined by the product of KF and A.
In most vascular beds of the body, filtration occurs across the arteriolar end of the capillary and reabsorption occurs across the venular end. In general, there is net filtration across capillary beds (i.e., filtration > reabsorption) that is picked up by the lymphatics. If an imbalance occurs where net filtration exceeds the capacity of the lymphatics, then edema results. The kidneys are an exception to these generalizations in that renal glomerular capillaries filter large amounts of fluid along their entire length. This results from higher glomerular capillary hydrostatic pressures and higher capillary permeabilities.