Which one of the following statements is correct regarding the movement of fluid across the capillary endothelium?
a. Water and electrolytes are actively transported across the capillary endothelium
b. Capillary hydrostatic pressure is greatest at the distal end of the capillary
c. Interstitial oncotic pressure is greater than capillary oncotic pressure
d. Interstitial hydrostatic pressure increases 10-fold along the length of the capillary
e. Fluid reabsorption may occur at the venous end of the capillary
The correct answer is e. In most capillary beds (but not all), fluid reabsorption occurs at the venous end of the capillary.
A series of hydrostatic and oncotic pressures, known as Starling’s forces (after the same Ernest Starling as discovered Starling’s law of the heart), control the bulk flow of water and accompanying solutes across the capillary endothelium. The four Starling’s forces, and typical values for a skeletal muscle capillary, are;
– capillary hydrostatic pressure (Pc). Produced by the pressurized flow of blood into capillaries from the arterial system, this is the ‘water pressure’ pushing fluid out of the capillary. Pc is greatest, around 35mmHg, at the arteriolar end of a typical tissue capillary but falls to 25mmHg by the time blood reaches the venous end.
– capillary oncotic pressure (πc). Maintains fluid within the capillary attracted by the high concentration of plasma proteins. It remains roughly constant over the length of the capillary.
– interstitial hydrostatic pressure (Pi). This tends towards pushing fluid back into the capillary from the extracellular space. This pressure is negligible at the arterial end of the capillary but increases to a few mmHg as fluid passes out into the interstitium generating an increasing backpressure with distance along the capillary.
– tissue oncotic pressure (πi). The force holding fluid in the extracellular space through its oncotic attraction to interstitial proteins. Due to the much lower protein concentration of the interstitium, πi is negligible.
In a typical tissue capillary, the net balance of the Starling forces at the arterial end of the capillary ([Pc-Pi] – [πc-πi]) is positive and fluid is driven from the capillary lumen into the tissue interstitium (fig 1). However, as blood flows along the capillary and more fluid leaves the capillary for the tissues, the outward hydrostatic pressure of the capillary falls while the hydrostatic pressure of the interstitium increases. This process continues down the length of the capillary until, toward the venous end, the balance of Starling’s forces reverses and fluid flows back across the capillary endothelium from tissue interstitium into the capillary lumen.
The amount of fluid filtering into the tissues in the proximal capillary always exceeds that re-absorbed at the venous end so that in all tissues, there is net movement of fluid from capillary to tissue. This excess tissue fluid, around 3L/day, is removed and ultimately returned to the vasculature by the lymphatic system.
Figure 1. The changing balance of Starling forces along the capillary.