It is already clear that the lymphatic system functions as an “overflow mechanism” to return excess proteins and excess fluid volume from the tissue spaces to the circulation. Therefore, the lymphatic system also plays a central role in controlling (1) the concentration of proteins in the interstitial fluids, (2) the volume of interstitial fluid, and (3) the interstitial fluid pressure. Let us explain how these factors interact.
First, remember that small amounts of proteins leak continuously out of the blood capillaries into the interstitium. Only minute amounts, if any, of the leaked proteins return to the circulation by way of the venous ends of the blood capillaries. Therefore, these proteins tend to accumulate in the interstitial fluid, which in turn increases the colloid osmotic pressure of the interstitial fluids.
Second, the increasing colloid osmotic pressure in the interstitial fluid shifts the balance of forces at the blood capillary membranes in favor of fluid filtration into the interstitium. Therefore, in effect, fluid is translocated osmotically outward through the capillary wall by the proteins and into the interstitium, thus increasing both interstitial fluid volume and interstitial fluid pressure.
Third, the increasing interstitial fluid pressure greatly increases the rate of lymph flow, which carries away the excess interstitial fluid volume and excess protein that has accumulated in the spaces.
Thus, once the interstitial fluid protein concentration reaches a certain level and causes comparable increases in interstitial fluid volume and pressure, the return of protein and fluid by way of the lymphatic system becomes great enough to balance the rate of leakage of these into the interstitium from the blood capillaries. Therefore, the quantitative values of all these factors reach a steady state, and they will remain balanced at these steady state levels until something changes the rate of leakage of proteins and fluid from the blood capillaries.
