Figure 1610

Relation between interstitial fluid pressure and lymph flow in the leg of a dog. Note that lymph flow reaches a maximum when the interstitial pressure, PT, rises slightly above atmospheric pressure (0 mm Hg). (Courtesy Drs. Harry Gibson and Aubrey Taylor.)

value of -6 mm Hg.Then, as the pressure rises to 0 mm Hg (atmospheric pressure), flow increases more than 20-fold. Therefore, any factor that increases interstitial fluid pressure also increases lymph flow if the lymph vessels are functioning normally. Such factors include the following:

• Elevated capillary pressure

• Decreased plasma colloid osmotic pressure

• Increased interstitial fluid colloid osmotic pressure

• Increased permeability of the capillaries

All of these cause a balance of fluid exchange at the blood capillary membrane to favor fluid movement into the interstitium, thus increasing interstitial fluid volume, interstitial fluid pressure, and lymph flow all at the same time.

However, note in Figure 16-10 that when the interstitial fluid pressure becomes 1 or 2 millimeters greater than atmospheric pressure (greater than 0 mm Hg), lymph flow fails to rise any further at still higher pressures. This results from the fact that the increasing tissue pressure not only increases entry of fluid into the lymphatic capillaries but also compresses the outside surfaces of the larger lymphatics, thus impeding lymph flow. At the higher pressures, these two factors balance each other almost exactly, so that lymph flow reaches what is called the "maximum lymph flow rate." This is illustrated by the upper level plateau in Figure 16-10.

Lymphatic Pump Increases Lymph Flow. Valves exist in all lymph channels; typical valves are shown in Figure

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