Physiology Of Lymph Formation

Lymph formation, which is essentially the filling of the lymphatic capillaries (initial lymphatics), is the main determinant of lymph flow. Once lymphatic fluid enters the lymphatic collecting vessels, it is propelled along either by active contractions or external forces, and flow is unidirectional.

The rate of movement of fluid and protein out of blood passing through capillaries (see Fig. 8) into the interstitial space [1-3] can be summarized by

where Vc is the volume of the capillary filtrate, Kc is the filtration coefficient, Pc is the capillary hydrostatic pressure, PTis the interstitial fluid hydrostatic pressure, Rp is the reflection coefficient of plasma protein, ITc is the oncotic pressure of plasma protein, and ITT is the oncotic pressure of interstitial fluid. As a result of imbalance between transcapillary hydrostatic and oncotic forces, a driving pres-

BLOOD CAPILLARY

BLOOD CAPILLARY

Interstitial Lymph Nodes

Figure 8 Blood capillary forces and lymph formation. Vc is the net volume of fluid out of the capillary; Pc is the capillary hydrostatic pressure,*nc is the oncotic pressure of plasma protein;PT is the interstitial fluid hydrostatic pressure,*nT is the oncotic pressure of interstitial fluid;VL is the net volume of fluid entering the initial lymphatics; PL is the hydrostatic pressure of the initial lymphatics;nL is the oncotic pressure of the lymphatic fluid.

Figure 8 Blood capillary forces and lymph formation. Vc is the net volume of fluid out of the capillary; Pc is the capillary hydrostatic pressure,*nc is the oncotic pressure of plasma protein;PT is the interstitial fluid hydrostatic pressure,*nT is the oncotic pressure of interstitial fluid;VL is the net volume of fluid entering the initial lymphatics; PL is the hydrostatic pressure of the initial lymphatics;nL is the oncotic pressure of the lymphatic fluid.

sure occurs and produces a continuous leakage of fluid from the capillary into the interstitium. This is usually balanced by an equal outflow of fluid from the interstitial space via the lymphatics.

In addition, there is protein movement across the capillary wall caused by bulk flow and diffusion. The transport of large molecules from plasma to lymph depends on their molecular size rather than their molecular weight.

Several mechanisms have been proposed to explain the entry of interstitial fluid into the initial lymphatics [2,3,8]. The most widely held view is that the overlapping interendothelial junctions of the initial lymphatic (lymphatic capillaries) act as valves and promote unidirectional movement of fluid from the interstitial space into the lumen of the lymphatics. The innermost overlapping edges are able to flap back and forth and hence act as flap valves. When the initial lymphatics are compressed by the surrounding tissues, by arterial pulsations or by the action of an active lymphatic pump, the flaplike edges of the endothelial cells are apposed and the contents of the initial lymphatics are squeezed forward (Fig. 9). As the initial lymphatics recoil following compression, the intraluminal pressure falls and the flaps open, promoting filling of the lymphatics. The anchoring filaments may pull on the endothelial cells as a result of recoil of the surrounding tissue, setting up a pressure difference across the lymphatic wall sufficient to remove fluid from the interstitial space.

Another mechanism proposed to explain the filling of initial lymphatics is the osmotic pressure theory (Fig. 10). The protein concentration in the initial lymphatics is about two to three times that in the interstitial space. This produces an inward osmotic pressure gradient which tends to move fluid into the initial lymphatics.

Osmotic Pressure Across Capillary Walls

CONTRACTILE SEGMENT Figure 9 Suction forces aiding lymphatic absorption.

Lymph Propulsion
Figure 10 Osmotic theory of lymphatic absorption: (a) at rest; (b) during tissue compression.

Observations of lymphatic flow patterns in the wing membranes of bats suggest that intraluminal pressures in the initial lymphatics fluctuate due to wall contractions and forward flow of lymph. The intraluminal pressures are negative or are tending toward negative values. The presence of lymphatic valves upstream ensures that retrograde flow does not interfere with the ''suction'' process.

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Responses

  • yasmin
    What formation depends upon interstitial fluid formation?
    8 years ago
  • OLLE
    How is tissue fluid and lymph formed?
    8 years ago

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