Lymphatic Drainage System

All of the fluid and protein that normally accumulate in the interstitial compartment as a consequence of capillary filtration are efficiently removed and carried back to the bloodstream via the lymphatic system. It is estimated that for a 70-kg man, the lymphatics return nearly 3 L of fluid and approximately 120 g of protein to the bloodstream every 24 hr. Because the recycling of plasma proteins is essential for maintenance of the normal oncotic pressure gradient between plasma and interstitial fluid, any injury or blockage of the main lymphatic channels may be life threatening.

Figure 4 illustrates some of the major functional elements of the lymphatic system. The sac-like terminal lymphatics are blind-ended endothelial tubes suspended in the interstitium by connective tissue anchoring filaments. The gaps between the endothelial cells of the terminal lymphatics are very large, which, in the absence of a basement membrane, allows ready access of interstitial fluid, along with its suspended particles (lipoproteins) and cells (lymphocytes), to the lymphatic vessel lumen. Hence, the composition of lymph is considered to be identical to that of interstitial fluid.

Smooth muscle

Smooth muscle

Anchoring filament

FIGURE 4 Schematic diagram of the elements making up a lymphatic bed.

Anchoring filament

FIGURE 4 Schematic diagram of the elements making up a lymphatic bed.

The terminal lymphatics coalesce into larger vessels, collecting lymphatics, that possess one-way valves similar to those in the veins. These lymph channels ultimately drain into the thoracic duct, which drains the lower part of the body, and the right lymph duct, which drains the upper body, and in turn empties into the venous system at the internal jugular and subclavian veins.

The principal driving force for the entry of interstitial fluid into the terminal lymphatics is the Pif. As fluid filtered from capillaries accumulates in the interstitium, Pif rises, thereby creating a hydrostatic pressure gradient between the interstitial compartment and the lymphatic vessel lumen. Expansion of the interstitium also causes the anchoring filaments to pull open the terminal lymphatic. This allows the elevated Pif to readily propel interstitial fluid into the lymph vessel (Fig. 5). Stretch-mediated, active contractions of lymphatics also favor the propulsion of lymph. When a collecting lymphatic becomes distended with interstitial fluid, a myogenic response is elicited that causes the lymphatic smooth muscle to automatically contract. The one-way pumping action of collecting lymphatics is the most likely explanation for the subatmospheric PIF measured in some tissues, because it literally sucks fluid out of the interstitial compartment. Additional lymph propulsive forces are generated when external compression causes local, transient increases in Pif. Because lymph vessels have valves that prevent backflow, external intermittent compression phenomena such as muscular contraction, respiratory movements, and even arterial pulsations tend to push the lymph toward the veins into which the lymphatics terminate. Despite these mechanisms to facilitate lymph flow, in most tissues lymph flow rate

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