In all organ systems, more fluid is filtered than absorbed by the capillaries, and plasma proteins diffuse into the interstitial spaces through the large pore system. By removing the fluid, the lymphatic vessels also remove proteins. This function is essential because the protein concentration is higher in plasma than in tissue fluid and only some form of convective transport can return the protein to the plasma. The ability of lymphatic vessels to change diameter— whether initiated by the lymphatic vessel or by forces generated within a contractile organ—is important for lymph
The arrangement of lymphatic vessels in the small intestine. The intestinal lymphatic vessels are unusual in that lymphatic valves are normally restricted to vessels about to exit the organ, whereas valves exist throughout the lymphatic system of the skin and skeletal muscles. (Modified from Unthank JL, Bohlen HG. Lymphatic pathways and role of valves in lymph propulsion from small intestine. Am J Physiol 1988;254:G389-G398.)
formation and protein removal. In the smallest lymphatic vessels and to some extent in the larger lymphatic vessels in a tissue, the endothelial cells are overlapped rather than fused together as in blood capillaries. The overlapped portions of the cells are attached to anchoring filaments, which extend into the tissue (Fig. 16.3). When stretched, anchoring filaments pull apart the free edges of the en-dothelial cells when the lymphatic vessels relax after a compression or contraction. The openings created in this process allow tissue fluid and molecules carried in the fluid to easily enter the lymphatic vessels.
The movement of fluid from tissue to the lymphatic vessel lumen is passive. When compressed or actively contracted lymphatic vessels are allowed to passively relax, the pressure in the lumen becomes slightly lower than in the interstitial space, and tissue fluid enters the lymphatic vessel. Once the interstitial fluid is in a lymphatic vessel, it is called lymph. When the lymphatic bulb or vessel again actively contracts or is compressed, the overlapped cells are mechanically sealed to hold the lymph. The pressure developed inside the lymphatic vessel forces the lymph into the next downstream segment of the lymphatic system. Because the anchoring filaments are stretched during this process, the overlapped cells can again be parted during relaxation of the lymphatic vessel.
The compression/relaxation cycle—whether controlled by lymphatic smooth muscle cells or the contractile lymphatic endothelial cells—increases in frequency and vigor when excess water is in the lymph vessels. Conversely, less fluid in the lymphatic vessels allows the vessels to become quiet and pump less fluid. This simple regulatory system ensures that the fluid status of the organ's interstitial environment is appropriate.
The active and passive compression of lymphatic bulbs and vessels also provides the force needed to propel the lymph back to the venous side of the blood circulation. To maintain directional lymph flow, microscopic lymphatic bulbs and vessels, as well as large lymphatic vessels, have one-way valves (see Fig. 16.3). These valves allow lymph to flow only from the tissue toward the progressively larger lymphatic vessels and, finally, into large veins in the chest cavity.
Lymphatic pressures are only a few mm Hg in the bulbs and smallest lymphatic vessels and as high as 10 to 20 mm Hg during contractions of larger lymphatic vessels. This progression from lower to higher lymphatic pressures is possible because, as each lymphatic segment contracts, it develops a slightly higher pressure than in the next lymphatic vessel and the lymphatic valve momentarily opens to allow lymph flow. When the activated lymphatic vessel relaxes, its pressure is again lower than that in the next vessel, and the lymphatic valve closes.
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