From one-third to two-thirds of the fluid entering a capillary passes into the tissue space. The cells in the tissue space are bathed in this interstitial fluid (ISF) (also known as extracellular fluid or ECF) that has been filtered from the blood. Here in the tissue space is where cells acquire all their needs and deposit all their products.
Fig. 126.96.36.199 Tissue Space
Most (~90%) of this interstitial fluid returns at the venule end of the capillary. The 10% that does not is picked up by tiny vessels called lymph capillaries. The cells forming the walls of the lymph capillaries are loosely fitted together, thus making the wall very porous. Any serum proteins that filtered through the capillary wall pass easily from the tissue space into the interior of the lymph capillary. The lymph capillaries of the intestinal villi, called lacteals, also pick up fat droplets. White blood cells (leukocytes) migrate from the tissue space into the lymph capillary squeezing between the cells that make up its wall.
The lymph capillaries drain into still larger collecting vessels. The flow through the collecting vessels is quite slow. Like blood in the veins, contraction of skeletal muscles compresses the collecting vessels and squeezes the fluid — now called lymph — along. Again, like the return of blood in the veins, the lymph can flow only in one direction because of valves in the vessels. All the lymph collected from the entire left side of the body, the digestive tract and the right side of the lower part of the body flows into a single major vessel, the thoracic duct.
The thoracic duct empties into the left subclavian vein. The lymph in the right side of the head, neck, and chest is collected by the right lymph duct and empties into the right subclavian vein. Together they empty 1–2 liters of lymph into the blood each day.
The collecting vessels of the lymphatic system are interrupted by lymph nodes. These are especially abundant in the groin, armpits, abdomen and neck. These contain cavities called sinuses into which the lymph flows bringing various leukocytes (e.g., lymphocytes and dendritic cells) and out of which pass antibodies and lymphocytes which then enter the blood at the subclavian veins.
The walls of the sinuses are lined with phagocytic cells, which engulf any foreign particles, e.g., bacteria, that might be present in the lymph. Tests have demonstrated that over 99% of the bacteria carried into a node are screened out before the lymph leaves the node on its return to the blood. This filtering mechanism is one of the most important body defenses against infectious disease. When combating a heavy infection, the lymph nodes enlarge producing "swollen glands".
The production of lymph is increased by
- increased blood pressure in the capillaries
- increased capillary permeability such as occurs if the adherens junctions between the cells lining the capillaries are damaged
- a decreased concentration of plasma proteins (such as occurs in prolonged malnutrition).
The lymphatic system may be unable to handle the increased volume of lymph, and it may accumulate in the tissues and distend them. This condition is known as edema.