The Lymphatic System

The small amount of interstitial fluid that remains 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. Even the large 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 - link to discussion.)

The lymph capillaries drain into still larger vessels that make up the lymphatic system. The flow through the lymph vessels is quite slow. Like blood in the veins, contraction of skeletal muscles compresses the lymph 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.

• the entire left side of the body,
• the digestive tract and
• the right side of the lower part of the body

The thoracic duct empties about 100 ml of lymph every hour 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.

Edema

• increased blood pressure in the capillaries and/or
• a decreased concentration of plasma proteins (such as occurs in prolonged malnutrition).

Lymph Nodes

• groin
• armpits
• abdomen and
• neck.

These contain cavities - called sinuses - into which the lymph flows. 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".

• antibodies and
• lymphocytes