The interstitial fluid contains water and other small molecules, but no RBCs, and less protein than found in plasma. A separate system of vessels—the *lymphatic system—returns interstitial fluid to the blood. Each capillary bed contains at least one blind-ended lymph capillary.

Interstitial fluid that enters the lymphatic vessels is called lymph. Fine lymphatic capillaries merge into progressively larger vessels and ultimately into the right and the left lymphatic ducts—that empty into large veins at the base of the neck (see Figure 41.2). The left lymphatic duct, also known as the thoracic duct, carries most of the lymph from the lower part of the body and is larger than the right lymphatic duct. Lymphatic vessels, like veins, have one-way valves that keep the lymph flowing toward the lymphatic ducts. Therefore lymph, like blood, is propelled toward the heart by skeletal muscle contractions and breathing movements. Lymph nodes along the major lymphatic vessels are a major site of lymphocyte production and of the phagocytic action that removes microorganisms and other foreign materials from the circulation.