Composition of Lymph and Its Functions

Roughly 90% of the fluid that leaks out of capillary walls diffuses straight back into the blood, but the remaining 10% takes a different route: it is picked up by a separate network of thin-walled tubes called lymph vessels (lymphatics), and once inside them it is called lymph. The lymphatic system as a whole is a network of these vessels together with filtering stations (lymph nodes) and clusters of lymphocytes distributed through various lymphoid organs. Lymphatic vessels run parallel to blood vessels through the skin and along the respiratory and digestive tracts, acting as one-way return ducts for fluid that continuously seeps out of blood capillaries into the tissues (see Figure 7.5). Before lymph re-joins the bloodstream it must pass through at least one lymph node, and these nodes are especially dense in the neck, groin (inguinal), armpit (axillary), and respiratory and digestive regions - inside each node, narrow channels called sinusoids are lined with macrophages that filter out and destroy invading microorganisms before they can reach the general blood supply. After passing through its nodes, lymph drains into progressively larger collecting ducts and finally empties into the large veins running beneath the collar bone (the subclavian veins), rejoining the blood. The main cells found in lymph are lymphocytes, which are picked up from the lymphatic fluid, carried around the body in the arterial blood, and eventually recycled back into the lymphatic system. Beyond this general drainage- …