Lymph vessels drain fluid diffused through capillary walls from tissues around the body. Some of this fluid is reabsorbed back into the capillaries; the rest is drained from cells surrounding the lymphatic vessels and back into venous circulation, as explained by The Merck Manual Home Health Handbook.
To ensure lymph moves in one direction, lymph vessels have valves that prevent back-flow. Lymphatic fluid consists of proteins, minerals and other nutrients. It passes through a number of lymph nodes before being drained into venous circulation. Lymph nodes help to get rid of damaged cells in the body, as well as cancer cells, infectious organisms and other foreign material.