Definition of Lymph
The fluid that circulates through the lymph vessels. Lymph is clear and colorless or white with fat, depending on its location. It contains about 90 percent water and carries proteins, globulins, glucose, electrolytes, and other chemicals dissolved within it.
Leukocytes, primarily lymphocytes and monocytes, circulate in the lymph, suspended in the fluid. Lymph originates from and returns to the blood. Fluid from the blood (plasma) seeps from the capillaries into the spaces between the cells.
This interstitial fluid carries the nutrientsfrom the blood, surrounding the cells in a bath from which they withdraw the nutrients they need. Leukocytes in the blood move freely between the lymph and the blood. Lymph capillaries draw the interstitial fluid back into the lymph vessels, which carry the lymph they collect through a network of lymph vessels. Ultimately the lymph vessels return the lymph to the blood via its portals into the right and left subclavian veins.
Compared to the blood the heartpumps through the circulation, the lymph moves leisurely through its network of vessels, achieving a top rate of about 100 milliliters an hour in the major trunk vessels (the lymphatic ducts). It flows primarily as a function of gravity, with some help from the massaging actions of contracting skeletal muscles during movement.
Because most of the body’s infection-fighting action takes place in the lymph nodes and other lymph tissues, the lymph is the primary pathway for transporting pathogens for destruction by macrophages and other leukocytes in the lymph nodes.
The lymph also is the primary channel for the body to carry the residue of infection to other structures and systems that eliminate it from the body (through phagocytosis as well as other means). Cancer cells can overload the lymph, hijacking it to become the pathway for their spread (metastasis) to other organs and parts of the body.
For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”