What does the lymph system do?

The lymph system is probably most familiar to people because doctors often check for "swollen lymph nodes" in the neck. The lymph nodes, however, are just one part of a system that reaches points throughout the body in much the same way blood vessels do. The main difference between blood flowing in the circulatory system and lymph flowing in the lymph system is that the heart pressurizes blood. The lymph system is passive. Bodies have no "lymph pump." Instead, fluids ooze into the lymph system and are pushed by normal body motions to the lymph nodes. This is like the water and sewer systems in a city. Water is actively pressurized, while sewage flows by gravity.

Lymph is a mostly clear liquid that replenishes cells with water and nutrients. Lymph is blood plasma -- the same liquid that makes up blood (minus red and white cells). Blood transfers water and nutrients to the lymph through capillary walls and lymph carries it to cells. The cells also make proteins and waste products. The lymph absorbs these products and carries them away. Random bacteria that enter the body also get into this inter-cell fluid. The lymph system also drains and filters these fluids to detect and remove bacteria. Small lymph vessels collect liquid and move it toward larger vessels so the fluid finally arrives at the lymph nodes where it can be processed.

Lymph nodes have filtering tissue and many lymph cells. When fighting certain bacterial infections, lymph nodes swell with bacteria as well as the cells fighting those bacteria, to the point where you can actually feel the nodes. Swollen lymph nodes are a good indication you have an infection.

Once the lymph nodes filter the lymph, it re-enters the bloodstream.

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