The lymphatic system works with the body's white blood cells to protect against pathogens. Swollen lymph nodes under the jaw or armpits can be painful and indicate that the body is fighting an intruder. This system plays a critical role in the maintenance of the body's fluid and osmotic balance. The lymphatic system is much like the venous portion of the circulatory system. It is a network of one-way vessels that collects the fluid that "leaks" from the capillaries into the interstitial (or extracellular) space between cells. It does not have a central 'pump' to circulate this fluid, so relies on skeletal muscles in the body to transport this fluid.