21.11: The lymphatic system plays a supporting role in circulation.

Our bodies have what appears to be another circulatory system (FIGURE 21-22). In addition to the cardiovascular system, and running close to it throughout the body, is the lymphatic system. Why the seeming redundancy? The lymphatic system is a network of vessels—though not a closed system—that has a supporting role in the process of circulation. It has three important functions (FIGURE 21-23).

1. Recycling. As diffusion occurs between the capillaries and the interstitial fluid surrounding cells, much fluid is lost from the blood. Here’s where the lymphatic system comes in. Intertwined around blood vessels, the lymphatic capillaries take in, by diffusion, fluid, proteins, and other substances that have leaked into the interstitial fluid from the blood. Once recovered, this fluid, now called lymph, travels through progressively larger lymphatic vessels that eventually join up with veins in the shoulders. At this point, the recovered proteins and fluid (which amounts to several liters each day) are returned to the blood as it makes its way back to the heart.

2. Fighting illness. As it moves through the lymphatic system, lymph passes through patches of connective tissue called lymph nodes. These compartmentalized sacs are filled with pathogen-fighting white blood cells that remove dangerous materials (including bacteria, cancer cells, and viruses) from the body. This is why your lymph nodes—including your tonsils, the largest lymph nodes of all—become swollen when your body is fighting an infection.

3. Retrieving nutrients. The lymphatic system has numerous little projections that extend into the small intestine and absorb lipids from the food you have eaten and shuttle them from the digestive tract to the bloodstream.

In humans, the lymphatic system accomplishes these tasks without a pump. There is no “lymph heart.” Rather, lymph is pushed through the system when muscles adjacent to lymph vessels contract and squeeze the fluid onward. Lymph vessels (like veins) have valves that keep the lymph flowing in one direction. When you sit for an extended period of time, lymph can accumulate in vessels. You can help move it along by contracting the muscles in your extremities and progressively contracting muscles closer and closer to your shoulders.

Infection by some parasitic worms can cause the scarring of lymph vessels. When lymph vessels are scarred, fluid recovered by the lymphatic system cannot be returned to the circulatory system, and the extremities swell, often tremendously (FIGURE 21-24). This disease is called elephantiasis. Because the parasitic worms causing elephantiasis are transmitted by mosquitoes, the condition is most common in tropical regions. It can be treated with antibiotics that kill the symbiotic bacteria necessary for the parasitic worm to live.