Part of maintaining proper water balance is to make sure that the same amount of water enters and exits the body. This homeostasis is controlled by the nervous system and kidneys. The other part of proper water balance is to make sure that water is distributed properly throughout the body.

Why this is important…Fluid accumulation leads to edema, which is a common sign of illness. You will also learn why doctors often check for swollen lymph nodes under a patient’s chin.

Foundations

As blood flows through the body, some plasma is squeezed out of capillaries and fills spaces around cells. The lymphatic system recovers this fluid and returns it into veins under your collar bone. Think of this like your body’s version of a watershed – small creeks flow gently into streams, which flow into rivers. Along the way, the water passes through lakes were the dirt settles out and the water is cleaned.

Key terms

Lymph – fluid that flows through the lymph vessels and nodes

Edema – condition of excess fluid in a body part, making it swollen or puffy

Details

Fluid can collect in areas of the body for a variety of reasons. Too many solutes can draw water out of the bloodstream by osmosis. High blood pressure can cause more plasma to exit capillaries by filtration than normal. Exercise can cause fluids to build up in muscles because of the higher metabolic activity. Even periods of inactivity can leave hands or feet somewhat swollen with extra fluid.

Whatever the cause, there needs to be a way of getting fluid back into the circulatory system. The water, electrolytes and proteins in this fluid are vital to keeping the circulatory system functioning properly. Lymph vessels are spread throughout your body collecting fluid. The smallest vessels are open tubes that collect fluid passively (Figure 12-2). Each tube has valves that keep the fluid moving in one direction.

Closeup of lymph vessels. Vessels are formed by overlapping cells with valves to direct flow in one direction.

FIGURE 12-2. Lymph vessel close-up. Notice the one-way valves that keep lymphatic fluid moving back toward the blood vessels. ("Lymphatic vessel" by DataBase Center for Life Science (DBCLS) is licensed under CC BY 4.0)

Lymph vessels get clogged occasionally. An unusual, but serious, blockage is caused by an infection of roundworms that make their way into lymph vessels, blocking the flow of lymph. This can result in severe edema, usually to the legs. You might see why it is called elephantiasis (Figure 12-3). Edema is usually far less severe, but can be a sign of various problems.

Picture of man's extremely swollen legs due to blocked lymph vessels.

FIGURE 12-3. Elephantiasis. This disease is usually caused by small roundworms invading the lymphatic system and blocking the flow of fluid. It is a gruesome illustration of how much fluid leaks out of blood vessels. ("Elephantiasis " by Centers for Disease Control and Prevention is in the Public Domain)

The lymphatic system has no pump to move the fluid. There is some contraction of the lymph vessels themselves. The skeletal muscles also squeeze the vessels, moving fluid upward.

Stop & Think #3: Sometimes feet get swollen if sitting for long periods of time, such as long car trips or plane flights. The cause is edema. Explain how flexing leg muscles would help lessen edema.

Since the vessels are open, anything can enter them. Along with the fluid, debris from dead or damaged cells, bacteria, viruses, and even dirt particles might enter the lymph vessels. Lymph nodes work like traps to clean up dirty lymph before returning it to the bloodstream. Several lymph vessels enter each node where the fluid goes through a chamber of net-like cells and fibers. Immune cells are also abundant here and gobble up 99% of the debris before the fluid returns to the bloodstream.

Stop & Think #4: Why might the environment in lymph nodes help immune cells capture bacteria?

Swollen lymph nodes can indicate infection. If your body has an infection, there will be more bacteria than normal in lymph nodes. This causes an immune response and the lymph nodes swell up with more fluid and cells. This is why doctors feel for swollen lymph nodes under your chin. There are many lymph nodes here and they are a convenient way to feel for signs of infection. You also have clusters of lymph nodes in armpits and the groin, which is why these areas might feel tender when you are ill.

Lymph nodes can also catch cancer cells. Most cancerous cells remain in one place and form a tumor. You may have heard about a cancer becoming metastatic or metastasizing – this is when cancerous cells become so abnormal that they break away from the tumor and try to find a new place to grow. Since cancer cells can also move through lymph vessels, they often get caught in lymph nodes and form new tumors there.

Learning objectives

Describe how the lymphatic system recovers fluid
Describe the role of lymph nodes

Stop & Think Questions Explained

3. Flexing leg muscles would squeeze lymphatic fluid upward. Since the vessels have one-way valves, the lymph would only travel upward.

4. Fluid moves more slowly in the lymph nodes and there are fibers that help the immune cells trap bacteria.