You’ve probably heard of your skeletal, immune, and cardiovascular systems. One biological system you may not have heard of is the lymphatic system. What is it, and what are lymph nodes?

The lymphatic system is made of thin tubes--not unlike veins or arteries--that carry a fluid called lymph, and lymph nodes. Other important components of the system are organs including the thymus, liver, and spleen. Those tubes, called lymphatic vessels, spread throughout the body just like veins and arteries do. The lymph they carry contains a high concentration of white blood cells called lymphocytes, which are important to immune function. They help fight disease and destroy damaged cells.

The lymphatic system isn’t just similar to the cardiovascular system, it also works alongside it. After blood leaves your heart it travels down arteries as they spread through the body. Arteries get smaller and smaller the further they get from the heart. Eventually they’re so small that the walls are only one cell thick and only one blood cell can travel through at a time. Once that small, the tubes are called capillaries.

Think of the blood stream as a conveyor belt made of fluids and blood cells. It carries all sorts of things with it: oxygen, nutrients, carbon dioxide, wastes, bacteria, and damaged cells. After leaving the heart, it mostly carries oxygen and nutrients. When it reaches the capillaries, the high pressure that results from the microscopic size of the tubes causes fluids, oxygen, and nutrients to leave the blood. At the same time, fluids which have picked up carbon dioxide, wastes, and damaged cells are entering the blood stream through the tiny capillaries. The blood carries them through the body's veins afterward, going through various filtration systems before it reaches the heart and gets re-oxygenated.

However, the high pressure in capillaries also means that not all the fluid carrying wastes and damaged cells can get back into the blood stream directly. The excess is picked up by lymphatic vessels, filtered in lymph nodes, and eventually dumped back into the blood. The lymphatic system has several functions but this is the one that’s important in cancer.

Cancer cells are fundamentally damaged. Once they begin breaking off from the main tumor they can travel with the fluid that leaves the capillaries and eventually picked up by the lymphatic system. They’re carried to the lymph nodes and can get stuck there as the nodes try to filter them out. It’s a bit like getting lost while driving around in a residential neighborhood. When lots of cancer cells get stuck in the nodes, this can make them swell.

This is why a doctor worried that a patient’s cancer may have spread turns to the lymph nodes as one of the first places they check.

What cancer cells are really looking for is the freeway: the blood stream. When cancer cells eventually get there, they can spread to distant places of the body with relative speed and begin spreading in their new areas. This is metastasis.

Sources:

“Capillary Exchange.”Open Learning Initiative: Register for a Course, Carnegie Mellon University, oli.cmu.edu/jcourse/workbook/activity/page?context=b880f0e580020ca600adef862c9adaf0.

“Heart & Blood Vessels: Blood Flow.”Cleveland Clinic, my.clevelandclinic.org/health/articles/17059-heart--blood-vessels-how-does-blood-travel-through-your-body.

“The lymphatic system and cancer.”Cancer Research UK, 13 Dec. 2017, www.cancerresearchuk.org/what-is-cancer/body-systems-and-cancer/the-lymphatic-system-and-cancer.