Certain facts about the human body are common knowledge.
Everyone knows, for example, that blood streams through a closed system of tubes not unlike garden hoses—the largest: an inch in diameter; others: so narrow that blood cells actually parade through in single file.
And, everyone knows that the blood’s circulation is powered by a remarkable pump called the heart.
But what about that other fluid—lymph—the stuff we mostly know about when our doctor palpates our neck and says, “you have swollen glands.”
Have you ever seen a neat diagram of that fluid circulating through the body? No. You haven’t. Lymph does not literally circulate. You might have seen a crazy lymph “drainage” diagram, though.
Lymph flows the way land-based tributaries do when they carry water, fish, and debris from the tiniest brooks to bigger creeks to rivers and then out to the sea. Lymph’s “sea” is the bloodstream.
Consider tissues, say, in your arm. There, and all over your body, something called “interstitial fluid” bathes and nourishes cells and carries away debris.
Thin, microscopic capillaries lying throughout the arm tissue continuously suck up interstitial fluid. Upon entering the capillaries, this fluid acquires the name lymph.
The lymph then begins its inexorable journey toward the heart. It moves from those straw-like capillaries to larger lymph trunks and then to even wider lymph ducts that dump it into the bloodstream at great veins near the collarbone.
That’s where lymph’s independent existence ends. At that point, it becomes an integral part of the blood.
At various sites throughout the body—in the neck, armpits, chest, gut—lymph encounters small glands, called lymph nodes, as well as larger lymphatic organs, the most famous of which is the spleen.
As lymph percolates through these organs and nodes, the bacteria, pollen, cancer cells, and cellular waste products that it has swept up along the way get mechanically filtered out and then physically destroyed by cells and antibodies of the immune system.
Swollen glands may hurt like crazy during bouts with viruses, but that swelling is a good sign, indicating that the immune system is in an active battle against the infection.
Swelling in tissues, though, is not salutary. That swelling is a signal that something has gone haywire with the uptake and recycling of the interstitial fluid.
Many years ago, my father’s secretary went to her doctor, because one of her hands was twice the size of the other.
“How long have you been wearing these bracelets?” the doctor asked her, pointing to the silver, gold, and platinum bracelets stacked tightly on her left arm from wrist to elbow.
“Thirty years,” she said. She had put them on when she was 25, and she had left them there.
The doctor concluded that her bangles had essentially stopped normal fluid communication between that hand and the rest of her body.
But now I can’t remember whether the hand with the bracelets was the big one or the little one.
I suppose that either possibility makes sense. If the bracelets had been cutting off lymph drainage, then her compromised hand would have been the bigger, swollen one. If they had cut off blood circulation, though, then her hand had probably atrophied, denying it essential oxygen and nutrients.
Although I mentioned that blood circulates through a closed system, that is only true for the cells and proteins that it transports. Ninety percent of its fluids move back and forth between the bloodstream (where the fluid component is called plasma) and the body’s tissues (interstitial fluid). The other ten percent travels through the lymph system, moving like a meandering river along what might be referred to, by analogy, as “the splenic route.”