Medicine: The Second Circulation

For once, the Greeks had no word for it: they neither understood nor named the body’s system of glands and connecting channels through which colorless fluids flow. The Romans did coin a name, but for the fluid only. They called it lympha, after a fancied resemblance to clear spring water. But nothing about the lymphatic system was clear then, or for another 2,000 years. Only now, says Tulane University’s Physiologist Hymen S. Mayerson in a report to the American College of Surgeons, are the workings of the lymphatic system beginning to be understood. The body’s second circulation system, he says, plays an essential role in keeping man alive and healthy by filtering gallons of fluid every day, transporting vital substances to the places where they are needed, and working to keep the body’s infinitely complex chemistry in balance.

The lymphatic system comprises up to a hundred scattered lymph glands, or “nodes.” Some lie deep within the body, others close to the skin. The lymph nodes produce some of the white cells that help the body fight off invading bacteria. They are favorite sites for colonization by cancer cells, and the whole lymphatic system may fall victim to a cancerlike process. (Best-known example: Hodgkin’s disease.) There may be miles of lymphatic ducts, but they are so fragile and elusive that nobody has measured them. Often buried in fat, they shrink to the vanishing point when not filled with fluid, and disappear at the gentlest touch of the anatomist’s probe.

Johhny-Come-Lately. Though anatomists have been mapping its geography for 300 years, the lymphatic system is a Johnny-come-lately on the evolutionary stage, says Dr. Mayerson. It is found only in higher animals; the higher the animal, the more complex the system. The complexity is necessary, says Dr. Mayerson, because in the mammal, nature evolved a closed, high-pressure blood-circulating system with conduits of diminishing thickness carrying blood and oxygen to thin-walled capillaries. “But here, nature ran into a snag: the high pressure made the capillaries leaky.” Even the large molecules of proteins slip through delicate capillaries. Nature had to invent a collection and drainage system so that whatever Leaked into the tissue spaces around the capillaries could be picked up and put back into circulation in the bloodstream. Nature’s solution to the problem was the lymphatic system.

Plasma (the blood’s clear fluid) is supposed to carry most of the blood’s circulating protein supply, but there is always some protein outside the bloodstream because it has leaked through the capillaries —surprisingly as much as half of the blood’s total stock. By a still-obscure method, the lymphatic system picks up this protein,, which then flows to lymphatic collection points. Biggest of these, in the abdomen, is the cisterna chyli. Others are in the chest. Through large lymph channels—notably the thoracic duct—the protein returns to the blood stream. Most surprising, Dr. Mayerson and fellow researchers found, is the sheer volume that the lymphatics handle. In the dog, and probably in man, the kidneys’ lymphatics process a volume of fluid almost equal to the kidneys’ output of urine.

A sharp drop in the plasma’s protein content (the result of drastic surgery, a burn or an accident) is one of the factors that make shock catastrophic and possibly fatal. So the long-mysterious lymph system is a means of combating shock.

Good Servant. There seems to be no end to the variety of vital biochemical substances that the lymphatic system carries. Dr. Mayerson believes that it conveys hormones into the bloodstream from the glands where they are produced. He is convinced that cholesterol gets into the blood through the lymph-system interchange. And circulating cholesterol is under indictment as a cause of atherosclerosis. In one of nature’s delicate balance mechanisms, a rise in blood pressure may push more fats through artery walls. If the lymphatic system cannot drain away all this fat from the tissue spaces around the arteries, some of it is likely to be left stuck in the arteries—also leading to atherosclerosis. Most victims of heart attacks suffer first from atherosclerosis.

When the lymphatic drainage system of a major organ is blocked, other organs may be affected by a chain reaction. In medicine’s present state of knowledge, says Dr. Mayerson, the lymphatic system “is a good servant, reliable and loyal, and does a capable job when all is going well. But its capacity is limited. It has not evolved to the point of being able to cope with abnormal stresses and strains.” Dr. Mayerson and his colleagues hope to learn how to help the lymphatic system do its job even in times of crisis.