In addition to the blood system, our body has another transport system called the lymphatic system. Clear fluid leaks out of blood capillaries and surrounding tissue cells. To cope with this leakage, fluid is able to drain into a system of tubes that increase in size from tiny lymph capillaries to lymph vessels to lymph ducts and finally link together into two large collecting ducts (the thoracic duct and the right lymphatic duct). The lymphatic fluid drains back into the blood system at the right and left subclavian veins just before the blood returns to the heart at the right atrium.
There is no pump in this drainage system – instead, muscles attached to your skeleton provide the force to move this fluid back to the blood system at the subclavian veins.
To help this one-way flow of fluid, there are semilunar valves in the veins that prevent lymph flowing backwards. When people are sitting in one place or their muscles are restricted from full movement, there can be a build-up of fluid in the tissue because the pumping action of the muscle is not able to aid the movement of fluid away from the tissue. When that happens, tissues may swell – this is called oedema.
One way of preventing edema is to move your skeletal muscles so there is not a build-up of tissue fluid. This is why you are requested to carry out leg exercises when you are flying long distances. Also, when you are standing still for very long periods, you should gently rock to and fro.
At certain points on the larger lymphatic vessels, there are lymph nodes. Their function is to produce cells called lymphocytes that protect the body against infection.
Lymph nodes are distributed throughout the body, and you may have noticed that, when you are sick, your doctor my feel the lymph nodes in your neck or ask about the lymph nodes in your pelvic region. When the lymph nodes are swollen, it is a sign that the body is actively fighting an infection with the arsenal of cells and substances produced by this lymph node tissue.
Nature of science
One way scientists communicate their findings is by careful drawings. Medical students check their dissections against the dissections by experts. Henry Gray’s Anatomy of the Human Body is a famous anatomy textbook that was first published in the UK in 1858. Others have contributed to this book, and later editions have been published since then.
Scientists have discovered that cancer cells travel from the tumour through the lymphatic system, and this process is called metastasis. Scientists have developed a technique to map the closest lymph node to the tumour. Once this lymph node is identified, surgeons are able to remove this lymph node and carry out a biopsy (cross section and chemical test) to see if the cancer cells have travelled this far. This technique is called sentinel node biopsy.
Because of this new technique of detection, biopsy and treatment, there is a much greater chance that cancers will be detected before they spread, so there is more incentive for people to check on unusual growths on their bodies rather than ignoring them or denying that there might be a problem.