Most people find it fairly easy to envisage the heart andvessels as a closed system of pipes and tubes, with circulating round and round. The lymphatic system is more difficult to understand. The lymphatic system consists of lymph vessels throughout the body, which carry a fluid called lymph. This lymph is essentialy tissue fluid which needs to be filtered of all kinds of debris and dead . After filtration the lymph returns to the stream. During the filtration of foreign particles or bacteria, antibodies can be formed in the lymph nodes. This is the immunological function of the lymphatic system.
The body fluid which bathes allis derived from the bloodstream, and is continuously renewed from it. Thousands of litres of fluid per day with various substances seep out of the capillaries into the , where exchange of nutrients and waste products takes place. Ninety per cent of the tissue fluid re-enters the veins again directly. About ten per cent of the tissue fluid is returned to the circulation by the lymphatic system, which drains all organs except the brain. The lymphatic system is – in contrast to the blood circulation – a one-way system, starting blindly in the lymphatic capillaries, which have extremely thin walls and are much more permeable than blood capillaries. Even particles as large as cells can penetrate through the pores of the lymphatic capillaries. The superficial lymphatics comprise the multitude of narrow vessels that lie just beneath the skin, close to the capillary veins. The deep lymphatics are larger and much less numerous, and are associated with the deep blood vessels.
The lymph vessels become progessively larger until they empty into the circulation by means of the two major lymph ducts.
These are the right lymphatic duct, which drains the right upper half of the body, and the thoracic duct which drains the rest of the body. From these major lymph ducts the lymph fluid passes into the large veins of the neck, thus restoring tissue fluids and proteins to the circulation. The lymph from the intestines carries nutrients, especially fats, to the bloodstream, and is called chyle because of its milky white colour (chylos is the Greek word for milky juice).
When lymph cannot be drained by the lymph vessels because of an obstruction, massive lymph oedema can develop. For example, as a result of an operation for breast cancer, the lymphatic channels of the arm can be damaged. The arm may then swell to gigantic proportions.
However before entering the veins, lymph passes through lymph nodes (called lacteals when they process chyle from the intestines). The lymph nodes are absorbent glands that function as filters in the lymphatic drainage system. A lymph node is 10-25 mm in diameter and is supplied by one or more lymph
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Vessels. Every lymph vessel passes through at least one lymph node.
Inside the capsule of the lymph node is a network of little canals, forming a sponge-like structure. The walls of this network, through which the lymph percolates, are formed by reticulum cells that can ingest foreign proteins, dead cells, bacteria and the like, thus cleansing the lymph. For example the lymph nodes in the lungs of smokers accumulate inhaled carbon particles (soot) and the lymph nodes can also trap cancer cells or bacterial agents such as tuberculosis bacteria.
The reticulum cells belong to the reticulo-endothelial system (RES), part of which are also the tonsils, spleen, thymus and Peyer’s patches in the intestines. The structure of the lymphatic system can be clearly shown by injecting an X-ray opaque dye into one of the small lymphatic channels in the cleft between the toes. This technique is called lymphangiography. With it the lymphatic system and any defects in it can be seen on X-rays. Enlarged lymph nodes can be revealed, even if located deep within the body. CAT (computerized axial tomography) scanning is an alternative method for detecting enlarged lymph nodes hidden indeep within the body. This procedure is a special type of X-ray examination and does not require any injection.