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Lymph
At the capillary end of the arteries, the walls are only one-cell thick. The pressure in the arterial capillaries is also quite high. These leads the plasma to leak into the interstitial space. The interstitial space is the space between the cells of the tissue. The fluid is plasma minus the proteins (as the proteins cannot pass through the walls of the capillaries) and is called the interstitial fluid or the tissue fluid.
Capillary Exchange
However, at the venous end of the capillaries, the pressure is not that high and some of the fluid goes back into the blood vessels. There is still some fluid that has to be returned to the blood vessel along with the dissolved solutes. This fluid if not cleared from the interstitial space will result in oedema or swelling. The fluid then enters the closed vessels called the lymph vessels and is called the
lymph.
Lymph Vessels
The lymph vessels also have walls that are single celled in thickness. The lymph vessels join together to form the bigger lymph ducts. The lymph vessels have no muscles but have valves that allow only unidirectional lymph flow. An example of a lymph vessel is the lacteal that are present in the villi of the small intestine. The digested fats enter the lacteals and are transported to the blood stream via the lymph ducts.
Circulation of Lymph
The lymph is circulated in the body with the help of the action of the muscles surrounding the lymph vessels. Thus, the movement of lymph is slow and uncertain. Exercise improves the rate of lymph flow. The lymph vessels are provided with valves to prevent backflow of the lymph. Throughout the lymphatic system, the lymphatic ducts enlarge at various places to form sinuses or cavities called the lymph glands or nodes. The lymph nodes are cavities lined with phagocytic cells and have numerous lymphocytes.
Human Lymphatic System
The lymphocytes produce antibodies against the bacteria and other germs and attack them. The phagocytes of the lymph node also attack the bacteria and the other foreign bodies. Their activity becomes more during an infection. This causes an increase in their size. For example, the tonsils enlarge and become painful during a bacterial infection. The lymph is returned to the venous blood near the heart.
