English on Duty page 191
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THE LYMPHATIC SYSTEM
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The Lymphatic System consists of a network of vessels that transport fluid
throughout the body, called lymph. Lymph is formed from a clear fluid
that bathes all the body tissues, mainly by filtration from the capillaries.
Lymphocytes and other substances, which are also part of the immune
system, are found in the lymph. All the organs, except for the heart and the
brain, have tiny lymphatic vessels. Foreign substances, proteins and
bacteria enter the lymphatics and join together to form larger vessels. The
lymph vessels contain valves which prevent the backflow of lymph. If a
lymphatic vessel becomes occluded, the buildup of lymph is termed
lymphedema. The lymphatics empty their contents into two large ducts
and from there the lymph reenters the circulation through the veins at the
base of the neck. Vessels draining the small intestine collect the digested
fat and pass it directly into the main blood circulation, bypassing the liver.
The lymph nodes are situated at frequent intervals along the lymph vessels
- which have two principal functions: 1) The production of lymphocytes
(white blood cells) and antibodies and 2) they act as a second line of
defense against bacterial infection, filtering off and destroying any that
bypass the inflammatory response at the infection site. In the case of
cancer cells, the lymph nodes cannot act as efficient barriers, they are only
partially effective against metastases. Swelling and pain normally occur
when a lymph node is stimulated by an infection. You may notice the
swelling even before the infection itself appears.
The lymph nodes are situated along the back and the sides of the neck, in
the armpits and groin. Other lymph nodes are found in the hilum of the
lungs, around the large vessels in the abdomen and pelvis. The tonsils and
spleen consist of lymphatic tissue and are considered part of the lymphatic
system. The Spleen lies within the abdominal cavity, and is situated under
the margin of the ribs on the left side. It is the size of an adult's fist. It is a
vascular organ, having a large arterial blood supply. The spleen produces
leukocytes, monocytes, lymphocytes and plasma cells. Lymphocytes are
produced in the white pulp and the latter is scattered throughout the red
pulp, which stores excess red blood cells and contains macrophages which
filter out old and damaged cells, debris and bacteria. It stores blood and if
an individual suffers severe blood loss, the spleen can increase the blood
volume from 340ml to 540ml in less than sixty seconds. It also stores iron
from the worn-out red cells, and like other lymphoid tissues, helps in the
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development of immunity. On entering the spleen, the blood flow slows
down as it enters a meshwork of dilated blood vessels. Lymphocytes are
found within these vessels. Another group of cells called phagocytes
which are capable of engulfing dead cells and foreign substances in the
blood and removing them from the circulation are also found here. The
arterial blood entering the spleen leaves it through the splenic vein and
passes to the liver through the portal vein. The function of the spleen is to
deal with the removal of abnormal or worn out blood cells from the
circulation.
The Tonsils are small rounded masses of tissue, especially lymphoid
tissue, which consist of the palatine tonsils in the oropharynx. Air reaches
the pharynx by passing through a section of lymphoid tissue in a ring
surrounding the back of the nose and throat. The adenoids are situated
posteriorly at the back of the nose, the two tonsils at the sides of the back
of the mouth and an adenoid-like tissue pad on the pharyngeal part of the
tongue. Bacteria and viruses are removed from inhaled air by the
lymphoid tissue.