The circulatory system is responsible for transporting nutrients and oxygen to cells through the blood vessels. The smallest of these vessels are the capillaries. In the capillaries the blood pressure forces plasma out into the tissue fluid that surrounds the cells in a given area.
Not all the fluid that leaves the capillaries is able to return to the heart via the circulatory system. Fluid leaves the vascular system across capillary walls at a greater rate than it returns. It is estimated that over a 24-hour period the difference in volume between plasma leaving the capillaries and that which returns to the veins is about three litres. If not for the lymphatic system this fluid would accumulate and cause large-scale swelling of the tissues. Fortunately, a system of lymph capillaries drains away this fluid so that it does not accumulate in the tissues of our body. The lymph fluid also contains substances such as fats, proteins, ions, clotting factors, lymphocytes, and cellular waste products that cannot return via the circulatory system. Once inside the lymph vessels, this fluid is collectively known as lymph.
Unlike blood vessels, which are a closed circuit, lymph capillaries are blind-ended tubes that start in the tissues of our body. They are found everywhere in our body except the brain and spinal cord. In the small intestine there are special lymph capillaries called lacteals, which are involved in the absorption of fats. All lymph capillaries join to form larger lymphatic vessels. These lymphatic vessels ultimately merge into large vessels that join into two major ducts: the thoracic duct and the right lymphatic duct. These ducts empty their contents back into the main veins, which bring blood back to the heart.
The lymphatic system does not have a heart to create pressure and cause fluid to move. A contraction of skeletal muscle squeezes lymphatic vessels and pushes lymph fluid along. Like veins, larger lymphatic vessels have valves that prevent lymph from flowing in the reverse direction.
At intervals along the lymphatic vessels are lymph nodes, which are primary lymphoid structures. They range in size from being nearly invisible to the naked eye to the size of a marble. The lymph capillaries, lymphatic vessels, and lymph nodes together make up the lymphatic system. Also included in the lymphatic system are three structures known as the lymphoid organs and include the tonsils, thymus, and spleen.
Cells within the lymph nodes, including white blood cells or lymphocytes, play a very important role within the immune system of our body by removing bacteria and other foreign matter from the lymph as it passes through the nodes. Some lymphocytes also produce antibodies, which play an important role in fighting infection. The organs and tissues of the lymphatic system are the major production sites of two main types of lymphocytes: T cells and B cells, which respond and proliferate in response to an infection. Also found within the lymph nodes are white blood cells called phagocytes, which engulf and destroy particles such as bacteria and dead cells by a process known as phagocytosis.
The lymphatic system is thus a vital system in not only maintaining fluid balance within our body but also as a crucial wall of defence against the many foreign bodies such as bacteria that enter our body every day.
Glossary
Antibody
A protective molecule circulated in the blood and lymph in response to invasion by a foreign particle or microbe. Antibodies, which are produced by B lymphocytes, are carried around the body by blood and lymph.
B cells
A type of lymphocyte that is produced and matures in the bone marrow.
Blood capillaries
Very thin walled vessels that allow for the rapid exchange of materials between the circulatory system and the fluid surrounding the tissue and cells.
Lacteals
Lymphatic vessels within the small intestine that are involved in the absorption of fats.
Lymph
a colourless liquid that moves within the lymphatic system and consists of water, fats, proteins, ions, clotting factors, lymphocytes, and cellular waste products.
Lymph capillaries
Very thin walled vessels of the lymphatic system. Lymph capillaries are blind-ended tubes that are very permeable to a wide range of substances, including proteins, fats, ions, and bacteria.
Lymph nodes
A small mass of lymphoid tissue varying in size from microscopic to the size of a marble, found within the lymphatic system. They produce lymphocytes, which respond to infection.
Lymph vessels
The larger vessels of the lymphatic system. They contain valves that only allow lymph fluid to flow in one direction.
Lymphocyte
A type of white blood cell that is a primary component of the immune response to infection. The main types of lymphocytes include B and T cells.
Lymphoid organs
Organs made from tissue in which the main types of cells are reticular cells and lymphocytes. These types of organs include the spleen, thymus, tonsils, and adenoids.
Pathogen
A specific causative agent of disease such as a bacterium, virus, or chemical.
phagocytes
White blood cells that engulf and destroy invading microorganisms, foreign particles, and dead cells.
Phagocytosis
The process by which cells such as phagocytes surround and engulf objects.
Plasma
The liquid component of blood in which particulates are suspended. Plasma consists of more than 90 percent water, about 7 percent protein, and small percentages of other substances, such as inorganic salts, glucose, urea, and other waste products of metabolism.
Right lymphatic duct
A large vessel that receives lymph from the right side of the head and neck and returns it to the bloodstream.
Spleen
The largest lymphoid organ in the human body; it is located in the abdomen. Its primary responsibility is to filter the blood. The spleen also contains regions for both lymphocyte production and red blood cell destruction.
T cells
A type of lymphocyte that is produced in the bone marrow but matures in the thymus gland.
Thoracic duct
The common trunk of all the lymphatic vessels. It drains the lymph from the lower extremities and returns it to venous circulation.
Thymus
A lymphoid organ located in the neck or chest. It is responsible for the production and maturation of lymphocytes.
Tonsils
Small masses of lymphatic tissue that produce lymphocytes. There are three sets in humans: on each side of the oral pharynx behind the mouth cavity (palatine tonsils); on the top wall of the nasal pharynx (adenoids); and at the base of the tongue (lingual tonsils). Tonsils can become infected and enlarged causing a condition known as tonsillitis.
Vein
A blood vessel that takes de-oxygenated blood from the tissues and returns it to the heart. Veins have thin walls and contain valves to maintain blood flow in one direction.
White blood cell
A general name for a group of cells that includes lymphocytes and phagocytes. They play a primary role in the immune response.
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