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Spleen

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Spleen
Spleen

Spleen

Image Credit: SEER Training Modules / U. S. National Institutes of Health, National Cancer Institute

Spleen

The spleen is the largest organ of the lymphatic system. Located in the upper left region of the abdominal cavity, the spleen's primary function is to filter blood of damaged cells, cellular debris, and pathogens such as bacteria and viruses. Like the thymus, the spleen houses and aids in the maturation of immune system cells called lymphocytes. Lymphocytes are white blood cells that protect against foreign organisms that have managed to infect body cells. Lymphocytes also protect the body from itself by controlling cancerous cells. The spleen is valuable to the immune response against antigens in the blood.

Spleen Anatomy

The spleen is often described as being about the size of a small fist. It is positioned under the rib cage, below the diaphragm, and above the left kidney. The spleen is rich in blood supplied via the splenic artery. Blood exits this organ through the splenic vein. The spleen also contains efferent lymphatic vessels, which transport lymph away from the spleen. Lymph is a clear fluid that comes from blood plasma that exits blood vessels at capillary beds. This fluid becomes the interstitial fluid that surrounds cells. Lymph vessels collect and direct lymph toward veins or other lymph nodes.

The spleen is a soft, elongated organ that has an outer connective tissue covering called a capsule. It is divided internally into many smaller sections called lobules. The spleen consists of two types of tissue: red pulp and white pulp. White pulp is lymphatic tissue that mainly consists of lymphocytes called B-lymphocytes and T-lymphocytes that surround arteries. Red pulp consists of venous sinuses and splenic cords. Venous sinuses are essentially cavities filled with blood, while splenic cords are connective tissues containing red blood cells and certain white blood cells (including lymphocytes and macrophages).

Spleen Function

The major role of the spleen is to filter blood. The spleen develops and produces mature immune cells that are capable of identifying and destroying pathogens. Contained within the white pulp of the spleen are immune cells called B and T-lymphocytes. T-lymphocytes are responsible for cell mediated immunity, which is an immune response that involves the activation of certain immune cells to fight infection. T-cells contain proteins called T-cell receptors that populate the T-cell membrane. They are capable of recognizing various types of antigens (substances that provoke an immune response). T-lymphocytes are derived from the thymus and travel to the spleen via blood vessels. B-lymphocytes or B-cells originate from bone marrow stem cells. B-cells create antibodies that are specific to a specific antigen. The antibody binds to the antigen and labels it for destruction by other immune cells. Both white and red pulp contain lymphocytes and immune cells called macrophages. These cells dispose of antigens, dead cells, and debris by engulfing and digesting them.

While the spleen functions chiefly to filter blood, it also stores red blood cells and platelets. In instances where extreme bleeding occurs, red blood cells, platelets, and macrophages are released from the spleen. Macrophages help to reduce inflammation and destroy pathogens or damaged cells in the injured area. Platelets are blood components that help blood clot to stop blood loss. Red blood cells are released from the spleen into blood circulation to help compensate for blood loss.

Summary

The spleen is a lymphatic organ that performs the valuable function of filtering blood. While it is an important organ, it can be removed when necessary without causing death. This is the case because other organs, such as the liver and bone marrow, can perform filtration functions in the body. A spleen may need to be removed if it becomes injured or enlarged. An enlarged spleen may occur for several reasons. Bacterial and viral infections, increased splenic vein pressure, vein blockage, as well as cancers may cause the spleen to become enlarged. Abnormal cells may also cause an enlarged spleen by clogging splenic blood vessels, decreasing circulation, and promoting swelling. A spleen that becomes injured or enlarged may rupture. Spleen rupture is life threatening because it results in serious internal bleeding.

Sources:
  • SEER Training Modules, Spleen. U. S. National Institutes of Health, National Cancer Institute. Accessed 26 June 2013 (http://training.seer.cancer.gov/)


  • The Spleen. Patient.co.uk. Updated 06/10/12 (http://www.patient.co.uk/health/the-spleen)

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