The largest organ in the lymphatic system is the spleen. It is similar to a large lymph node and acts as a blood filter for the body. The spleen is an integral part of the immune system. Not only does it synthesize antibodies, but it also acts as a reservoir for many materials. There exists a method to remember the spleen’s measurements and location. This is the 1x3x5x7x9x11 rule. The approximate measurements of a spleen are one inch wide, three inches tall, and five inches long. It weighs roughly seven ounces and sits between the ninth and eleventh ribs.
The spleen lies underneath the left portion of the thoracic diaphragm. The side of the spleen facing the diaphragm is smooth, while the opposite side has a ridge that divides it into two parts: the anterior gastric portion and the posterior renal portion. A thin outer coating called a capsule protects the organ. Additionally, the spleen contains two regions of tissue -- white pulp and red pulp. The spleen is not part of the digestive system, despite connecting to the blood vessels of the stomach and pancreas.
Phagocytes are cells that protect the body by consuming foreign materials such as bacteria and other particles and ingesting old red blood cells. The body’s immune system uses these cells to fight infections, respond to inflammation, and other functions. The spleen contains many of these cells, primarily monocytes and macrophages, cells that respond to inflammation then turn into macrophages to perform defense mechanisms. They also play important roles in adaptive immunity.
Part of the immune system that uses phagocytes is the mononuclear phagocyte system. Though this concept is slightly dated, it is a way to understand the roles of certain cells that exist in the spleen and other lymphoid organs. The spleen is the largest part of this process. The phagocytes in the spleen and its surrounding connective tissue make up the majority of the cells of the mononuclear phagocyte system. Its functions include creating new red and white blood cells, destroying old ones, creating plasma proteins, and storing iron.
The spleen contains red pulp that consists of connective tissue -- the cords of Billroth -- and blood vessels with open pores or sinusoids. The red pulp makes up over 75 percent of the spleen and acts as a storage area for monocytes. Billroth’s cords hold the monocytes in clusters. Many other types of cells such as white blood cells, splenic cells, macrophages, and red blood cells. The red pulp macrophages, like all phagocytes, protect the blood by ingesting foreign materials and aged red blood cells.
The remaining 25 percent of the spleen is white pulp, which consists of lymphatic tissue. Unlike the red pulp, the white pulp doesn’t contain a variety of cells. Instead, the white pulp contains areas that create antibodies and harbor T cells and macrophages. A T cell is a lymphocyte, a specific type of white blood cell that assists with the innate immune system. The macrophages of the white pulp are more of a mystery to researchers. Experts do not know much about their origin or lifespan, though they exist in all of the secondary lymphoid organs.
The cells that carry oxygen through the blood are the erythrocytes or red blood cells. Typically, red blood cells form in the bone marrow. However, up to the fifth month in the womb, a child’s spleen is also creating red blood cells. This process, hematopoiesis, ceases in the spleen after birth. However, the spleen retains its ability to create lymphocytes, making it an integral part of the immune system. In some rare cases, a disorder will enable the spleen to continue producing the red blood cells after birth.
Not only does the spleen create lymphocytes and red blood cells, but it also acts as a storage unit for them, as well as platelets, which are responsible for coagulation. The spleen contains more of these materials than are active in the human body. This excess exists in case an emergency infection or illness kills a large number of cells or requires more. The spleen holds up to a cup of red blood cells at any given moment, along with a quarter of the body’s white blood cells, and a third of its platelets.
Rarely, a child may grow another spleen while developing in the womb. This is typically a small nodule near the actual spleen. Around ten percent of the population has these accessory spleens, though they are only around a centimeter in diameter, so most people do not realize it. Some children may develop more than one accessory spleen as part of the congenital disease polysplenia. This can prevent the development of a functional, full-sized spleen.
The spleen is one of the few organs without which the human body can still survive. Asplenia is a general term that refers to a non-functioning spleen. This may be congenital, trauma-related, the result of a disease, or it may refer to someone who had their spleen surgically removed. Not having a spleen can cause vaccines to have a weaker effect and the body becomes slightly more susceptible to infections. Sepsis is also more likely to occur.
In ancient Greece and throughout the medieval period, many physicians thought the spleen was an organ crucial to human life. They believed that the body required four humors kept in balance to stay healthy. One of these four humors was black bile, and physicians thought the spleen created it. If a person had too much of this bile, they would become depressed. However, because doctors believed the spleen could cleanse excess bile as well as create it, they viewed the spleen in a positive light.
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