Spleen structure and function

Spleen is the largest lymphoid organ in the body and it is highly vascular. It is situated in left hypochondrial region, i.e. upper left part of the abdomen, behind the stomach and just below the diaphragm. About 10% of people have one or more accessory spleens which are situated near the main spleen. Spleen is covered by an outer serous coat and an inner fibromuscular capsule. From the capsule, the trabeculae and trabecular network arise. All the three structures, viz. capsule, trabeculae and trabecular network contain collagen fibers, elastic fibers, smooth muscle fibers and reticular cells. The parenchyma of spleen is divided into red and white pulp.

RED PULP

Red pulp consists of venous sinus and cords of structures like blood cells, macrophages and mesenchymal cells.

WHITE PULP

The structure of white pulp is similar to that of lymphoid tissue. It has a central artery, which is surrounded by splenic corpuscles or Malpighian corpuscles. These corpuscles are formed by lymphatic sheath containing lymphocytes and macrophages.

1. FORMATION OF BLOOD CELLS

Spleen plays an important role in the hemopoietic function in embryo. During the hepatic stage, spleen

produces blood cells along with liver. In myeloid stage, it produces the blood cells along with liver and bone marrow.

2. DESTRUCTION OF BLOOD CELLS

Older RBCs, lymphocytes and thrombocytes are destroyed in the spleen. When the RBCs become old

(120 days), the cell membrane becomes more fragile. Diameter of most of the capillaries is less or equal to that of RBC. The fragile old cells are destroyed while trying to squeeze through the capillaries because, these cells cannot withstand the stress of squeezing. Destruction occurs mostly in the capillaries of spleen because the splenic capillaries have a thin lumen. So, the spleen is known as ‘graveyard of RBCs’.

3. BLOOD RESERVOIR FUNCTION

In animals, spleen stores large amount of blood. However, this function is not significant in humans. But,

a large number of RBCs are stored in spleen. The RBCs are released from spleen into circulation during the emergency conditions like hypoxia and hemorrhage.

4. ROLE IN DEFENSE OF BODY

Spleen filters the blood by removing the microorganisms. The macrophages in splenic pulp destroy the microorganisms and other foreign bodies by phagocytosis. Spleen contains about 25% of T lymphocytes and 15% of B lymphocytes and forms the site of antibody production.

SPLENOMEGALY AND HYPERSPLENISM

Splenomegaly refers to enlargement of spleen. Increase in the activities of spleen is called hypersplenism. Some diseases cause splenomegaly resulting in hypersplenism.

Diseases which cause splenomegaly:

1. Infectious diseases such as malaria, typhoid and tuberculosis

2. Inflammatory diseases like rheumatoid arthritis

3. Pernicious anemia

4. Liver diseases

5. Hematological disorders like spherocytosis

6. Cysts in spleen

7. Hodgkin’s disease

8. Glandular fever.

Effects of Splenomegaly

1. Hemolysis resulting in anemia

2. Leukopenia

3. Thrombocytopenia

4. Increase in plasma volume.

HYPOSPLENISM AND ASPLENIA

Hyposplenism or hyposplenia refers to diminished functioning of spleen. It occurs after partial removal of

spleen due to trauma or cyst. Asplenia means absence of spleen. Functional asplenia means normal functions of spleen. It occurs in the following conditions:

1. Congenital absence of spleen function (congenital asplenia).

2. Acquired through surgical removal of spleen (splenectomy).

3. Acquired through some diseases, which destroy spleen to such an extent that it becomes nonfunctional. This process is called autosplenectomy. The diseases which cause autosplenectomy are

sickle cell anemia and spherocytosis

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