Table of Contents
Definition of Spleen, Function
Fibrous ligaments anchor the spleen to the stomach, colon, and left kidney.
The spleen holds about 300 milliliters of blood, roughly 4 percent of the body’s total blood supply, and contains about a third of the body’s platelets (the cells responsible for coagulation).
Its high blood content gives the spleen a dark red color and a somewhat porous texture. The spleen has two main structural and functional sections that filter the blood for different substances, the red pulp and the white pulp.
The white pulp consists of nodules and follicles, similar to those of other lymphatic tissues such as the lymph nodes, arranged in sheathlike structures that encase each of the tiny blood vessels (arterioles) within the spleen. The white pulp has two primary roles, to filter antigens from the circulating blood and to produce lymphocytes (a type of leukocyte).
These functions are interrelated in that the lymphocytes bear antibodies specific to the antigens the white pulp traps. When the lymphocytes enter the circulation of the blood or lymph, their antibodies allow them to intercept and destroy pathogens such as viruses or bacteria that carry the antigens.
The red pulp surrounds the white pulp. In the developing fetus the red pulp produces the majority of blood cells, erythrocytes (red blood cells) and leukocytes (white blood cells) alike, until about the fifth month of pregnancy, after which the red bone marrow takes over erythropoiesis (erythrocyte production).
Hematopoietic capability of the red pulp remains available but dormant after birth. Throughout life, the red pulp serves as an extramedullary (out of the marrow) resource that the body can press into action to produce erythrocytes.
The red pulp also filters the blood, culling outdated, defective, or damaged erythrocytes from circulation. Phagocytic cells called macrophages that reside within the red pulp break down the erythrocytes, sending components such as hemoglobin and bilirubin back into the blood for transportation to the liver, which recycles them. The red pulp also filters other cellular debris from the blood.
Potential Health Conditions Involving the Spleen
The spleen’s primary vulnerability is trauma, which can cause life-threatening hemorrhage (uncontrolled bleeding). A blow to the upper abdomen, such as may occur in motor vehicle accidents or with aggressive contact sports, can cause a rupture injury to the spleen. Such a blow also can fracture a rib, causing a penetrating wound to the spleen.
Other lymphatic structures and the liver can partially compensate for the spleen’s loss, though the risk for serious infection significantly increases. There are many health conditions that can cause the spleen to enlarge (splenomegaly). The spleen also enlarges when fighting systemic infections such as infectious mononucleosis and in some cancers.
For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”