The spleen is an organ sitting in the left upper quadrant of the abdomen. It is usually small and sits inside the rib case. It is similar to the liver in many ways, which sits a similar position in the right upper quadrant of the abdomen.
Blood supply
The spleen receives blood from the splenic artery, which is a branch of the celiac artery. The celiac artery branches from the abdominal aorta.
The spleen is drained by the splenic vein, into the portal hepatic vein. This blood then moves through the liver and eventually returns to the heart through the inferior vena cava.
The spleen usually contains a large volume of blood and consequently, a rupture of the capsule is a medical emergency.
Systemic functions
The spleen is involved because it contains a huge reservoir of monocytes in red pulp. Red pulp macrophages eat cellular debris to remove badly damaged or dysfunctional erythrocytes (red blood cells) from the body.
The spleen is involved because it contains specialised lymphoid tissues in which lymphocytes are created, trained, and organised. A large proportion of serum immunoglobulin type M (IgM) is produced in the spleen.
There are two distinct tissues within the spleen which are specialised to carry out these functions: red pulp and white pulp.
Histology
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| Simplified representation of the functional organisation of the spleen |
The splenic artery branches into many trabecular arteries. These pass through both the white pulp and the red pulp. The tissues are drained by trabecular veins which drain into the splenic vein.
White pulp
White pulp comprises approximately 25% of splenic tissue. It is not truly white but it appears less red than red pulp. Blood reaches the tissue through a central arteriole. Each arteriole is surrounded by Periarteriolar Lymphoid Sheaths (PALS). These contain T lymphocytes.
The PALS are surrounded by lymph follicles with dividing B lymphocytes. IgM and IgG2 are produced here. These are antibodies which are particularly effective against encapsulated bacteria.
Furthest from the central arteriole you can find marginal zone tissue. It lies between white pulp and red pulp. It contains antigen presenting cells (including dendritic cells, macrophages, and metallophillic macrophages). It appears to be specialised for the trapping of particles from the systemic circulation. Immuno-stimulatory antigens are then trafficked into the white pulp for processing. It also appears to be proficient at the removal of viral particles from the blood stream.
Red pulp
Red pulp is engorged with blood, giving it a red appearance. It is made of connective tissue organised into cords of Bilroth and sinusoids. Incoming blood must pass these cords and is forced through narrow gaps in the endothelium (the lining) of the sinusoids. Fragile, inflexible erythrocytes are trapped or destroyed by the strain. Healthy erythrocytes should pass intact into the sinusoids, which drain into the trabecular veins, into the splenic vein and back into systemic circulation. The red pulp contains a huge reservoir of monocytes and specialised macrophages which consume the debris.
Splenic capsule
Finally the spleen is surrounded by a tough capsule made of connective tissue.
Comparison to other organs
- The liver
Both organs are part of the reticuloendothelial system. They both contain a huge volume of blood and remove waste from the circulation. They sit in equivalent positions in the upper abdomen/lower thorax. They both receive blood from the same branch of the systemic circulation (the celiac artery and abdominal aorta). The splenic vein drains into the liver, allowing the venous blood to be processed a second time before it returns to the heart.
- Lymph nodes
The spleen and lymph nodes are useful sites of activity for the lymphatic system. The microscopic organisation of the white pulp is very similar to the microscopic organisation of lymph nodes. Within both structures, immune cells can share antigens and undergo priming. Cells within lymph nodes and within the spleen are able to express various chemokines. These are immunological proteins which guide chemotaxis (movement) of specific immune cells towards the relevant tissues. The state of an immune cell determines which chemokines it will migrate towards and the ability of the cell to detach itself from a tissue or migrate through a tissue.
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