The spleen is a soft, highly vascular organ that plays an important role in the immune system and blood filtration. It is the largest lymphoid organ in the body, containing white and red pulp, which are involved in immune responses and the removal of old or damaged red blood cells. The spleen is also a storage site for platelets and plays a role in recycling iron from hemoglobin.
Location
The spleen is located in the upper left quadrant of the abdomen, beneath the diaphragm and posterior to the stomach. It lies between the 9th and 11th ribs and is protected by the ribcage. The spleen is in close proximity to other structures such as the left kidney, the splenic flexure of the colon, and the tail of the pancreas.
Structure and Anatomy
The spleen is a soft, highly vascular, fist-sized organ that plays a vital role in the lymphatic and circulatory systems. It is encased in a fibrous capsule and consists of two main types of tissue—red pulp and white pulp—which perform distinct functions related to blood filtration and immune response. Below is a detailed breakdown of the spleen’s anatomical structure.
External Structure
The spleen is a smooth, oval-shaped organ with a distinctive notch on its anterior surface. It is surrounded by a fibrous capsule and has several surfaces and ligaments that connect it to adjacent structures.
Capsule
The capsule of the spleen is a dense, fibrous layer of connective tissue that surrounds the entire organ. This capsule provides structural support and protection, and it extends inward to form trabeculae, which partition the spleen into compartments and provide an internal framework.
Surfaces of the Spleen
- Diaphragmatic Surface: The convex surface of the spleen that faces the diaphragm, fitting snugly against it.
- Visceral Surface: The concave surface that faces the abdominal cavity, and it is in contact with surrounding organs such as the stomach, left kidney, and pancreas.
Hilum
The hilum is the indentation on the visceral surface of the spleen where blood vessels, nerves, and lymphatic vessels enter and exit. The splenic artery and splenic vein pass through the hilum, providing the blood supply to the spleen. The hilum is also the point where lymphatic drainage occurs.
Ligaments
The spleen is connected to adjacent organs by peritoneal ligaments, which help maintain its position in the upper abdomen:
- Gastrosplenic Ligament: Connects the spleen to the greater curvature of the stomach and contains the short gastric and left gastroepiploic vessels.
- Splenorenal (Lienorenal) Ligament: Connects the spleen to the left kidney and contains the splenic vessels and the tail of the pancreas.
- Phrenicosplenic Ligament: Attaches the spleen to the diaphragm.
Internal Structure
The internal structure of the spleen is divided into two main regions: red pulp and white pulp, which perform distinct but complementary roles related to blood filtration and immune defense.
Red Pulp
The red pulp makes up the majority of the spleen’s internal structure (about 75-80%) and is responsible for filtering the blood. It is composed of splenic cords (cords of Billroth) and splenic sinusoids:
- Splenic Cords: These are made up of a meshwork of reticular fibers, which contain a high concentration of macrophages, red blood cells, platelets, and other immune cells. The splenic cords serve as the site for trapping and breaking down old or damaged red blood cells.
- Splenic Sinusoids: These are wide, irregularly shaped blood vessels that are lined with specialized endothelial cells. The sinusoids allow blood to flow through the red pulp, where it is filtered and returned to the circulatory system.
White Pulp
The white pulp forms approximately 20-25% of the spleen’s tissue and is involved in immune function. It is organized around the central arterioles and consists of lymphoid tissue, including lymphoid follicles and periarteriolar lymphoid sheaths (PALS):
- Periarteriolar Lymphoid Sheaths (PALS): These are areas of densely packed T lymphocytes that surround the central arterioles. They play a key role in cell-mediated immune responses.
- Lymphoid Follicles: These structures are rich in B lymphocytes and often contain germinal centers, where B cells proliferate and differentiate in response to antigens.
Marginal Zone
The marginal zone is the area between the red pulp and white pulp. It contains macrophages, dendritic cells, and a mix of B and T lymphocytes. The marginal zone serves as the first point of contact for antigens entering the spleen and plays a role in the initiation of immune responses.
Blood Supply
The spleen is highly vascular and relies on an extensive network of blood vessels to carry out its functions of filtering blood and supporting immune responses.
Splenic Artery
The splenic artery is a major branch of the celiac trunk and supplies oxygenated blood to the spleen. It enters the spleen at the hilum and branches into smaller arterioles as it moves through the spleen, ultimately giving rise to the central arterioles that supply the white pulp.
Central Arterioles
The central arterioles are small arteries that branch from the splenic artery and pass through the white pulp. These arterioles are surrounded by periarteriolar lymphoid sheaths (PALS) and serve as the primary blood supply for the white pulp.
Splenic Sinusoids and Venous Drainage
Blood flows from the arterioles into the splenic sinusoids of the red pulp, where it is filtered. After passing through the sinusoids, the blood is collected into small venules and then drains into the splenic vein, which exits the spleen at the hilum and drains into the portal vein of the liver.
Lymphatic Drainage
The spleen contains lymphatic vessels that help drain lymph fluid from the organ and transport it to nearby lymph nodes for further filtration and immune processing.
Lymphatic Vessels
Lymphatic drainage from the spleen occurs through lymphatic vessels located at the hilum. These vessels carry lymph away from the spleen and into the pancreaticosplenic lymph nodes and other nearby lymph nodes.
Immune Cell Migration
Lymphocytes and other immune cells produced or activated in the spleen can migrate through the lymphatic system to other parts of the body, where they participate in immune responses.
Nerve Supply
The spleen receives autonomic innervation, which regulates blood flow and immune activity within the organ.
Sympathetic Innervation
The sympathetic nerve fibers that supply the spleen originate from the celiac plexus. These nerves travel alongside the splenic artery and control vasoconstriction, influencing the volume of blood filtered by the spleen.
Parasympathetic Innervation
The vagus nerve provides parasympathetic innervation to the spleen, although its role is less prominent compared to sympathetic input.
Function
The spleen plays a multifaceted role in the immune system and blood regulation. It is responsible for filtering blood, recycling red blood cells, storing blood components, and mounting immune responses. Below is a detailed breakdown of the functions of the spleen, organized into specific categories.
Blood Filtration
The spleen acts as a filter for the blood, removing old or damaged red blood cells and other blood components while also capturing pathogens and foreign particles.
Removal of Old or Damaged Red Blood Cells
The red pulp of the spleen contains splenic cords and splenic sinusoids, which filter red blood cells (RBCs). As blood flows through the sinusoids, old, damaged, or abnormal RBCs are trapped and engulfed by macrophages. This process helps maintain healthy blood cell levels by removing defective or senescent RBCs from circulation.
Recycling of Iron and Hemoglobin
The breakdown of old red blood cells releases hemoglobin, which is further processed to recover iron. The iron is recycled and transported to the bone marrow for the production of new RBCs, while the remaining hemoglobin is converted into bilirubin, which is transported to the liver for excretion.
Clearance of Blood-Borne Pathogens and Debris
The spleen also removes pathogens, immune complexes, and cellular debris from the blood. Macrophages in the red pulp phagocytose bacteria, viruses, and dead cells, preventing their spread and further infection.
Immune Response
The spleen is a key lymphoid organ involved in initiating and regulating immune responses to blood-borne pathogens. It contains both innate and adaptive immune cells, which work together to detect and eliminate foreign invaders.
Antigen Capture and Presentation
The spleen’s white pulp is rich in immune cells, including B cells, T cells, macrophages, and dendritic cells. These cells are responsible for capturing and presenting antigens (foreign substances) to initiate an immune response. Dendritic cells and macrophages in the marginal zone capture antigens from the blood and present them to T lymphocytes in the white pulp.
Activation of T and B Lymphocytes
- The periarteriolar lymphoid sheaths (PALS) in the white pulp are rich in T lymphocytes. When T cells encounter antigens, they become activated and either assist B cells in producing antibodies (helper T cells) or directly attack infected cells (cytotoxic T cells).
- B lymphocytes, located in lymphoid follicles, are activated when they encounter antigens. Once activated, B cells proliferate and differentiate into plasma cells, which produce antibodies to neutralize pathogens.
Production of Antibodies
Plasma cells, derived from activated B cells, reside in the white pulp of the spleen and secrete antibodies into the bloodstream. These antibodies bind to pathogens, marking them for destruction by other immune cells or neutralizing them directly.
Immune Memory
The spleen helps develop immune memory by maintaining a population of memory B and T cells. These memory cells remain in the spleen and circulate in the blood, ready to mount a quicker and stronger immune response if the same pathogen is encountered again.
Hematopoiesis (During Fetal Development)
In early fetal development, the spleen plays a critical role in the formation of blood cells, a process known as hematopoiesis. Although this function is later taken over by the bone marrow, the spleen serves as a major site of hematopoiesis during embryonic and fetal stages.
Fetal Hematopoiesis
The spleen produces red blood cells, white blood cells, and platelets during early development. This process continues until about the fifth month of gestation, after which the bone marrow becomes the primary site of hematopoiesis. However, in certain conditions, such as severe anemia, the spleen can resume its hematopoietic function, a process known as extramedullary hematopoiesis.
Storage of Blood Components
The spleen serves as a reservoir for blood, storing red blood cells, platelets, and other immune components, which can be released into circulation when needed.
Platelet Storage
The spleen stores about one-third of the body’s platelets, which are essential for blood clotting. In response to injury or blood loss, the spleen releases these stored platelets into the bloodstream to help form clots and stop bleeding.
Red Blood Cell Storage
In some animals (such as horses and dogs), the spleen can store a significant amount of red blood cells. While this function is less pronounced in humans, the spleen does hold a small reserve of red blood cells that can be released into the bloodstream in response to situations such as hypoxia (low oxygen levels).
Rapid Response to Hemorrhage
In cases of significant blood loss or hemorrhage, the spleen contracts to release stored blood components, including platelets and red blood cells, into the circulation. This helps maintain blood volume and pressure during emergencies.
Regulation of Blood Volume
The spleen plays a role in regulating blood volume by acting as a blood reservoir and filtering out excess cells or fluid from circulation.
Removal of Excess Cells
In conditions where there are excessive numbers of circulating blood cells (e.g., polycythemia), the spleen can remove these excess cells from the bloodstream, helping maintain a balanced blood volume and composition.
Blood Reservoir
The spleen can store blood and release it into circulation when necessary, particularly in situations of blood loss or during physical activity. This ability helps regulate overall blood volume and pressure.
Recycling of Iron
The spleen plays an essential role in the recycling of iron from hemoglobin during the breakdown of old or damaged red blood cells.
Recovery of Iron from Hemoglobin
Macrophages in the spleen’s red pulp break down hemoglobin from degraded red blood cells. The iron released from hemoglobin is captured and returned to the body’s iron stores, primarily in the liver and bone marrow, where it is used for the production of new red blood cells.
Iron Storage and Transport
The spleen stores iron temporarily before it is transported to the liver and bone marrow via transferrin, a blood protein responsible for iron transport. This process ensures a constant supply of iron for hematopoiesis, the production of new red blood cells.
Defense Against Blood-Borne Pathogens
The spleen is particularly effective at defending the body against blood-borne pathogens, such as encapsulated bacteria.
Filtering Bacteria and Pathogens
The spleen filters the blood to trap and destroy encapsulated bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis), which are more difficult for the immune system to detect and eliminate. Macrophages and other immune cells in the red pulp engulf and digest these pathogens.
Protection Against Sepsis
By filtering pathogens from the bloodstream, the spleen helps prevent the spread of infection and reduces the risk of sepsis, a life-threatening condition where infection spreads throughout the body.
Lymphocyte Production and Activation
The spleen serves as a site for the production, maturation, and activation of lymphocytes—the key players in the immune system.
B Cell Activation
The white pulp of the spleen is rich in B lymphocytes, which produce antibodies in response to pathogens. When activated by an antigen, B cells proliferate and differentiate into plasma cells, which release antibodies into the bloodstream.
T Cell Activation
T lymphocytes in the periarteriolar lymphoid sheaths (PALS) are activated by antigen-presenting cells such as dendritic cells and macrophages. These T cells can then assist B cells in antibody production or directly attack infected cells.
Clinical Significance
The spleen plays a crucial role in immune defense, blood filtration, and maintaining healthy blood cell levels. Its clinical significance is particularly evident in conditions like splenomegaly (enlarged spleen), which can occur due to infections (e.g., mononucleosis), liver diseases (e.g., cirrhosis), or hematologic disorders (e.g., leukemia and lymphoma). Asplenia (absence of spleen) or splenectomy (surgical removal of the spleen) can leave individuals more vulnerable to infections, particularly from encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae, because the spleen is essential in filtering these pathogens from the blood.
Patients without a spleen are at higher risk for overwhelming post-splenectomy infection (OPSI), a potentially fatal condition. Therefore, individuals without a functioning spleen require vaccinations and prophylactic antibiotics to prevent infections. The spleen’s role in blood cell recycling and immune response highlights its importance in overall health, and disorders affecting the spleen often require careful medical management.
