Spleen

Introduction

The spleen is a vital organ located in the upper left quadrant of the abdomen. It plays an essential role in blood filtration, immune response, and hematologic regulation. Despite its importance, the spleen is often overlooked in discussions of systemic physiology.

Anatomy of the Spleen

Gross Anatomy

The spleen is a soft, vascular organ with a roughly ovoid shape. It is positioned beneath the diaphragm and protected by the ribs on the left side of the body. Its size and shape can vary between individuals.

• Location and Relations: The spleen lies lateral to the stomach, posterior to the left kidney, and adjacent to the diaphragm. Its position can influence clinical examination and imaging.
• Surfaces and Borders: The diaphragmatic surface faces the ribs and diaphragm, while the visceral surface faces the stomach, kidney, and colon. Borders are classified as superior, inferior, anterior, and posterior.
• Ligaments and Peritoneal Attachments: The spleen is supported by the gastrosplenic, splenorenal, and phrenicosplenic ligaments, which stabilize it in the abdominal cavity and carry blood vessels.
• Vascular Structures: The splenic artery, a branch of the celiac trunk, supplies oxygenated blood. Venous blood drains via the splenic vein into the portal vein.

Microscopic Anatomy

Microscopically, the spleen is organized into red and white pulp regions, which support its hematologic and immune functions.

• White Pulp: Composed of lymphoid tissue surrounding central arterioles, white pulp is involved in immune surveillance and lymphocyte activation.
• Red Pulp: Contains venous sinusoids and cords of Billroth, responsible for filtering blood, removing aged or damaged red blood cells, and storing platelets.
• Marginal Zone: A transitional region between red and white pulp that plays a critical role in antigen trapping and initiating immune responses.
• Reticular Framework and Stromal Cells: Provides structural support for hematopoietic and immune cells and facilitates the movement of blood through the spleen.

Physiology of the Spleen

Hematologic Functions

The spleen plays a key role in maintaining healthy blood composition and managing cellular components.

• Red Blood Cell Filtration and Destruction: Aged, damaged, or abnormal red blood cells are identified and removed from circulation in the red pulp.
• Platelet Storage and Regulation: The spleen stores a portion of circulating platelets and releases them as needed to maintain hemostasis.
• Hematopoiesis in Fetal Life: During fetal development, the spleen serves as a primary site for red and white blood cell production before the bone marrow takes over this function after birth.

Immune Functions

The spleen is a critical lymphoid organ that contributes to both innate and adaptive immunity.

• White Pulp-Mediated Immune Response: Lymphocytes in the white pulp recognize and respond to blood-borne antigens, producing antibodies and activating immune cells.
• Antigen Presentation and Lymphocyte Activation: Antigen-presenting cells capture antigens and stimulate T and B lymphocytes to mount a targeted immune response.
• Response to Encapsulated Bacteria: The spleen is especially important in defending against encapsulated organisms such as Streptococcus pneumoniae and Haemophilus influenzae, which can cause severe infections if splenic function is impaired.

Reservoir Functions

The spleen serves as a reservoir for blood components, providing additional supply during physiological stress or hemorrhage.

• Blood Reservoir for Emergencies: The red pulp stores red blood cells and platelets that can be released into circulation during acute blood loss or hypovolemic states.
• Regulation of Circulating Blood Volume: By adjusting the release of stored blood cells and plasma, the spleen helps maintain stable blood volume and pressure.

Blood Supply and Lymphatics

The spleen receives a rich blood supply and has an extensive lymphatic network that supports its hematologic and immune functions.

• Splenic Artery and Vein: The splenic artery provides oxygen-rich blood from the celiac trunk, while venous blood drains through the splenic vein into the portal vein.
• Collateral Circulation: Collateral vessels ensure continuous perfusion in case of arterial obstruction or surgical intervention.
• Lymphatic Drainage Pathways: Lymph from the spleen drains into the pancreaticosplenic and celiac lymph nodes, contributing to immune surveillance and fluid balance.

Innervation of the Spleen

The spleen receives autonomic innervation, which helps regulate vascular tone, blood flow, and immune activity within the organ.

• Sympathetic Innervation: Originates from the celiac plexus, controlling vasoconstriction and influencing the release of stored blood cells during stress or hemorrhage.
• Parasympathetic Innervation: Provided by the vagus nerve, contributing to modulation of immune functions and maintaining basal vascular tone.
• Clinical Relevance of Innervation: Damage or disruption of splenic innervation can affect blood reservoir capacity and immune responses, influencing outcomes in trauma or surgery.

Development and Embryology

The spleen develops from mesenchymal tissue during early embryogenesis and plays a critical role in fetal hematopoiesis.

• Origin from Mesenchymal Tissue: The spleen arises from mesodermal cells in the dorsal mesogastrium, forming the initial primordium during the fifth week of gestation.
• Fetal Hematopoietic Role: During fetal life, the spleen produces red and white blood cells, supporting oxygen transport and immune development before the bone marrow becomes the primary hematopoietic organ.
• Development of Vascular and Lymphoid Structures: The splenic artery, vein, and lymphoid tissue develop concurrently, establishing the organ’s blood supply and immune functionality by birth.

Common Spleen Disorders

Splenomegaly

Splenomegaly refers to enlargement of the spleen, which can result from various systemic and localized conditions.

• Causes: Includes infectious diseases such as mononucleosis, hematologic disorders like hemolytic anemia, and congestive conditions such as portal hypertension.
• Clinical Implications: Enlarged spleen can cause abdominal discomfort, early satiety, and increased risk of rupture.

Hypersplenism

Hypersplenism is characterized by overactivity of the spleen, leading to excessive sequestration and destruction of blood cells.

• Pathophysiology: Often associated with splenomegaly, leading to pancytopenia due to increased removal of red blood cells, white blood cells, and platelets.
• Hematologic Consequences: Can result in anemia, leukopenia, and thrombocytopenia, increasing susceptibility to infections and bleeding complications.

Traumatic Injuries

The spleen is vulnerable to injury due to its location and delicate structure, especially in blunt abdominal trauma.

• Splenic Rupture and Laceration: Can result from motor vehicle accidents, falls, or sports injuries, leading to internal bleeding and hypovolemic shock.
• Management and Surgical Interventions: Treatment may include observation for minor injuries, embolization, or surgical procedures such as splenectomy in severe cases.

Functional Asplenia and Hyposplenism

Functional asplenia and hyposplenism refer to reduced or absent splenic function, often predisposing individuals to infections.

• Causes and Risks: Can result from congenital conditions, sickle cell disease, or surgical removal of the spleen.
• Infection Susceptibility: Patients are at increased risk for infections by encapsulated bacteria, requiring vaccinations and prophylactic antibiotics in some cases.

Neoplasms

The spleen can be affected by both primary and secondary tumors, which may alter its structure and function.

• Primary Splenic Tumors: Rare and include lymphomas, hemangiomas, and angiosarcomas, which can cause splenomegaly or hematologic abnormalities.
• Secondary Involvement from Systemic Malignancies: The spleen may be involved in metastatic disease from cancers such as melanoma, breast, or lung carcinoma, often detected incidentally on imaging.

Diagnostic Evaluation of Spleen Disorders

Accurate assessment of spleen function and pathology requires a combination of clinical examination, laboratory tests, and imaging studies.

• Physical Examination and Palpation: Enlargement or tenderness of the spleen can be detected through careful abdominal examination.
• Laboratory Investigations: Complete blood count, peripheral smear, and specific tests for hemolytic or infectious conditions help identify functional abnormalities.
• Imaging Studies: Ultrasound, CT, and MRI provide detailed visualization of splenic size, lesions, and vascular status.
• Histopathology When Indicated: Biopsy or post-surgical examination allows for definitive diagnosis of neoplasms or infiltrative disorders.

Treatment and Management

Management of spleen disorders depends on the underlying condition, severity, and patient-specific factors. Treatment strategies may be medical, surgical, or preventive.

• Medical Management: Includes antibiotics for infections, medications to manage hematologic abnormalities, and monitoring for mild splenomegaly or hypersplenism.
• Surgical Management: Partial or total splenectomy may be necessary for traumatic rupture, severe hypersplenism, or neoplasms.
• Vaccinations and Preventive Measures for Asplenic Patients: Vaccines against encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis are essential. Prophylactic antibiotics may also be indicated.
• Lifestyle and Monitoring Considerations: Regular follow-up, avoidance of high-risk activities that may cause abdominal trauma, and patient education are important for long-term care.

References

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