Section 2.2: Spleen Anatomy, Functions, and Reticuloendothelial System
Key Takeaways
- The spleen is an intraperitoneal organ located in the left hypochondrium, bounded by the diaphragm, stomach, left kidney, and pancreatic tail.
- The splenic white pulp is responsible for immunologic responses and antibody synthesis, while the red pulp filters and removes damaged erythrocytes.
- Culling is the removal and destruction of abnormal or senescent erythrocytes, whereas pitting extracts intraerythrocytic inclusion bodies without cell destruction.
- Normal adult splenic length is less than 12 to 13 cm, and splenomegaly is diagnosed when the length measures 13 cm or greater.
Section 2.2: Spleen Anatomy, Functions, and Reticuloendothelial System
The spleen is a highly vascular, intraperitoneal organ located in the left hypochondrium (left upper quadrant) of the abdomen. It is the largest single mass of lymphoid tissue in the body and is a vital component of the reticuloendothelial system (RES). The spleen is covered by a dense fibroelastic capsule, except at the splenic hilum, where the splenic artery enters and the splenic vein exits.
Anatomical Location and Relations
The spleen lies posterolateral to the stomach, with its long axis parallel to the 10th rib. Its position is maintained by several peritoneal ligaments: the gastrosplenic ligament (connecting the spleen to the stomach), the splenorenal (lienorenal) ligament (connecting the spleen to the left kidney), and the phrenicocolic ligament.
The anatomical relationships of the spleen are critical for sonographers to identify, as displacement can indicate pathology:
- Superior-Lateral Border: The diaphragm forms a smooth, convex boundary separating the spleen from the left pleural cavity and lung.
- Medial-Anterior Border: The stomach lies medial and anterior to the spleen.
- Medial-Inferior Border: The left kidney lies medial and inferior to the spleen.
- Medial-Hilar Border: The tail of the pancreas lies medial and extends toward the splenic hilum. It is located anterior to the splenic vein and is an important landmark when evaluating the pancreatic tail or hilum.
Histology and Functional Zones
The splenic parenchyma is divided into two distinct functional zones: the white pulp and the red pulp.
White Pulp
The white pulp constitutes about 20% of the spleen and is composed of lymphatic tissue. It is distributed throughout the spleen in tiny islands surrounding the splenic arterioles. These islands are composed of lymphocytes and macrophages, organized into malpighian corpuscles. The white pulp performs the primary immune functions of the spleen, including antigen presentation, lymphocyte proliferation, and antibody (immunoglobulin G) production. It is highly active in fighting blood-borne pathogens, particularly encapsulated bacteria.
Red Pulp
The red pulp makes up the remaining 80% of the parenchyma and is composed of splenic sinuses (sinusoids) and splenic cords (cords of Billroth). The red pulp is a vast filter that screens circulating blood. The cords of Billroth are packed with macrophages that destroy old, damaged, or abnormal blood cells. The sinuses are highly distensible vascular spaces where erythrocytes are squeezed and inspected.
| Splenic Zone | Anatomical Structure | Primary Function | Sonographic Features |
|---|---|---|---|
| White Pulp | Malpighian corpuscles, lymphocytes | Immune response, antibody production, lymphocyte defense | Not individually resolvable; contributes to overall homogeneous texture |
| Red Pulp | Splenic cords (Billroth), sinuses | Erythrocyte filtration, culling, pitting, iron recycling | Forms the vascular background; responds to portal congestion |
Physiology and Splenic Functions
The spleen performs several complex physiological roles essential for hematology and immunology:
- Culling: The identification and destruction of senescent, malformed, or damaged erythrocytes. Normal red blood cells are highly pliable and can squeeze through the narrow walls of the splenic sinuses. Aged or rigid cells (e.g., in spherocytosis) are trapped in the cords of Billroth and phagocytosed by macrophages.
- Pitting: The process by which macrophages pluck out intraerythrocytic inclusion bodies (such as Howell-Jolly bodies, which are nuclear remnants, or Heinz bodies, which are denatured hemoglobin) without destroying the erythrocyte itself. The repaired cell is then returned to the circulation.
- Hematopoiesis: During the 5th and 6th months of fetal development, the spleen is an active site of erythrocyte and leukocyte production. While this function ceases before birth, the spleen retains its hematopoietic potential. In adults with severe bone marrow disorders (e.g., myelofibrosis), the spleen can reactivate blood cell production, a process known as extramedullary hematopoiesis.
- Blood Reservoir: The spleen stores red blood cells and up to 30% of the body's platelets within its sinuses. During acute hemorrhage or exercise, sympathetic stimulation causes the splenic capsule to contract, expelling these cells into the systemic circulation.
Normal Measurements and Splenomegaly
- Length: The normal adult spleen measures less than 12 cm or 13 cm in length. A splenic length of 13 cm or greater is the primary diagnostic criterion for splenomegaly.
- Width: 7 to 8 cm.
- Thickness: 3 to 4 cm.
In clinical practice, splenomegaly is most commonly caused by portal hypertension (resulting from liver cirrhosis), which leads to passive venous congestion of the red pulp. Other causes include lymphoma, leukemia, infectious mononucleosis, and hemolytic anemias.
Congenital Variants
- Accessory Spleen (Splenule): This is the most common splenic variant, found in up to 30% of individuals. It appears as a small, round, homogeneous mass, usually less than 2 cm in diameter. It is most commonly located at the splenic hilum or near the tail of the pancreas. Its echogenicity and texture are identical to the main spleen. It is clinically significant because it can be mistaken for a pancreatic tumor or enlarged lymph node, and it can hypertrophy to take over splenic function after a splenectomy.
- Wandering Spleen (Splenoptosis): An embryological anomaly where the supporting ligaments fail to develop, allowing the spleen to migrate into the lower abdomen or pelvis. It presents as a mobile mass and is prone to splenic torsion, which can cause acute infarction.
- Polysplenia and Asplenia (Heterotaxy):
- Polysplenia: Characterized by multiple small spleens, associated with bilateral left-sidedness, interrupted IVC, and biliary atresia.
- Asplenia: Characterized by the absence of splenic tissue, associated with bilateral right-sidedness, midline liver, and severe congenital heart defects.
During an abdominal ultrasound, a 1.5 cm, well-demarcated, homogeneous nodule is visualized at the splenic hilum. The echogenicity of the nodule is identical to the splenic parenchyma. What is the most likely diagnosis?
Which of the following processes describes the spleen's ability to identify and destroy old, abnormal, or damaged red blood cells?
A patient with suspected heterotaxy syndrome is referred for an ultrasound. The sonographer notes the complete absence of a spleen, a midline gallbladder, and a reversed orientation of the aorta and IVC. Which condition is associated with these findings?