The adult spleen weighs about 100–150 g. Try cupping your hand- your spleen would fit nicely there! This encapsulated organ is situated in the left upper quadrant of the abdomen.
It is made up of:
- Supporting tissue
- White pulp
- Red pulp
- Its vascular system
The spleen lies in the left hypochondrium between the 9th and 11th ribs. It is situated obliquely between the fundus of the stomach and the left diaphragm; it is vascular, and reddish purple in colour.
It weighs approximately 150 g and measures approximately 12 cm in length, 7 cm in width and 3-4 cm in depth.
The spleen is a lymphatic organ which appears during the 5th developmental week as a focus of mesenchymal proliferation between the layers of the dorsal mesogastrium between the stomach and dorsal aorta. As the stomach rotates, the left part of the dorsal mesogastrium comprises the gastrolienal and lienorenal ligaments.
Clusters of the mesenchymal cells merge to form the lobulated fetal spleen. These lobulations may persist in adult life seen as clefts or notches. The dorsal mesogastrium containing the spleen is then forced to the left by the rotation of the greater curvature of the stomach.
Following this, fusion occurs between the surface of the dorsal mesogastrium posterior to the spleen and the opposing parietal peritoneum, which overlies the left kidney, forming the splenorenal ligament. Therefore, the splenic artery lies within the retroperitoneum before entering the splenorenal ligament.
By week 12 the spleen has taken up its final position within the peritoneum. The spleen produces red blood cells during gestational weeks 16 to 32 in conjunction with its role of antibody, monocyte and lymphocyte production. The production of red blood cells then ceases and the spleen continues the role to which we are more familiar.
The spleen functions as a haematopoietic centre until late fetal life.
The spleen is intraperitoneal with peritoneum covering the whole organ except for its vascular pedicle.
The visceral surface of the spleen is related to:
- The anterior surface of the left kidney
- The splenic flexure of the colon
- The fundus of the stomach
- The tail of the pancreas attaches to the splenorenal ligament and extends to the splenic hilum, making the pancreas quite vulnerable in splenectomy.
- The diaphragmatic surface is related to the diaphragm; the diaphragm separates the spleen from the pleura and the lung, lying craniolaterally and posterior and following the curvature of the left diaphragm and costophrenic recess from the 9th – 11th ribs.
The visceral surface is moulded to the impressions of the surrounding organs and the centrally placed hilum providing the entry point for the splenic vessels, nerves and lymphatics.
The visceral surface is divided by a ridge, into dorsal and ventral portions
- The dorsal portion faces caudomedially and is closely related to the left kidney and left adrenal
- The ventral portion of the visceral surface faces craniomedially and is related to the posterior wall of the stomach and the pancreatic tail. Anterior to the spleen is the splenic flexure of the left colon.
The spleen is suspended in position by the gastrosplenic, splenorenal and phrenicocolic ligaments. These ligaments are formed from fusions of two surfaces in the visceral peritoneum:
- The gastrosplenic ligament, carrying the short gastric arteries and the left gastroepiploic artery, connects the greater curve of the stomach to the splenic hilum. Here it divides, coating the spleen before rejoining to form the phrenicocolic ligament that extends to the splenic flexure of the mesocolon
- The splenorenal ligament extends from the anterior aspect of the left kidney to the hilum of the spleen and encloses the splenic blood vessels, lymphatics, nerves as well as the pancreatic tail
The splenic artery originates from the coeliac axis. It is 13-32 cm long and follows a tortuous course along the superior boarder of the pancreas, giving off branches to form the short gastric, left gastroepiploic and pancreatic arteries before dividing into between 2-5 branches as it approaches the splenic hilum in the splenorenal ligament.
The intrasplenic arterial branches do not connect, which explains why branch artery obstruction leads to wedge infarction.
Variants in the origin of the splenic artery include arising from the common hepatic or left gastric arteries or even the aorta.
The splenic vein is formed from 4-5 branches at the hilum and passes anteriorly in the splenorenal ligament before running retroperitoneally posterior to the body of the pancreas.
The splenic vein receives many tributaries, including the inferior mesenteric vein before merging with the superior mesenteric vein to form the portal vein posterior to the head of the pancreas at the level of the first and second lumbar vertebrae (L1/L2).
The superior polar vein is a variant of the splenic vein, draining the upper splenic pole before joining the splenic vein proximally. In the case of splenic vein occlusion, collateral veins in the region of the stomach appear.
The splenic vein receives the short gastric and left gastroepiploic veins proximally whilst further along it receives pancreatic branches and the inferior mesenteric vein before merging with the superior mesenteric vein (SMV).
Lymphatic drainage of the spleen is via the pancreaticosplenic nodes. This chain drains to the coeliac nodes.
The splenic nerves originate from the coeliac plexus. These sympathetic nerves follow closely the course of the splenic artery and fibres innervate the splenic capsule, trabeculae and vasculature.
Encasing the spleen is a capsule consisting of fibro-elastic tissue and a small amount of smooth muscle. Trabeculae extend into the organ and divide the splenic pulp: red and white. In brief: the white pulp is responsible for the splenic lymphoid function. It is a major site of antibody, lymphocyte and monocyte production. The red pulp, which constitutes the majority of the spleen, carries out a phagocytic function and is responsible for the destruction of aged and abnormal blood cells.
Functions of the Spleen
The spleen carries out these functions:
- Immune responses
- Storage of red blood cells