The spleen is a lymphoid organ lying in the left hypochondrium posterior to the stomach. The fresh spleen is purple in colour and variable in size and shape. Since it lies entirely behind the midaxillary line and under cover of the left lower ribs, the normal spleen cannot be palpated in the living subject, even during full inspiration. The spleen is soft and very vascular and can be damaged by blunt or penetrating injuries resulting in life-threatening intra-peritoneal haemorrhage. The blood may irritate the peritoneum lining the abdominal surface of the diaphragm, producing pain referred to the left shoulder region (p. 205).
The spleen is oval in shape when viewed from its anterior aspect (Fig. 4.46) and its long axis lies parallel to the left tenth rib. The two extremities of the organ are connected by superior and inferior borders. The superior border often possesses one or more notches near its anterior end, while the inferior border is usually smooth. The organ has two easily distinguishable surfaces. The diaphragmatic surface faces backwards and laterally and is smoothly convex (Fig. 4.47). The visceral surface faces anteromedially and is characterized by ridges and depressions. The centrally placed hilum is perforated by numerous blood vessels together with lymphatics and nerves. The depressions around the hilum accommodate adjacent organs.
The spleen is an intraperitoneal organ and most of its capsule is covered by peritoneum of the greater sac. However, there is a small bare area near the hilum, which gives attachment to two peritoneal folds or ligaments. The splenorenal (lienorenal) ligament runs medially to reach the left kidney, while the gastrosplenic ligament connects the spleen to the greater curvature of the stomach. Part of the omental bursa lies between these two ligaments and extends to the left as far as the splenic hilum (Fig. 4.38).
Arching above the spleen and descending posterior and lateral to it, the left dome of the diaphragm is responsible for movements of the organ during ventilation (Fig. 4.48). The diaphragm separates the spleen from the left lung and pleura, and from the ninth, tenth and eleventh ribs.
On the visceral surface of the spleen, above the hilum, is the gastric impression, which accommodates part of the posterior surface of the stomach. Below the medial half of the hilum is the renal impression, which abuts the superior pole of the left kidney. Near the lateral extremity of its visceral surface, the spleen may possess a small colic impression, which lies against the left colic flexure. The tail of the pancreas extends laterally into the splenorenal ligament and its tip may reach the splenic hilum (Fig. 4.48).
Fig. 4.48 Spleen and its vessels and relationship to the diaphragm, pancreas and left kidney. The stomach and part of the colon and peritoneum have been removed.
The splenic artery is a direct branch of the coeliac trunk (p. 161). It follows a tortuous course along the upper border of the pancreas, giving off several pancreatic branches. The artery traverses the splenorenal ligament and divides into its terminal branches near the hilum of the spleen. Several splenic branches enter the hilum, while the short gastric arteries and the left gastro-omental artery enter the gastrosplenic ligament to supply the fundus and greater curvature of the stomach, respectively. Additional clumps of splenic tissue (splenunculi) may be present along the course of the artery.
Veins accompany the terminal branches of the splenic artery and unite adjacent to the hilum of the spleen to form the splenic vein. Running to the right, this vein lies posterior to the tail of the pancreas within the splenorenal ligament and continues retroperitoneally posterior to the body of the gland and inferior to the splenic artery. It then crosses the anterior aspects of the left kidney and renal vessels and receives several small tributaries from the pancreas. Posterior to the neck of the pancreas, the splenic vein unites with the superior mesenteric vein to form the portal vein and, close to its termination, is usually joined from below by the inferior mesenteric vein. If the pressure rises abnormally in the portal venous system (portal hypertension), the spleen may become enlarged (splenomegaly).