Chapter 69: Spleen Anatomy

The spleen, a large and complex mass of vascular and lymphoid tissue situated in the upper left quadrant of the abdomen, functions crucially in immunological defense, phagocytosis, metabolism, and the maintenance of circulating blood elements. Though not essential for life, its role is vital, particularly in the fetus where it is a major site of blood cell formation. The adult organ typically measures 9–14 cm in length and weighs between 150 and 350 g in vivo. Its variable, wedge-like shape is molded by adjacent structures, including the respiratory diaphragm (superolateral surface), and the stomach, left kidney, and left colic flexure (inferomedial visceral surface). The visceral surface contains impressions reflecting these nearby organs, as well as the splenic hilum, which is penetrated by the splenic vessels, nerves, and lymphatics. Notably, the spleen is only palpable on examination once it has tripled in size due to conditions like splenomegaly. Anatomical variations include the frequent presence of notches on the superior border and the finding of a fully functional accessory spleen in approximately 14% of individuals. The spleen is supported by peritoneal ligaments, such as the gastrosplenic, splenorenal, phrenicosplenic, and splenocolic ligaments. The vascular supply is derived from the highly tortuous splenic artery, typically arising from the coeliac trunk, which delivers blood to segmental arteries with relatively limited collateral circulation. The venous return collects into the splenic vein, which runs behind the pancreas before joining the superior mesenteric vein to form the hepatic portal vein. The parenchyma is distinctly divided into the white pulp (about 20% of volume), composed of T and B lymphocytes organized in peri-arteriolar lymphoid sheaths (PALS) and follicles, and the red pulp (about 75% of volume), which serves as a filtration device consisting of splenic cords (of Billroth) and venous sinusoids. Blood circulation in the human red pulp is generally understood to be open, meaning blood percolates slowly through the cord tissue before re-entering the specialized sinusoids. Clinically, the spleen is vulnerable to blunt trauma, which can cause lacerations and hemorrhage, sometimes resulting in referred left shoulder pain (Kehr’s sign). Surgical management often involves splenectomy, which requires careful ligation of the splenic artery (usually first) and vein (usually last). The most significant long-term risk of spleen removal is severe, life-threatening sepsis. Consequently, conservative or partial splenectomy techniques are favored to preserve some splenic function whenever possible.