Physiology

Physiology

The splenic parenchyma consists of white and red pulp that is surrounded by serosa and a collagenous capsule with smooth muscle fibres. These penetrate the parenchyma as trabeculae of dense connective tissue fibres rich in collagen and elastic tissue. These, with the reticular framework, support the cells of the spleen and surround the vessels in the splenic pulp. The white pulp comprises a central trabecular artery surrounded by nodules with germinal centres and periarterial lymphatic sheaths that provide a framework filled with lymphocytes and macrophages. Arteries from the central artery and the periph - eral ‘penicillar’ arteries pass into the marginal zone that lies at the edge of the white pulp. Plasma-rich blood that has passed through the central lymphatic nodules is filtered as it passes through the sinuses within the marginal zone, and particles are phagocytosed. Immunoglob ulins produced in the lymphatic nodules enter the circulation through the sinuses in the marginal zone, beyond which lies the red pulp, which consists of cords and sinuses. Cell-concentrated blood passes in the trabecular artery through the centre of the white pulp to the red pulp cords. Red cells must elongate and become thinner to pass from the cords to the sinuses, a process that removes abnormally shaped cells from the circulation ( Figure 70.3 ). As 90% of the blood passing through the spleen moves through an open circula tion in which blood flows from arteries to cords, and thence sinuses, splenic pulp pressure reflects the pressure through - out the portal system. The remaining 10% of the blood flow through the spleen bypasses the cords and sinuses by direct arteriovenous communications. The overall flow rate of b lood is about 300 /uni00A0 mL/min. -

Greater sac Stomach (lesser curvature) Stomach (greater curvature) Gastrosplenic ligament Short gastric arteries Spleen Splenorenal ligament Splenic artery Left kidney Figure 70.1 Transverse section (craniocaudal view) showing the important ligaments of the spleen containing the blood vessels (courtesy of Dr Tusharindra Lal, Chennai, India). Coeliac trunk Aorta Common hepatic artery Portal vein Duodenum Pancreas (head) Splenic vein (retropancreatic) Superior mesenteric vein Uncinate process Tail of pancreas Figure 70.2 Schematic representation showing relations at the hilum of the spleen with the blood supply and venous drainage. Note the avascular splenocolic ligament that needs to be divided carefully to free the inferior pole of the spleen from the splenic /f_l exure (courtesy of Dr Tusharindra Lal, Chennai, India). Aorta Falciform ligament Free edge of the lesser omentum Bile duct Hepatic portal vein Epiploic foramen Inferior vena cava Peritoneum (parietal layer) Right kidney Lesser sac Body of Left gastric pancreas Short gastric arteries artery Splenic artery Hilum of spleen Left gastroepiploic artery Splenocolic ligament Splenic /f_l exure of colon

Capsule Trabecula Vascular sinusoid Red pulp Figure 70.3 Functional anatomy of the spleen. Blood from a central trabecular artery passes through the white pulp into the surrounding red pulp and hence to the vascular cords and sinuses that drain into the trabecular vein.