Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovsky 2008

Special Part
Hematopoietic and Immune System Organs - General Information
Secondary Lymphoid Organs - Spleen

The Spleen, splen {lien}, is an unpaired, bean-shaped organ (Fig. 117) located in the pregastric bursa, bursa pregastrica, of the Abdominal cavity. The spleen is a large lymphoid organ where lymphocytes are produced and phagocytosis of senescent or damaged erythrocytes and platelets takes place. Along with this, the spleen plays an important role in humoral and cellular immune responses. The spleen serves as a Blood reservoir: up to 8-12 % of the total blood volume can accumulate in its venous sinuses, and it can alter its size and shape. Its dimensions are: length - 12-15 cm, width - 8-10 cm, thickness - 4-5 cm. The average weight of the spleen is 150-200 g.

It has two surfaces: diaphragmatic and visceral; as well as two borders - superior and inferior, and two extremities - anterior and posterior. The PROJECTION OF THE spleen is at the level of the 9th to 12th Ribs. Inferiorly, the spleen touches the upper pole of the left Kidney, while its superior surface lies adjacent to the Diaphragm.

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Fig. 117. Spleen, lien (anterior view)

On the visceral surface of the spleen is its hilum, hilum splenicum, through which the splenic artery and nerves enter the organ, and the Veins emerge.

The spleen is surrounded by Peritoneum on all sides, except for the hilum. The peritoneum forms the gastrosplenic ligament, lig. gastrosplenicum, the phrenicosplenic ligament, lig. phrenicosplenicum, and the splenorenal ligament, lig. splenorenale, which suspend the spleen beneath the diaphragm near The Stomach and colon.

Externally, the spleen is covered by a Connective Tissue capsule consisting of Fibrous connective tissue, tunica fibrosa, from which numerous trabeculae, trabeculae splenicae, extend into the parenchyma, forming the stroma of the organ. The trabeculae divide the splenic pulp, pulpa splenica, into separate areas containing splenic lymphoid follicles, folliculi lymphatici splenici.

The spleen is distinguished into white and red pulp (Fig. 117 A). The white pulp, pulpa alba, consists of lymphoid tissue in the form of spherical aggregations, or nodules. The white pulp also includes periarterial lymphatic sheaths, which are accumulations of lymphocytes and plasma Cells along the course of the pulp artery. The red pulp, pulpa splenis rubra, is formed by reticular tissue containing Blood Cells and sinusoid-type Blood Vessels, which give it its red color.

Blood supply. The intraorgan vasculature of the spleen has several morphofunctional features. The Blood supply to the spleen is provided by the splenic artery, arteria lienalis. This is a muscular-type artery with a prominent internal elastic membrane. The splenic artery runs from right to left behind the parietal peritoneum along the superior border of the Pancreas and reaches the splenic hilum. From the hilum, the Arteries run inward within the large trabeculae (trabecular arteries) and enter the pulp. The white pulp of the spleen is located along the arteries that emerge from the trabeculae. At each site where the sheath expands into a lymphoid follicle, the artery branches to supply the follicle itself. These branches are called follicular arteries. The follicular artery further divides, forming a capillary network of the lymphoid follicle, and then exits the follicle into the red pulp. Here, the follicular artery divides into 2-6 branches that run in different directions. Because these branches diverge like a brush, they are called penicillar arterioles. Each of the penicillar arterioles divides into 2-3 arterioles (Asołnitzky, 1937), which terminate as ellipsoidal (sheathed) arterioles. Ellipsoidal arterioles are surrounded by unique "sheaths" — sleeves of accumulated reticular cells and reticular fibers that act as arterial sphincters of the spleen. Through The system of blood capillaries, the ellipsoidal arterioles connect with the venous sinuses of the spleen. From the venous sinuses, blood flows into the VEINS OF THE red pulp, then into the trabecular veins, and finally into the splenic vein. In the wall of the venous sinuses, there are clusters of myocytes that form the venous sphincters of the spleen.

Fig. 117 A. Internal Structure of the spleen (diagram)

A - White and red pulp; B - Terminal vascular pathway of the spleen

Lymphatic drainage. Lymphatic vessels and Lymph Nodes of the spleen are concentrated in the region of the hilum and accompany the arteries entering the spleen. Lymph from the spleen drains into the celiac lymph nodes.

Innervation. Innervation of the spleen is provided by sensory nerve fibers from the Neurons of the spinal ganglia and postganglionic sympathetic nerve fibers from the celiac plexus, which form a prominent subserous plexus and a finer plexus in the region of the hilum. Upon entering the spleen, the nerves form intratrabecular plexuses of varying density.