Anatomy of spleen

THE SPLEEN
The spleen is a wedge-shaped organ lying mainly in the left hypochondrium, and partly in the epigastrium. It is wedged in between the fundus of the stomach rand the diaphragm. Sometimes the spleen is tetrahedral in shape .The spleen is soft, highly vascular and dark purple in colour.
                 The size and weight of the spleen are markedly variable. On an average the spleen is 1 inch or 2.5 cm thick, 3 inches or 7.5 cm broad, 5 inches or 12.5 cm long, 7 ounces in weight, and is related to 9th to 11th ribs. Normally, the spleen is not palpable (odd numbers 1, 3, 5, 7, 9, 11). 

Spleen Antomy

Surface Marking:

 (a) It is marked on the left side of the back, with its long axis corresponding with that of the 10th rib
 (b) The upper border corresponds to the upper border of the 9th rib, and the lower border to the lower border of the 11th rib.
 (c) The medial end lies 4 to 5 cm from the midline; and the lateral end at the midaxillary line.

The spleen lies obliquely along the long axis of the 10th rib. Thus it is directed downwards, forwards and laterally, making an angle of about 45 degrees with the horizontal plane

External Features

The spleen has two ends, three borders and two surfaces .Two Ends The anterior end is expanded and is more like a border. It is directed downwards and forwards, and reaches the midaxillary line. The posterior end is rounded. It is directed upwards, backwards and medially, and rests on the upper pole of the left kidney
      Three Borders :The superior border is characteristically notched near the anterior end. The inferior border is rounded. The intermediate border is also rounded and is directed to the right.
      Two Surfaces :The diaphragmatic surface is convex and smooth. The visceral surface is concave and irregular.

       It bears the following impressions:

 The gastric impression, for the fundus of the stomach, lies between the superior and intermediate borders. It is the largest and most concave impression .The renal impression, for the left kidney, lies between the inferior and intermediate borders. The colic impressions, for the splenic flexure of the colon, occupies a triangular area adjoining the anterior end of the spleen. Its lower part is related to the phrenicocolic ligament. The pancreatic impression, for the tail of the pancreas, lies between the hilum and the colic impression.
                              The hilum lies on the inferomedial part of the gastric impression along the long axis of the spleen. It transmits the splenic vessels and nerves, and provides attachment to the gastrosplenic and lienorenal ligaments.

 Peritoneal Relations :

                         The spleen is surrounded by peritoneum, and is suspended by following ligaments:
1. The gastrosplenic ligament extends from the hilum of the spleen to the greater curvature of the somach. It contains the short gastric vessels and associated lymphatics and sympathetic nerves.

2.The lienorenal  extends from the hilum of the spleen to the anterior surface of the left kidney. It contains the tail of the pancreas, the splenic vessels, and associated pancreaticosplenic lymph nodes, lymphatics and I sympathetic nerves.

3.The phrenicocolic ligament is not attached to the spleen, but supports its anterior end. It is a horizontal fold of peritoneum extending from the splenic flexure of the colon to the diaphragm opposite the 11th rib in the midaxillary line. It limits the upper end of the left paracolic gutter.

Visceral Relations:

     1. Visceral Surface
  The visceral surface is related to the fundus of the stomach, the anterior surface of the left kidney, the splenic flexure of the colon and the tail of the pancreas .
     2.Diaphragmatic Surface 
The diaphragmatic surface is related to the diaphragm which separates the spleen from the costo-diaphragmatic recess of pleura, lung and 9th, 10th and 11th ribs of the left side .

 Venous Drainage :

The splenic vein is formed at the hilum of the spleen. It runs a straight course behind the pancreas. It joins the superior mesenteric vein behind the neck of the pancreas to form the portal vein. Its tributaries are the short gastric, left gastroepiploic, pancreatic and inferior mesenteric vein.

 Arterial Supply:

 The spleen is supplied by the splenic artery which is the largest branch of the coeliac trunk. The artery is tortuous in its course to allow for movements of the spleen. It passes through the lienorenal ligament to reach the hilum of the spleen where it divides into five or more branches. These branches enter the spleen to supply it. Within the spleen it divides repeatedly to form successfully the straight vessels called penicilli, ellipsoids and arterial capillaries. Further course of the blood is uncertain.

According to "closed" theory of splenic circulation, the capillaries are continuous with the venous sinusoids that lie in the red pulp; the sinusoids join together to form veins. However, according to "open" theory of splenic circulation, the capillaries end by opening into the red pulp from where the blood enters the sinusoids through their walls. Still others believe in a "compromise" theory, where the circulation is open' in distended spleen and closed' in contracted spleen. The splenic circulation is adapted for the mechanism of separation and storage of the red blood cells. On the basis of its blood supply, the spleen is said to have superior and inferior vascular segments. The two segments are separated by an avascular plane. Each segment may sometimes be subdivided into one to two disc-like middle segments and a cap-like pole segment.
                              Apart from its terminal branches, the Lymphatic Drainage Splenic tissue proper has no lymphatics. A few lymphatics arise from the connective tissue of the capsule and trabeculae and drain into the pancreaticosplenic lymph nodes situated along the splenic artery.

Nerve Supply :

Sympathetic fibres are derived from the coeliac plexus. They are vasomotor in nature. They also supply some smooth muscle present in the capsule.

Functions of the Spleen :

1. Phagocytosis:
 The spleen is an important component of the reticuloendothelial system. The splenic phagocytes include
 (a) the reticular cells and free macrophages of the red pulp.
 (b) modified reticular cells of the ellipsoids.
 (c) free macrophages and endothelial cells of the venous sinusoids, and
 (d) surface reticular cells of the lymphatic follicle.

 The phagocytes present in the organ remove cell debris and old and effete RBCs, other blood cells and microorganisms, and thus filter the blood. Phagocytosis of circulating antigens initiates humoral and cellular immune responses.

2. Haemopoiesis:
                    The spleen is an important haemopoietic organ during foetal life. Lymphopoiesis continues throughout life. The lymphocytes manufactured in it take part in immune responses of the body. In the adult spleen, haemopoiesis can restart in certain diseases, like chronic myeloid leukaemia and myelosclerosis.
 3. Immune responses:
                  Under antigenic stimulation, there occurs increased lymphopoiesis for cellular responses, and increased formation of plasma cells for the humoral responses. 4. Storage ofRBCs: Red blood cells can be stored in the spleen and released into the circulation when needed. This function is better marked in animals than in man

                                                           HISTOLOGY

                          Histologically, spleen is made up of the following four component parts:

 1. Supporting tissue, fibroelastic forming the capsule, coarse trabeculae and a fine reticulum. In human, the smooth muscle cells in the capsule and trabeculae are few, and the contraction and distension of spleen are attributed to constriction or relaxation of the blood vessels, which regulate the blood flow in the organ.
2. White pulp consisting of lymphatic nodules arranged around an eccenteric arteriole called Malpighian corpuscle.
3. Red pulp is formed by the collection of cells in the interstices of reticulum, in between the sinusoids .The cell population includes (a) all types of lymphocytes (small, medium and large), (b) all three types of blood cells (RBC, WBC and platelets), and (c) the fixed and free macrophages. Lymphocytes are freely transformed into plasma cells which can produce large amounts of antibodies, the immunoglobulins.
4. Vascular system trasverses the organ and permeates it thoroughly.
                   

                                                   CLINICAL ANATOMY

1. Palpation of the spleen: A normal spleen is not palpable. An enlarged spleen can be felt under the left costal margin during inspiration. Palpation is assisted by turning the patient to his right side. Note that the spleen becomes palpable only after it has enlarged to about twice its normal size.

2. Splenomegaly: Enlargement of the spleen is called splenomegaly. It may occur in a number of diseases. Sometimes the spleen becomes very large. It then projects towards the right iliac fossa in the direction of the axis of the tenth rib.

 3. Splenectomy: Surgical removal of the spleen is called splenectomy. During this operation damage to the tail of the pancreas has to be carefully avoided, as the tail of pancreas is very rich in islets of Langerhans.

 4. Splenic puncture: Spleen can be punctured through the 8th or 9th intercostal space in the midaxillary line using a lumbar puncture needle. When enlarged, it can be punctured through the anterior abdominal wall. To avoid laceration of spleen, the patient must hold his breath during the procedure. Intrasplenic pressure is an indirect record of the portal pressure.
                      Splenic venography reveals and confirms the enlarged portosystemic communications in cases of portal hypertension.

5. Splenic infarction: The smaller branches of splenic artery are end arteries. Their obstruction (embolism) therefore results in splenic infarction which causes referred pain in the left shoulder (Kehr's sign).