Spleen Anatomy PDF

Spleen The spleen is an organ shaped like a shoe that lies relative to the 9th and 11th ribs and is located in the left hypochondria and partly in the epigastrium. Thus, the spleen is situated between the fundus of the stomach and the diaphragm. The spleen is very vascular and reddish purple in colo...

Spleen The spleen is an organ shaped like a shoe that lies relative to the 9th and 11th ribs and is located in the left hypochondria and partly in the epigastrium. Thus, the spleen is situated between the fundus of the stomach and the diaphragm. The spleen is very vascular and reddish purple in color; its size and weight vary. A healthy spleen is not palpable. Gross Anatomy The spleen's 2 ends are the anterior and posterior end. The anterior end of the spleen is expanded and is more like a border; it is directed forward and downward to reach the midaxillary line. The posterior end is rounded and is directed upward and backward; it rests on the upper pole of the left kidney. The spleen's 3 borders are the superior, inferior, and intermediate. The superior border of the spleen is notched by the anterior end. The inferior border is rounded. The intermediate border directs toward the right. The 2 surfaces of the spleen are the diaphragmatic and visceral. The diaphragmatic surface is smooth and convex, and the visceral surface is irregular and concave and has impressions. The gastric impression is for the fundus of the stomach, which is the largest and most concave impression on the spleen. The renal impression is for the left kidney and lies between the inferior and intermediate borders. The colic impression is for the splenic flexure of the colon; its lower part is related to the phrenicocolic ligament. The pancreatic impression for the tail of the pancreas lies between the hilum and colic impression Hilum The hilum can be found on the inferomedial part of the gastric impression. The hilum transmits the splenic vessels and nerves and provides attachment to the gastrosplenic and splenorenal (lienorenal) ligaments. Visceral relations The visceral surface of the spleen contacts the following organs: • Anterior surface of the left kidney • Splenic flexure of the colon • The fundus of the stomach • Tail of the pancreas The diaphragmatic surface is related to the diaphragm; the diaphragm separates the spleen from the pleura and the lung. Vascular supply The splenic artery supplies blood to the spleen. This artery is the largest branch of the celiac trunk and reaches the spleen's hilum by passing through the splenorenal ligament. It divides into multiple branches at the hilum. The splenic circulation is adapted for the separation and storage of the red blood cells. The spleen has superior and inferior vascular segments based on the blood supply. The 2 segments are separated by an avascular plane. Its terminal branches aside, the splenic artery also gives off branches to the pancreas, 5-7 short gastric branches, and the left gastro-omental (gastroepiploic) artery Functions of the Spleen Immune responses After antigenic stimulation, increased formation of plasma cells for humoral responses and increased lymphopoiesis for cellular responses occurs. Phagocytosis One of the spleen's most important functions is phagocytosis. The spleen is a component of the reticuloendothelial system. The splenic phagocytes include reticular cells, free macrophages of the red pulp, and modified reticular cells of the ellipsoids. Phagocytes in the spleen remove debris, old and effete red blood cells (RBCs), other blood cells, and microorganisms, thereby filtering the blood. Phagocytosis of circulating antigens initiates the humoral and cellular immune responses. Hematopoiesis The spleen is an important hematopoietic organ during fetal life; lymphopoiesis continues throughout life. The manufactured lymphocytes take part in immune responses of the body. In the adult spleen, hematopoiesis can restart in certain diseases such as chronic myeloid leukemia and myelosclerosis. Storage of red blood cells The RBCs are stored in the spleen. Approximately 8% of the circulating RBCs are present within the spleen; however, this function is seen better in animals than humans. The Adrenal Glands The adrenal glands lie retroperitoneal above each kidney. They are each enclosed within the perirenal fascia but in a separate compartment from the kidney. Each gland is composed of a body and medial and lateral limbs. The adrenal glands have an outer cortex derived from mesoderm and an inner medulla (10% of the weight of the gland), which is derived from the neural crest and is related to the sympathetic nervous system. The right adrenal gland tends to have a consistent location. It lies posterior to the inferior vena cava, medial to the right lobe of the liver and lateral to the right diaphragmatic crus It is lower and more medial in relation to the spine than the left. On cross-section, it is linear or V-shaped, with a larger medial limb and a smaller lateral limb. The left adrenal gland lies posterior to the splenic vein and lateral to the diaphragmatic crus, but its position is less consistent than that of the right side. The left adrenal gland is more semilunar than the right and it extends down the superomedial border of the kidney towards the hilum. On crosssection it is triangular or Y-shaped Embryologically, the adrenals do not develop with the kidneys. They develop in the retroperitoneum and descend, whereas the kidneys develop in the pelvis and ascend. In cases where the kidneys fail to ascend normally, the adrenal glands are still found in the expected position although their shape may be more discoid owing to lack of moulding by the kidneys during development At birth the adrenal glands are relatively much larger than in the adult – one-third the size of the kidney at birth and one-thirtieth in the adult. The size of the gland is somewhat variable, but as a rule of thumb on crosssectional imaging the limbs of the adrenal should not be thicker than the diameter of the adjacent crus of the diaphragm Arterial supply Three arteries supply these glands on each side, namely: • the superior adrenal artery from the inferior phrenic artery. • the middle adrenal artery from the aorta. • the inferior adrenal artery from the renal artery. Venous drainage One vein drains the adrenal gland on each side. The right adrenal vein drains to the IVC and the left adrenal vein drains to the left renal vein Variants Small masses of adrenal cortical tissue called cortical bodies are often found near the adrenal glands These may become attached to other organs early in embryology and migrate with these organs to be found in such places as the broad ligament of the uterus, the spermatic cord and even the epididymis Radiological features of the adrenal glands Plain films of the abdomen The adrenal glands are visible only if calcified, and they are then seen to be lateral to the spine at the level of the upper pole of the kidneys. Computed tomography With modern scanners, the adrenal glands can be identified in almost all subjects. The shape of the adrenal gland on CT cuts is variable, with a linear, inverted V shape being commonest on the right and a triangular or Y shape commonest on the left. Its craniocaudal extent is less than 4 cm and limb thickness is usually less than 1 cm The right adrenal gland is seen posterior to the IVC More laterally it lies between the liver and the crura of the diaphragm. The left adrenal is higher and extends more laterally anterior to the kidney, from which it is separated by perirenal fat. Contrast medium may help to distinguish it from the adjacent splenic vessels Magnetic resonance imaging The adrenals are very well seen on MRI because of surrounding fat (more easily than with CT) They are iso or slightly hypointense compared to liver on both T1- and T2-weighted images They lose signal on fat suppression or fat subtraction techniques, depending on the cholesterol content of the adrenal cortex (cortisol is derived from cholesterol esters) Ultrasound In thin individuals, the adrenal glands can sometimes be seen between the kidney and liver on the right and between the kidney and pancreatic tail on the left using high-resolution scanning. They are readily seen in neonates and usually seen in children The kidneys l The kidneys lie retroperitoneally in the paravertebral gutters of the posterior abdominal wall. l They lie obliquely with their upper poles more medial and more posterior than their lower. l The kidneys measure 10–15 cm in length, the left being commonly 1.5 cm longer than the right Their size is approximately that of three-and-a-half lumbar vertebrae and their associated discs on a radiograph l On coronal cross-section each kidney is seen to have an outer cortex and an inner medulla l Extensions of the cortex centrally as columns of Bertin separate the medulla into pyramids whose apices, jutting into the calyces, are called the papillae There are usually seven pairs of minor calyces, each pair having an anterior and a posterior calyx, although there is wide variation Minor calyx pairs combine to form two or three major calyces, which in turn drain via their infundibula to the pelvis This arrangement is quite variable, l The pelvis may be intrarenal or partially or entirely extrarenal The gap between the renal substance and the pelvis is called the renal sinus and is filled with fat l A simple calyx has one papilla indenting it; a compound calyx has more than one Compound calyces are said to be less efficient at preventing intrarenal reflux of urine from the calyx and are more common in the upper pole l The hilum of the kidney lies medially, that of the left at L1 vertebral level and that of the right slightly lower at L1/L2 level, owing to the bulk of the liver above At the hilum, l The functional subunit of the kidney is called the nephron and consists of a glomerulus in the cortex and a tubule in the medulla This drains to a collecting duct, which empties into the calyx at the tip of the medulla The kidney has approxi- mately 1 million nephrons The relations of the kidneys • Posteriorly: upper third, diaphragm and twelfth rib and the costodiaphragmatic recess of the pleura lower third, medial to lateral: psoas, quadratus lumborum and transversus abdominis muscles • Superiorly: • the adrenal gland – more medial on the right kidney • Anteriorly: Right kidney: liver, second part of the duodenum, ascending colon, small intestinal loops Left kidney: stomach, pancreas and its vessels, spleen, splenic flexure of the colon, jejunal loops Blood supply of the kidneys The renal arteries normally arise from the aorta at L1/L2 level The right renal artery is longer and lower than the left and passes posterior to the IVC Both renal arteries usually have two divisions: one passes posterior to the renal pelvis and supplies the posterior upper part of the kidney; another anterior branch supplies the upper anterior kidney; a branch of the anterior division passes inferiorly and supplies the entire lower part of the kidney Within the hilum, the renal arteries divide inconsistently into five segmental branches which cross the renal sinus anterior and posterior to the pelvis and pierce the medulla in between the pyramids These are termed interlobar arteries, as they pass between the lobes or pyramids At the cortico- medullary junction and the base of the pyramids the interlobar arteries become the arcuate arteries They do not anastomose, but form arcades around the bases of the pyramids Branches of the arcuate arteries give off the interlobular arteries, Venous drainage l There is extensive anastomosis between the veins of the kidney Five or six interlobular veins unite at the hilum to form the renal vein l The renal vein lies anterior to the pelvis at the hilum The renal veins drain directly to the IVC The left renal vein is much longer than the right and passes anterior to the aorta to reach the IVC It also receives the inferior phrenic, adrenal and gonadal veins of that side The right renal vein receives no extrarenal tributaries Lymphatic drainage Lymph drainage follows the arteries to para-aortic nodes Fascial spaces around the kidneys A true fibrous capsule surrounds the kidney This, in turn, is surrounded by perirenal fat, which separates the kidney from the surrounding organs including the adrenal gland A conden- sation of fibroareolar tissue around this fat forms the renal fascia Thus the retroperitoneum is divided into three compartments: the perirenal space around the kidneys within the renal fascia, and the anterior and posterior pararenal spaces anterior and posterior to the renal fascia l The perirenal space has an anterior (Gerota’s) fascia and a posterior (Zuckerkandl’s) fascia These fascial layers are fused laterally as the lateral conal fascia, which is continuous with the fascia on the deep surface of the transversalis abdominis muscle, and this defines the lateral limit of the space Above, the layers do not join but are open to the bare area of the liver on the right and the subdiaphragmatic extraperitonal subdiaphragmatic space on the left (Fig 5 53) l The anterior pararenal space lies anterior to the anterior renal fascia and behind the posterior peritoneum It is continuous across the midline and contains the pancreas, duodenum and ascending and descending colon l The posterior pararenal space lies posterior to the posterior renal fascia and anterior to the muscles of the posterior abdominal wall This is limited medially by the attachment of the renal fascia to the psoas muscle, but is continuous laterally with the extraperitoneal fatty tissue (properitoneal fat plane) deep to the transversalis fascia It extends inferiorly to the fat anterior to the iliacus muscle and the pelvic extraperitoneal spaces It contains only fat Intravenous urogram (IVU):The right kidney has three major calyces The left kidney has two major calyces, and a bifid pelvis Note the course of the lower ureters The distal part of the ureter passes behind the bladder 1. Right upper-pole (major) calyx 3. Right lower-pole (major) calyx 5. Left lower-pole (major) calyx 7. Papilla 9. Fornix 11. Right renal pelvis 13. Left ureter: vascular impression 15. Right psoas outline 17. Gas in transverse colon 2. Right middle (major) calyx 4. Left upper-pole (major) calyx 6. Minor calyx (infundibulum of) 8. Infundibulum 10. Bifid left renal pelvis 12. Right ureter 14. Upper pole right kidney 16. Gas in body of stomach 18. Intravesical ureter