Kidneys Each kidney has a characteristic bean-like shape. It has a convex lateral margin; and a concavity on the medial side which is called the hilum. It has upper and lower ends and anterior and posterior surfaces. Terminal branches of the renal artery enter the kidney at the hilum, and the veins emerge from it. The hilum also gives attachment to the upper expanded end of the renal pelvis. Because of the presence of the liver on the right side, the right kidney lies slightly lower than the left kidney. The hilum of each kidney lies more or less in the transpyloric plane, a little medial to the tip of the ninth costal cartilage. The vertical axis of the kidney is placed obliquely so that its upper end is nearer the median plane than the lower end. The upper end is about 2.5 cm (one inch) from the median plane, while the lower end is about 7.5 cm (three inches) from it. The bean-shaped kidneys are retroperitoneal in the posterior abdominal region. They lie in the extraperitoneal connective tissue immediately lateral to the vertebral column. The hilum of the kidney lies at the level of the first lumbar spine, the upper pole at the level of the llth thoracic spine (TXI), and the lower pole at the level of the third lumbar spine (LIII). The area in which the kidney lies can be represented as a parallelogram (Morrison's parallelogram). The upper and lower boundaries of this parallelogram are formed by transverse lines drawn through the eleventh thoracic and third lumbar spines. Although the width of the kidney is actually about 6 cm it appears to be only 4.5 cm when viewed from the front (or back) because of foreshortening. Relationships to other structures Posterior relations - Each kidney rests on the diaphragm, the corresponding psoas major, quadratus lumborum, and the origin of the corresponding transversus abdominis muscle. - The medial and lateral lumbocostal arches (from which some fibres of the diaphragm take origin) also lie behind the kidney. - The diaphragm separates the upper part of the kidney from the pleura and from the twelfth rib. - The left kidney being higher is also separated by the diaphragm from the eleventh rib Anterior relations of right kidney The anterior surface of the right kidney is related to numerous structures, some of which are separated from the kidney by a layer of peritoneum and some of which are directly against the kidney: a small part of the superior pole is covered by the right suprarenal gland; moving inferiorly, a large part of the rest of the upper part of the anterior surface is against the liver and is separated from it by a layer of peritoneum; medially, the descending part of the duodenum is retroperitoneal and contacts the kidney; the inferior pole of the kidney, on its lateral side, is directly associated with the right colic flexure and, on its medial side, is covered by a segment of the intraperitoneal small intestine. Anterior relations of the left kidney The anterior surface of the left kidney is also related to numerous structures, some with an intervening layer of peritoneum and some directly against the kidney: a small part of the superior pole, on its medial side, is covered by the left suprarenal gland; the rest of the superior pole is covered by the intraperitoneal stomach and spleen; moving inferiorly, the retroperitoneal pancreas covers the mid-part of the kidney; on its lateral side, the lower half of the kidney is covered by the left colic flexure and the beginning of the descending colon, and, on its medial side, by the parts of the intraperitoneal jejunum. Posterior Relations Posteriorly, the right and left kidneys are related to similar structures. - - - Superiorly is the diaphragm and inferior to this, moving in a medial to lateral direction, are psoas major, quadratus lumborum, and transversus abdominis muscles. The superior pole of the right kidney is anterior to rib XII, while the same region of the left kidney is anterior to ribs XI and XII. The pleural sacs, and specifically, the costodiaphragmatic recesses, therefore extend posterior to the kidneys. Also passing posterior to the kidneys are the subcostal vessels and nerves and the iliohypogastric and ilio-inguinal nerves. Renal fat and fascia The kidneys are enclosed in, and associated with, a unique arrangement of fascia and fat. Immediately outside the renal capsule, there is an accumulation of extraperitoneal fat-the perinephric fat (perirenal fat), which completely surrounds the kidney. Enclosing the perinephric fat is a membranous condensation of the extraperitoneal fascia (the renal fascia). The suprarenal glands are also enclosed in this fascial compartment, usually separated from the kidneys by a thin septum. The renal fascia must be incised in any surgical approach to this organ. At the lateral margins of each kidney, the anterior and posterior layers of the renal fascia fuse . This fused layer may connect with the transversalis fascia on the lateral abdominal wall. Above each suprarenal gland, the anterior and posterior layers of the renal fascia fuse and blend with the fascia that covers the diaphragm. Medially, the anterior layer of the renal fascia continues over the vessels in the hilum and fuses with the connective tissue associated with the abdominal aorta and the inferior vena cava. In some cases, the anterior layer may cross the midline to the opposite side and blend with its companion layer. The posterior layer of the renal fascia passes medially between the kidney and the fascia covering the quadratus lumborum muscle to fuse with the fascia covering the psoas major muscle. Inferiorly, the anterior and posterior layers of the renal fascia enclose the ureters. In addition to perinephric fat and the renal fascia, a final layer of paranephric fat (pararenal fat) completes the fat and fascias associated with the kidney. This fat accumulates posterior and posterolateral to each kidney. Kidney structure Each kidney has a smooth anterior and posterior surface covered by a fibrous capsule, which is easily removable except during disease. On the medial margin of each kidney is the hilum of kidney, which is a deep vertical slit through which renal vessels, lymphatics, and nerves enter and leave the substance of the kidney. Internally, the hilum is continuous with the renal sinus. Perinephric fat continues into the hilum and sinus and surrounds all structures. Each kidney consists of an outer renal cortex and an inner renal medulla. The renal cortex is a continuous band of pale tissue that completely surrounds the renal medulla. Extensions of the renal cortex (the renal columns) project into the inner aspect of the kidney, dividing the renal medulla into discontinuous aggregations of triangular-shaped tissue (the renal pyramids). The bases of the renal pyramids are directed outward, towards the renal cortex, while the apex of each renal pyramid projects inward, towards the renal sinus. The apical projection (renal papilla) is surrounded by a minor calyx. The minor calices receive urine and represent the proximal parts of the tube that will eventually form the ureter. In the renal sinus, several minor calices unite to form a major calyx, and two or three major calices unite to form the renal pelvis, which is the funnelshaped superior end of the ureters. Renal vasculature and lymphatics A single large renal artery, a lateral branch of the abdominal aorta, supplies each kidney. These vessels usually arise just inferior to the origin of the superior mesenteric artery between vertebrae LI and LII. The left renal artery usually arises a little higher than the right, and the right renal artery is longer and passes posterior to the inferior vena cava. Accessory renal arteries are common. They originate from the lateral aspect of the abdominal aorta, either above or below the primary renal arteries, enter the hilum with the primary arteries or pass directly into the kidney at some other level, and are commonly called extrahilar arteries. Each renal artery usually divides into five segmental arteries that enter the hilum of the kidney. They are distributed to different segments or areas of the kidney. Lobar arteries arise from each segmental artery, one for each renal pyramid. Before entering the renal substance, each lobar artery gives off two or three interlobar arteries. The interlobar arteries run towards the cortex on each side of the renal pyramid. At the junction of the cortex and the medulla, the interlobar arteries give off the arcuate arteries , which arch over the bases of the pyramids. The arcuate arteries give off several interlobular arteries that ascend in the cortex. The afferent glomerular arterioles arise as branches of the interlobular arteries. Multiple renal veins contribute to the formation of the left and right renal veins, both of which are anterior to the renal arteries. They drain into the inferior vena cava. Lymphatic Drainage All lymph vessels from the kidneys drain directly into the lateral aortic nodes. There are three sets of vessels: - From the perirenal fat - From a superficial plexus under the capsules - Deep vessels from the renal tissue Nerve supply of the kidney The nerve supply is the renal sympathetic plexus. The afferent fibres that travel through the renal plexus enter the spinal cord in the 10 th, 11th and 12th thoracic nerves. Clinical Significance: The longer left renal vein crosses the midline anterior to the abdominal aorta and posterior to the superior mesenteric artery and can be compressed by an aneurysm in either of these two vessels. Renal Mobility The kidneys are maintained in their normal position by intra-abdominal pressure and by their connections with the perirenal fat and renal fascia. Each kidney moves slightly with respiration. Should the amount of perirenal fat he reduced, the mobility of the kidney may become excessive and produce symptoms of renal colic caused by kinking of the ureter. Kidney Trauma The kidneys are well protected by the lower ribs, the lumbar muscles, and the vertebral column. However, a severe blunt injury applied to the abdomen may crush the kidney against the last rib and the vertebral column. Depending on the severity of the blow, the injury varies from a mild bruising to a complete laceration of the organ. Penetrating injuries are usually caused by stab wounds or gunshot wounds and often involve other viscera. Because 25% of the cardiac outflow passes through the kidneys, renal injury can result in rapid blood loss. Kidney Tumors Malignant tumors of the kidney have a strong tendency to spread along the renal vein. The left renal vein receives the left testicular vein in the male, and this may rarely became blocked, producing left-sided varicocele. Transplanted Kidneys The iliac fossa on the posterior abdominal wall is the usual site chosen for transplantation of the kidney.