Tuesday, August 12, 2008

Gross and Microscopic Anatomy

The kidneys serve a number of important functions required to maintain normal human physiologic function. They are the primary organ for maintaining fluid and electrolyte balance, and they play a large role in maintaining acid-base balance. They produce renin, which plays a vital role in controlling blood pressure, and erythropoietin, affecting red blood cell production. They affect calcium metabolism, in particular calcium absorption, by converting a precursor of vitamin D to the most active form 1,25-dihydroxyvitamin D.

Grossly, the kidneys are bilaterally paired reddish brown organs (see Figs. 1-1 and 1-22 [1] [22]). Typically each kidney weighs 150 g in the male and 135 g in the female. The kidneys generally measure 10 to 12 cm vertically, 5 to 7 cm transversely, and 3 cm in the anteroposterior dimension. Because of compression by the liver, the right kidney tends to be somewhat shorter and wider. In children, the kidneys are relatively larger and possess more prominent fetal lobations. These lobations are present at birth and generally disappear by the first year of life, although occasionally they persist into adulthood. An additional common feature of the gross renal anatomy is a focal renal parenchymal bulge along the kidney's lateral contour, known as a dromedary hump. This is a normal variation without pathologic significance. It is more common on the left than the right and is believed to be caused by downward pressure from the spleen or liver.

Figure 1-22 Internal structure of the kidney. (From Drake RL, Vogl W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005, p 323.)

As one proceeds centrally from the peripherally located reddish brown parenchyma of the kidney, the renal sinus is encountered. Here the vascular structures and collecting system coalesce before exiting the kidney medially. These structures are surrounded by yellow sinus fat, which provides an easily recognized landmark during renal procedures such are partial nephrectomy. At its medial border, the renal sinus narrows to form the renal hilum. It is through the hilum that the renal artery, renal vein, and renal pelvis exit the kidney and proceed to their respective destinations.

Both grossly and microscopically there are two distinct components within the renal parenchyma: the medulla and the cortex. Unlike the adrenal gland, the renal medulla is not a contiguous layer. Instead, the medulla is composed of multiple, distinct, conically shaped areas noticeably darker in color than the cortex ( Fig. 1-22 ). These same structures are also frequently called renal pyramids, making the terms renal medulla and renal pyramid synonymous. The apex of the pyramid is the renal papilla, and each papilla is cupped by an individual minor calyx.


The kidney is divided into cortex and medulla. The medullary areas are pyramidal, more centrally located, and separated by sections of cortex. These segments of cortex are called the columns of Bertin.
Orientation of the kidney is greatly affected by the structures around it. Thus the upper poles are situated more medially and posteriorly than the lower poles. Also, the medial aspect of the kidney is more anterior than the lateral aspect.
Gerota's fascia envelops the kidney on all aspects except inferiorly, where it is not closed but instead remains an open potential space.
From anterior to posterior, the renal hilar structures are the renal vein, renal artery, and collecting system.
The renal artery splits into segmental branches. Typically, the first branch is the posterior segmental artery, which passes posterior to the collecting system. There are generally three to four anterior segmental branches that pass anteriorly to supply the anterior kidney.
The progression of arterial supply to the kidney is as follows: renal artery ➙segmental artery ➙interlobar artery ➙arcuate artery ➙interlobular artery ➙afferent artery.
The venous system anastomoses freely throughout the kidney. The arterial supply does not. Thus, occlusion of a segmental artery leads to parenchymal infarction but occlusion of a segmental vein is not problematic because there are many alternate drainage routes.
Anatomic variations in the renal vasculature are common, occurring in 25% to 40% of kidneys.
Each renal pyramid terminates centrally in a papilla. Each papilla is cupped by a minor calyx. A group of minor calyces join to form a major calyx. The major calyces combine to form the renal pelvis. There is great variation in the number of calyces, calyceal size, and renal pelvis size. The only way to determine pathologic from normal is by evidence of dysfunction.

The renal cortex is lighter in color than the medulla and not only covers the renal pyramids peripherally but also extends between the pyramids themselves. The extensions of cortex between the renal pyramids are given a special name: the columns of Bertin. These columns are significant surgically because it is through these columns that renal vessels traverse from the renal sinus to the peripheral cortex, decreasing in diameter as the columns move peripherally. It is because of this anatomy that percutaneous access to the collecting system is made through a renal pyramid into a calyx, thus avoiding the columns of Bertin and the larger vessels present within them.

Relations and Investing Fascia Anatomic Relationships

The position of the kidney within the retroperitoneum varies greatly by side, degree of inspiration, body position, and presence of anatomic anomalies ( Fig. 1-23 ). The right kidney sits 1 to 2 cm lower than the left in most individuals owing to displacement by the liver. Generally, the right kidney resides in the space between the top of the 1st lumbar vertebra to the bottom of the 3rd lumbar vertebra. The left kidney occupies a more superior space from the body of the 12th thoracic vertebral body to the 3rd lumbar vertebra.

Figure 1-23 Variation between individuals in level of the kidneys relative to the spinal column. Lighter lateral lines represent upper poles of kidneys; darker lateral lines represent lower poles. (From Anson BJ, Daseler EH: Common variations in renal anatomy, affecting blood supply, form, and topography. Surg Gynecol Obstet 1961;112:439-449.)

Of surgical importance are the structures surrounding the kidney (see Figs. 1-9 and 1-24 [9] [24]). Both kidneys have similar muscular surroundings. Posteriorly, the diaphragm covers the upper third of each kidney, with the 12th rib crossing at the lower extent of the diaphragm. Also important to note for percutaneous renal procedures and flank incisions is that the pleura extends to the level of the 12th rib posteriorly. Medially the lower two thirds of the kidney lie against the psoas muscle, and laterally the quadratus lumborum and aponeurosis of the transversus abdominis muscle are encountered. The effect of the muscular relations on the kidneys is severalfold ( Fig. 1-25 ). First, the lower pole of the kidney lies laterally and anteriorly relative to the upper pole. Second, the medial aspect of each kidney is rotated anteriorly at an angle of approximately 30 degrees. An understanding of this renal orientation is again of particular interest for percutaneous renal procedures in which kidney orientation influences access site selection.

Figure 1-9 Structures related to the posterior surface of the kidney. (From Drake RL, VogL W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005, p 322.)

Figure 1-24 Structures related to the anterior surfaces of each kidney. (From Drake RL, Vogl W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005, p 321.)

Figure 1-25 Normal rotational axes of the kidney. A, Transverse view showing approximate 30-degree anterior rotation of the left kidney from the coronal plane, relative positions of the anterior and posterior rows of calyces, and location of the relatively avascular plane separating the anterior and posterior renal circulation. B, Coronal section demonstrating slight inward tilt of the upper poles of the kidneys. C, Sagittal view showing anterior displacement of the lower pole of the right kidney.

Anteriorly, the right kidney is bordered by a number of structures (see Fig. 1-24 ). Cranially, the upper pole lies against the liver and is separated from the liver by the peritoneum except for the liver's posterior bare spot. The hepatorenal ligament further attaches the right kidney to the liver because this extension of parietal peritoneum bridges the upper pole of the right kidney to the posterior liver. Also at the upper pole, the right adrenal gland is encountered. On the medial aspect, the descending duodenum is intimately related to the medial aspect of the kidney and hilar structures. Finally, on the anterior aspect of the lower pole lies the hepatic flexure of the colon.

The left kidney is bordered superiorly by the tail of the pancreas with the splenic vessels adjacent to the hilum and upper pole of the left kidney. Also cranial to the upper pole is the left adrenal gland and further superolaterally, the spleen. The splenorenal ligament attaches the left kidney to the spleen. This attachment can lead to splenic capsular tears if excessive downward pressure is applied to the left kidney. Superior to the pancreatic tail, the posterior gastric wall can overlie the kidney. Caudally, the kidney is covered by the splenic flexure of the colon.

Gerota's Fascia

Interposed between the kidney and its surrounding structures is the perirenal or Gerota's fascia (Figs. 1-26 through 1-28 [26] [27] [28]). This fascial layer encompasses the perirenal fat and kidney and encloses the kidney on three sides: superiorly, medially, and laterally. Superiorly and laterally Gerota's fascia is closed, but medially it extends across the midline to fuse with the contralateral side. Inferiorly, Gerota's fascia is not closed and remains an open potential space. Gerota's fascia serves as an anatomic barrier to the spread of malignancy as well as a means of containing perinephric fluid collections. Thus, perinephric fluid collections can track inferiorly into the pelvis without violating Gerota's fascia.

Figure 1-28 Posterior view of Gerota's fascia on the right side, rotated medially with the contained kidney, ureter, and gonadal vessels, exposing the muscular posterior body wall covered by the transversalis fascia. (From Tobin CE: The renal fascia and its relation to the transversalis fascia. Anat Rec 1944;89:295-311.)

Figure 1-27 Anterior view of Gerota's fascia on the right side, split over the right kidney (which it contains), and showing inferior extension enveloping the ureter and gonadal vessels. The ascending colon and overlying peritoneum have been reflected medially. (From Tobin CE: The renal fascia and its relation to the transversalis fascia. Anat Rec 1944;89:295-311.)

Figure 1-26 Organization of the fat and fascia surrounding the kidney. (From Drake RL, Vogl W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005, p 322.)

Renal Vasculature

The renal pedicle classically consists of a single artery and a single vein that enter the kidney via the renal hilum (see Fig. 1-22 ). These structures branch from the aorta and inferior vena cava just below the superior mesenteric artery at the level of the second lumbar vertebra. The vein is anterior to the artery. The renal pelvis and ureter are located further posterior to these vascular structures.

Renal Artery

Specifically, the right renal artery leaves the aorta and progresses with a caudal slope under the IVC toward the right kidney. The left renal artery courses almost directly laterally to the left kidney. Given the rotational axis of the kidney (see Fig. 1-25 ), both renal arteries move posteriorly as they enter the kidney. Also, both arteries have branches to the respective adrenal gland, renal pelvis, and ureter.

Upon approaching the kidney, the renal artery splits into four or more branches, with five being the most common. These are the renal segmental arteries ( Fig. 1-29 ). Each segmental artery supplies a distinct portion of the kidney with no collateral circulation between them ( Fig. 1-30 ). Thus, occlusion or injury to a segmental branch will cause segmental renal infarction. Generally, the first and most constant branch is the posterior segmental branch, which separates from the renal artery before it enters the renal hilum. There are typically four anterior branches, which from superior to inferior are apical, upper, middle, and lower. The relationship of these segmental arteries is important because the posterior segmental branch will pass posterior to the renal pelvis while the others pass anterior to the renal pelvis. Ureteropelvic junction obstruction caused by a crossing vessel can occur when the posterior segmental branch passes anterior to the ureter causing occlusion. This division between the posterior and anterior segmental arteries has an additional surgical importance in that between these circulations is an avascular plane (see Figs. 1-25 and 1-30 [25] [30]). This longitudinal plane lies just posterior to the lateral aspect of the kidney. Incision within this plane results in significantly less blood loss than outside this plane. However, there is significant variation in the location of this plane, requiring delineation before incision. This can be done with either preoperative angiography or intraoperative segmental arterial injection of a dye, such as methylene blue.

Figure 1-29 A and B, Segmental branches of the right renal artery demonstrated by renal angiogram.

Figure 1-30 Typical segmental circulation of the right kidney, shown diagrammatically. Note that the posterior segmental artery is usually the first branch of the main renal artery and it extends behind the renal pelvis.

Once in the renal sinus, the segmental arteries branch into lobar arteries, which further subdivide in the renal parenchyma to form interlobar arteries ( Fig. 1-31 ). These interlobar arteries progress peripherally within the cortical columns of Bertin, thus avoiding the renal pyramids but maintaining a close association with the minor calyceal infundibula. At the base (peripheral edge) of the renal pyramids, the interlobar arteries branch into arcuate arteries. Instead of moving peripherally, the arcuate arteries parallel the edge of the corticomedullary junction. Interlobular arteries branch off the arcuate arteries and move radially, where they eventually divide to form the afferent arteries to the glomeruli.

Figure 1-31 Intrarenal arterial anatomy.

The 2 million glomeruli within each kidney represent the core of the renal filtration process. Each glomerulus is fed by an afferent arteriole. As blood flows through the glomerular capillaries, the urinary filtrate leaves the arterial system and is collected in the glomerular (Bowman's) capsule. Blood flow leaves the glomerular capillary via the efferent arteriole and continues to one of two locations: secondary capillary networks around the urinary tubules in the cortex or descending into the renal medulla as the vasa recta.

Renal Veins

The renal venous drainage correlates closely with the arterial supply. The interlobular veins drain the post glomerular capillaries. These veins also communicate freely via a subcapsular venous plexus of stellate veins with veins in the perinephric fat. After the interlobular veins, the venous drainage progresses through the arcuate, interlobar, lobar, and segmental branches, with the course of each of these branches paralleling the respective artery. After the segmental branches, the venous drainage coalesces into three to five venous trunks that eventually combine to form the renal vein. Unlike the arterial supply, the venous drainage communicates freely through venous collars around the infundibula, providing for extensive collateral circulation in the venous drainage of the kidney ( Fig. 1-32 ). Surgically, this is important, because unlike the arterial supply, occlusion of a segmental venous branch has little effect on venous outflow.

Figure 1-32 Venous drainage of the left kidney showing potentially extensive venous collateral circulation.

The renal vein is located directly anterior to the renal artery, although this position can vary up to 1-2 cm cranially or caudally relative to the artery. The right renal vein is generally 2 to 4 cm in length and enters the right lateral to posterolateral edge of the IVC. The left renal vein is typically 6 to 10 cm in length and enters the left lateral aspect of the IVC after passing posterior to the superior mesenteric artery and anterior to the aorta ( Fig. 1-33 ). Compared with the right renal vein, the left renal vein enters the IVC at a slightly more cranial level and a more anterolateral location. Additionally, the left renal vein receives the left adrenal vein superiorly, lumbar vein posteriorly, and left gonadal vein inferiorly (see Fig. 1-32 ). The right renal vein typically does not receive any branches.

Figure 1-33 Renal vasculature. Note path of left renal vein under the superior mesenteric artery. (From Drake RL, Vogl W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005, p 324.)

Common Anatomic Variants

Anatomic variations in the renal vasculature are common, occurring in 25% to 40% of kidneys. The most common variation is supernumerary renal arteries, with up to five arteries reported. This occurs more often on the left. These additional arteries can enter through the hilum or directly into the parenchyma. Lower pole arteries on the right tend to cross anterior to the IVC whereas lower pole arteries on either side can cross anterior to the collecting system, causing a ureteropelvic junction obstruction. When the kidney is ectopic, supernumerary arteries are even more common and their origin even more varied, with the celiac trunk, superior mesenteric artery, or iliac arteries all possible sources of ectopic renal arteries. Supernumerary veins occur as well, but this is a less common entity. The most common example is duplicate renal veins draining the right kidney via the right renal hilum. Polar veins are quite rare. Finally, the left renal vein may course behind the aorta or divide and send one limb anterior and one limb posterior to the aorta, resulting in a collar-type circumaortic formation.

Renal Lymphatics

The renal lymphatics largely follow blood vessels through the columns of Bertin and then form several large lymphatic trunks within the renal sinus. As these lymphatics exit the hilum, branches from the renal capsule, perinephric tissues, renal pelvis, and upper ureter drain into these lymphatics. They then empty into lymph nodes associated with the renal vein near the renal hilum. From here, the lymphatic drainage between the two kidneys varies (Figs. 1-34 and 1-35 [34] [35]). On the left, primary lymphatic drainage is into the left lateral para-aortic lymph nodes including nodes anterior and posterior to the aorta between the inferior mesenteric artery and the diaphragm. Occasionally, there will be additional drainage from the left kidney into the retrocrural nodes or directly into the thoracic duct above the diaphragm. On the right, drainage is into the right interaortalcaval and right paracaval lymph nodes, including nodes located anterior and posterior to the vena cava, from the common iliac vessels to the diaphragm. Occasionally, there will be additional drainage from the right kidney into the retrocrural nodes or the left lateral para-aortic lymph nodes.

Figure 1-35 Regional lymphatic drainage of the right kidney. Dark nodes, anterior; light nodes, posterior. Solid lines, anterior lymphatic channels; dashed lines, posterior lymphatic channels. Arrow leads to the thoracic duct.

Figure 1-34 Regional lymphatic drainage of the left kidney. Dark nodes, anterior; light nodes, posterior. Solid lines, anterior lymphatic channels; dashed lines, posterior lymphatic channels. Arrows lead to the thoracic duct.

Renal Collecting System Microscopic Anatomy from Glomerulus to Collecting System

Microscopically, the renal collecting system originates in the renal cortex at the glomerulus as filtrate enters into Bowman's capsule ( Fig. 1-36 ). Together the glomerular capillary network and Bowman's capsule form the renal corpuscle (malpighian corpuscle) ( Fig. 1-37 ). The glomerular capillary network is covered by specialized epithelial cells called podocytes that, along with the capillary epithelium, form a selective barrier across which the urinary filtrate must pass. The filtrate is initially collected in Bowman's capsule and then moves to the proximal convoluted tubule. The proximal tubule is composed of a thick cuboidal epithelium covered by dense microvilli. These microvilli greatly increase the surface area of the proximal tubule, allowing a large portion of the urinary filtrate to be reabsorbed in this section of the nephron.

Figure 1-36 Electron micrograph of the renal corpuscle. The glomerular capillary network is enveloped by podocytes and contained within Bowman's capsule. CS, capsular space of Bowman; PL, parietal layer of Bowman's capsule; Po, podocyte. Arrows indicate cytoplasmic extensions from podocytes that enwrap the glomerular capillaries. (From Kessel RG, Kardon RH: Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. Copyright 1979 by WH Freeman and Co.)

Figure 1-37 Renal nephron and collecting tubule. (From Netter F: Atlas of Human Anatomy, 2nd ed. Summit, NJ, Novartis Corp., Plate 317.)

The proximal tubule continues deeper into the cortical tissue where it becomes the loop of Henle. The loop of Henle extends variable distances into the renal medulla. Within the renal medulla, the loop of Henle reverses cource and moves back toward the periphery of the kidney. As it ascends out of the medulla the loop thickens and becomes the distal convoluted tubule. This tubule eventually returns to a position adjacent to the originating glomerulus and proximal convoluted tubule. Here the distal convoluted tubule turns once again for the interior of the kidney and becomes a collecting tubule. Collecting tubules from multiple nephrons combine into a collecting duct that extends inward through the renal medulla and eventually empties into the apex of the medullary pyramid, the renal papilla.

Renal Papillae, Calyces, and Pelvis

The renal papillae are the tip of a medullary pyramid and constitute the first gross structure of the renal collecting system. Typically, there are 7 to 9 papillae per kidney, but this number is variable, ranging from 4 to 18. The papillae are aligned in two longitudinal rows situated approximately 90 degrees from one another. There is an anterior row that owing to the orientation of the kidney faces in a lateral direction and a posterior row that extends directly posterior (see Figs. 1-25 and 1-38 [25] [38]). Each of these papillae is cupped by a minor calyx (see Fig. 1-22 ). At the upper and lower poles, compound calyces are often encountered. These compound calyces are the result of renal pyramid fusion and because of their anatomy are more likely to allow reflux into the renal parenchyma ( Fig. 1-39 ). Clinically, this can result in more severe scarring of the parenchyma overlying compound calyces.

Figure 1-38 The renal collecting system (left kidney) showing major divisions into minor calyces, major calyces, and renal pelvis. A, anterior minor calyces; C, compound calyces at the renal poles; P, posterior minor calyces.

Figure 1-39 Diagram demonstrating structural and functional distinctions between simple and compound renal papillae. Back pressure causes closure of the collecting ducts in a simple papilla, effectively preventing reflux of urine into the renal parenchyma. The structure of the compound papilla allows intrarenal reflux of urine with sufficient back pressure.

After cupping an individual papillae, each minor calyx narrows to an infundibulum. Just as there is frequent variation in the number of calyces, the diameter and length of the infundibula varies greatly. Infundibuli combine to form two or three major calyceal branches. These are frequently termed the upper, middle, and lower pole calyces, and these calyces in turn combine to form the renal pelvis. The renal pelvis itself can vary greatly in size, ranging from a small intrarenal pelvis to a large predominantly extrarenal pelvis. Eventually the pelvis narrows to form the ureteropelvic junction, marking the beginning of the ureter.

On close examination, it is clear that there is significant variation in the anatomy of the renal collecting system (Figs. 1-40 through 1-42 [40] [41] [42]). Number of calyces, diameter of the infundibuli, and size of the renal pelvis all vary significantly between normal individuals. Even in the same individual, the renal collecting systems may be similar but are rarely identical. Because of this variation, it can be difficult to distinguish pathologic from normal based on anatomy alone. Instead, it is demonstrated dysfunction that is needed to make the diagnosis of a pathologic anatomic formation within the renal collecting system.

Figure 1-40 Normal bilateral renal collecting systems, demonstrated by excretory urography.

Figure 1-41 Significant variation between two normal renal pelves, demonstrated by excretory urography. A, Large, extrarenal pelvis. B, Narrow, completely intrarenal pelvis, barely larger in caliber than the ureter.

Figure 1-42 Examples of normal variations in the architecture of the renal collecting system, demonstrated by excretory urography. A, Absence of calyces. B, Minor calyces arising directly from the renal pelvis. C, Megacalyces. D, “Orchid” calyces. E, Multiple minor calyces and nearly absent renal pelvis.

Renal Innervation

Sympathetic preganglionic nerves originate from the eighth thoracic through first lumbar spinal segments and then travel to the celiac and aorticorenal ganglia. From here, postganglionic fibers travel to the kidney via the autonomic plexus surrounding the renal artery. Parasympathetic fibers originate from the vagus nerve and travel with the sympathetic fibers to the autonomic plexus along the renal artery. The primary function of the renal autonomic innervation is vasomotor, with the sympathetics inducing vasoconstriction and the parasympathetics causing vasodilation. Despite this innervation, it is important to realize that the kidney functions well even without this neurologic control, as evidenced by the successful function of transplanted kidneys.

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