Kidney and ureter

Renal fascia

The renal fascia (Gerota’s fascia), is a dense, elastic connective tissue sheath that envelops each kidney and suprarenal gland, together with a layer of surrounding perirenal fat that is thickest at the renal borders and extends into the renal sinus at the hilum of the kidney ( Fig. 72.4 ; see Fig. 61.2 ).

The renal fascia was originally described as two separate entities, the posterior fascia of Zuckerkandl and the anterior fascia of Gerota, which fused laterally to form the lateroconal fascia ( ). According to this view, the lateroconal fascia continued anterolaterally behind the colon to blend with the parietal peritoneum. However, showed that the renal fascia is a single multi-laminated structure that is fused posteromedially with the muscular fasciae of psoas major and quadratus lumborum. It then extends anterolaterally behind the kidney as a bilaminated sheet, which, at a variable point, divides into a thin anterior layer, passing anterior to the kidney, and a thicker posterior layer that continues anterolaterally as the lateroconal fascia and fuses with the parietal peritoneum.

Classically, the anterior layer of renal fascia was thought to blend into the dense mass of connective tissue surrounding the great vessels in the root of the mesentery behind the duodenum and pancreas, thereby preventing communication between perirenal spaces across the midline. However, inspection of computed tomography (CT) images or of anatomical sections of cadavers, following injection of contrast or coloured latex, respectively, into the perirenal space, revealed that fluid could extend across the midline at the third to fifth lumbar levels through a narrow channel measuring 2–10 mm in the anteroposterior dimension. In the midline superiorly, the anterior and posterior layers of renal fascia fuse and are attached to the crura of the respiratory diaphragm. Inferiorly, the fasciae are separate for a variable craniocaudal distance. The posterior layer of renal fascia fuses with the psoas fascia, while the anterior layer of renal fascia extends across the midline anterior to the aorta and inferior vena cava; communication between the two sides is permitted, although it is very rarely of clinical significance. Below this level, the two fasciae once again merge and are attached to the aorta and inferior vena cava or iliac vessels. The containment of fluid to one side of the perirenal space that is observed in over two-thirds of clinical cases is attributed to the presence of fibrous septa.

Above the suprarenal glands, the anterior and posterior layers of renal fascia were previously said to fuse with each other and to the diaphragmatic fascia. This description of a closed superior cone is not universally accepted. Cadaveric dissections have shown that the superior aspect of the perirenal space is open and in continuity with the bare area of the liver on the right and the subphrenic space on the left. The posterior fascial layer blends bilaterally with the psoas fascia and fascia over quadratus lumborum, and with the diaphragmatic fascia. The anterior fascial layer on the right blends with the inferior aspect of the right coronary ligament at the level of the upper pole of the kidney and the bare area of the liver. On the left, the anterior layer fuses with the gastrosplenic ligament at the level of the suprarenal gland.

There is some debate concerning the inferior fusion of the renal fascia. Many investigators believe that, inferiorly, the anterior and posterior layers of the renal fascia fuse to produce an inverted cone that is open to the pelvis at its apex. Laterally, the anterior and posterior layers fuse with the iliac fascia; medially, they fuse with the periureteric connective tissue. The apex of the cone is open anatomically towards the iliac fossa but rapidly becomes sealed in inflammatory disease. An alternative view based on dissections of recently deceased cadavers after injections of coloured latex into the perirenal space, has shown that the anterior and posterior layers of renal fascia merge to form a single multilaminar fascia that contains the ureter in the iliac fossa. Anteriorly, this common fascia is loosely connected to the parietal peritoneum, and so denies free communication between the perirenal space and the pelvis, and between the perirenal and pararenal spaces.

A simple nephrectomy for benign disease removes the kidney from within the renal fascia; a radical nephrectomy (for cancer) classically removes the entire contents of the perirenal space, including the renal fascia and suprarenal gland, in order to give adequate clearance around the tumour.

Relations

The superior poles of both kidneys are thick and round and related to their respective suprarenal glands. The inferior poles are thinner and extend to within 2.5 cm of the iliac crests. The lateral borders are convex. The medial borders are convex adjacent to the poles and concave between them, and slope inferolaterally. In each, a deep vertical fissure opens anteromedially as the hilum of the kidney, which is bounded by anterior and posterior lips and contains the renal vessels and nerves, and the renal pelvis. The relative positions of the main hilar structures are the renal vein (anterior), the renal artery (intermediate) and the renal pelvis (posterior) ( Fig. 72.5 ). Usually, an anterior branch from the renal artery runs over the superior margin of the renal pelvis to enter the hilum on the posterior aspect of the renal pelvis, and often leaves the hilum in the same plane. Above the hilum the medial border is related to the suprarenal gland and below to the origin of the ureter.

The convex anterior surface of the kidney faces anterolaterally and its posterior surface faces posteromedially. The relations of the left and right kidneys are shown in Fig. 72.6 .

A small area of the superior pole of the right kidney is in contact with the right suprarenal gland, which can overlap the upper part of the medial border of the right kidney ( Fig. 72.7 ). A large area below this is immediately related to the right lobe of the liver, separated by a layer of parietal peritoneum. A narrow medial area is directly related to the descending part of the duodenum. Inferiorly, the anterior surface is directly in contact laterally with the right colic flexure and medially with part of the small intestine.

A small medial area of the superior pole of the left kidney is related to the left suprarenal gland (see Fig. 72.7 ). The lateral half of the superior pole of the anterior surface is related to the spleen, from which it is separated by a layer of peritoneum. A central quadrilateral area lies in direct contact with the pancreas and the splenic vessels. Above this, a small, variable, triangular region, between the suprarenal and splenic areas, is in contact with the stomach, separated by a layer of peritoneum. Below the pancreatic and splenic areas a narrow lateral strip that extends to the lateral border of the kidney is directly related to the left colic flexure and the beginning of the descending colon. An extensive medial area is related to the jejunum. The gastric area is covered with the peritoneum of the omental bursa, and the splenic and jejunal areas are covered by the peritoneum of the greater sac. Behind the peritoneum covering the jejunal area, branches of the left colic vessels are related to the kidney.

The posteromedial surface of the kidneys is embedded in fat and devoid of peritoneum. The right and left kidneys are related to similar structures. Superiorly are the respiratory diaphragm and the medial and lateral arcuate ligaments. More inferiorly, moving in a medial to lateral direction, are psoas major, quadratus lumborum and the aponeurotic tendon of transversus abdominis, the subcostal vessels and the subcostal, iliohypogastric and ilioinguinal nerves. The upper pole of the right kidney is level with the twelfth rib, and that of the left with the eleventh and twelfth ribs. The respiratory diaphragm covered with parietal peritoneum separates the kidney from the parietal pleura, which descends to form the costodiaphragmatic recess; the diaphragm is sometimes defective or absent in the lumbocostal triangle immediately above the lateral arcuate ligament, and this allows perirenal adipose tissue to contact the diaphragmatic pleura.

Internal macrostructure

The postnatal kidney has a thin fibrous capsule composed of collagen-rich connective tissue with some elastic and smooth muscle fibres. In renal disease, the capsule can become adherent.

The kidney itself can be divided into an internal renal medulla and external renal cortex ( Fig. 72.8 ). The renal medulla consists of pale, striated, conical renal pyramids; their bases are peripheral, and their apices converge to the renal sinus. At the renal sinus, they project into calyces as papillae.

The renal cortex (see Fig. 72.13 ) is subcapsular, arching over the bases of the renal pyramids and extending between them towards the renal sinus as renal columns. Its peripheral regions are termed cortical arches and are traversed by radial, lighter-coloured medullary rays, separated by darker tissue, the convoluted part. The rays taper towards the renal capsule and are peripheral prolongations from the bases of renal pyramids. The cortex is histologically divisible into outer and inner zones. The inner zone is demarcated from the renal medulla by tangential blood vessels (arcuate arteries and veins), which lie at the junction of the two; however, a thin layer of cortical tissue (subcortex) appears on the medullary side of this zone. The cortex close to the renal medulla is sometimes termed the juxtamedullary cortex.

Renal pelvis and calyces

The hilum of the kidney leads into a centrally located renal sinus, lined by the renal capsule and almost filled by the renal pelvis and renal vessels; the remaining space is filled by fat. Dissection into this plane can be challenging but is important in surgery on the renal pelvis, particularly open stone surgery. Within the renal sinus the collecting tubules of the nephrons of the kidney open on to the summits of the renal papillae to drain into minor calyces, which are funnel-shaped expansions of the upper urinary tract. The renal capsule covers the external surface of the kidney and continues through the hilum to line the renal sinus and fuse with the adventitial coverings of the minor calyces. Each minor calyx surrounds either a single renal papilla or, more rarely, groups of two or three renal papillae. The minor calyces unite with their neighbours to form two, or possibly three, larger chambers: the major calyces. There is wide variation in the arrangement of the calyces. As the posterior aspect of the kidney rotates laterally during its ascent in utero , the calyces that were lateral in utero become positioned anteriorly and the medial calyces move more posteriorly. The major calyces drain into the infundibula ( ). The renal pelvis is normally formed from the junction of two infundibula – one from the upper and one from the lower pole greater calyces – but there may be a third, which drains the calyces in the mid portion of the kidney. The major calyces are usually grouped so that three pairs drain into the upper pole infundibulum and four pairs into the lower one. If there is a middle infundibulum, the distribution is normally three pairs at the upper pole, two in the middle, and two at the lower pole. There is considerable variation in the arrangement of the infundibula and in the extent to which the renal pelvis is intrarenal or extrarenal. The funnel-shaped renal pelvis tapers as it passes inferomedially, traversing the hilum to become continuous with the ureter (see Figs 72.8–72.9 ). It is rarely possible to determine precisely where the renal pelvis ceases and the ureter begins; the region is usually extrahilar and normally lies adjacent to the lower part of the medial border of the kidney. Rarely, the entire renal pelvis has been found to lie inside the renal sinus so that the junction between it and the ureter occurs either in the vicinity of the hilum or completely within the renal sinus. FLOAT NOT FOUND

The calyces, renal pelvis and ureter are well demonstrated radiologically following an intravenous injection of radio-opaque contrast that is excreted in the urine (computed tomographic urography, CTU) ( Fig. 72.9 ), or after the introduction of radio-opaque contrast into the ureter by catheterization through a cystoscope (ascending or retrograde pyelography). Normal cupping of the minor calyces by projecting renal papillae can be obliterated by conditions that cause hydronephrosis, e.g. chronic distension of the ureter and renal pelvis due to upper or lower urinary tract obstruction, resulting in elevated pressure of the renal pelvis. An appreciation of the rotation of the kidneys, which results when the posterior calyces lie relatively medially and the anterior calyces lie laterally, is essential for interpreting contrast imaging of the collecting system of the kidneys.

Vascular supply and lymphatic drainage

Renal arteries

The paired renal arteries take about 20% of the cardiac output to supply organs that represent less than 1% of total body weight. They branch laterally from the aorta at the right-angles just below the origin of the superior mesenteric artery (see Fig. 59.5A ; Fig. 72.10A, C )). Both cross the corresponding crus of the respiratory diaphragm. The right renal artery is longer and often slightly higher, passing posterior to the inferior vena cava, right renal vein, head of the pancreas, and descending part of the duodenum. The left renal artery is often a little lower and passes behind the left renal vein, the body of the pancreas and the splenic vein. It may be crossed anteriorly by the inferior mesenteric vein.

A single renal artery to each kidney is present in approximately 70% of individuals ( Fig. 72.11 ). The arteries vary in their level of origin and in their calibre, obliquity and precise relations ( ). In its extrarenal course, each renal artery gives off one or more inferior suprarenal arteries, ureteric and capsular branches, and branches that supply perirenal tissue and the renal pelvis. Near the hilum of the kidney, each artery divides into an anterior and a posterior branch, and these divide into segmental arteries supplying the renal vascular segments. Accessory renal arteries are common (in approximately 30% of individuals) and usually arise from the abdominal aorta above or below (most commonly below) the renal artery and follow it to the hilum ( Figs 72.10B, D ). Accessory vessels to the inferior pole cross anterior to the ureter and can, by obstructing the ureter, cause hydronephrosis ( Fig. 72.12 ). In children with ureteropelvic junction obstruction, a crossing vessel is found in 28% of cases ( ). Three anatomical variants of aberrant lower pole crossing vessels have been described: either anterior to the dilated renal pelvis or ureteropelvic junction, or inferior to the ureteropelvic junction, causing kinking of the ureter ( ). Rarely, accessory renal arteries arise from the coeliac or superior mesenteric arteries near the aortic bifurcation, or from the common iliac arteries.

FLOAT NOT FOUND

The subdivisions of the renal arteries are described sequentially as segmental, lobar, interlobar, arcuate and interlobular (cortical radiate) arteries, and afferent and efferent glomerular arterioles ( Fig. 72.13 ).

Segmental arteries

Renal vascular segmentation was originally recognized by John Hunter in 1794, but the first detailed account of the primary pattern was produced in the 1950s from casts and radiographs of injected kidneys. Five arterial segments of the kidney have been identified ( Fig. 72.14A ). The superior segment occupies the anteromedial region of the superior pole. The anterior superior segment includes the rest of the superior pole and the central anterosuperior region. The inferior segment encompasses the whole inferior pole. The anterior inferior segment lies between the anterior and inferior segments. The posterior segment includes the whole posterior region between the superior and inferior segments. This is the pattern most commonly seen, and although there can be considerable variation, it is the pattern that clinicians most frequently encounter when performing partial nephrectomy. Anatomical and radiological analyses have suggested that there could be up to nine arterial segments for each kidney ( ) ( Fig. 72.14B ). Whatever pattern is present, it is important to emphasize that vascular segments are supplied by virtual end arteries. In contrast, the larger intrarenal veins have no segmental organization and anastomose freely.

Brödel described a relatively avascular longitudinal zone (the ‘bloodless’ line of Brödel) along the convex renal border, which was proposed as the most suitable site for surgical incision ( ). However, many vessels cross this zone and it is far from ‘bloodless’; planned radial or intersegmental incisions are therefore preferable ( ). Knowledge of the vascular anatomy of the kidney is important when partial nephrectomy is undertaken for renal cell carcinoma. In this surgery, the branches of the renal artery are defined so that the surgeon can safely excise the tumour while not compromising the vascular supply to the remaining renal tissue ( ).

Lobar, interlobar, arcuate and interlobular arteries

The initial branches of segmental arteries are lobar, usually one to each renal pyramid. Before reaching the renal pyramid they subdivide into two or three interlobar arteries, extending towards the cortex around each pyramid. At the junction of the renal cortex and medulla, interlobar arteries dichotomize into arcuate arteries, which diverge at right angles. As they arch between the renal cortex and medulla, each divides further, ultimately supplying interlobular arteries that diverge radially into the cortex. The terminations of adjacent arcuate arteries do not anastomose but end in the renal cortex as additional interlobular arteries. Though most interlobular arteries come from arcuate branches, some arise directly from arcuate, or even terminal, interlobar arteries (see Fig. 72.13 ). Interlobular arteries ascend towards the superficial cortex or can branch occasionally en route. Some are more tortuous and recurve towards the renal medulla at least once before proceeding towards the renal surface. Others traverse the surface as perforating arteries to anastomose with the capsular plexus (which is also supplied from the inferior suprarenal, renal and gonadal arteries).

Afferent and efferent glomerular arterioles

Afferent glomerular arterioles are mainly the lateral rami of interlobular arteries. A few arise from arcuate and interlobar arteries when they vary their direction and angle of origin: deeper ones incline obliquely back towards the renal medulla, the intermediate pass horizontally and the more superficial approach the surface of the kidney obliquely before ending in a glomerulus (see Fig. 72.13 ). Efferent glomerular arterioles from most glomeruli (except at juxtamedullary and, sometimes, intermediate cortical levels) soon divide to form a dense peritubular capillary plexus around the proximal and distal convoluted tubules; there are therefore two sets of capillaries – glomerular and peritubular – in series in the main renal cortical circulation, linked by efferent glomerular arterioles. The vascular supply of the renal medulla is largely from efferent arterioles of juxtamedullary glomeruli, supplemented by some from more superficial glomeruli, and ‘aglomerular’ arterioles (probably from degenerated glomeruli). Efferent glomerular arterioles passing into the renal medulla are relatively long, wide vessels, and contribute side branches to neighbouring capillary plexuses before entering the renal medulla, where each divides into 12–25 descending vasa recta. As their name suggests, these run straight to various depths in the renal medulla, contributing side branches to a radially elongated capillary plexus applied to the descending and ascending limbs of renal loops and to collecting ducts. The venous ends of capillaries converge to the ascending vasa recta, which drain into arcuate or interlobular veins. An essential feature of the vasa recta (particularly in the outer medulla) is that both descending and ascending vessels are grouped into vascular bundles, within which the external aspects of both types are closely apposed, bringing them close to the limbs of the nephron loops (of Henle) and collecting ducts. As these bundles converge centrally into the renal medulla, they contain fewer vessels; some terminate at successive levels in neighbouring capillary plexuses. This proximity of descending and ascending vessels to each other and to adjacent collecting ducts provides the structural basis for the countercurrent exchange and multiplier phenomena ( Fig. 72.15 ). These complex renal vascular patterns show regional specializations that are closely adapted to the spatial organization and functions of renal corpuscles, tubules and ducts (see below).

Renal veins

Fine radicles from the venous ends of the peritubular plexuses converge to join interlobular veins, one with each interlobular artery. Many interlobular veins begin beneath the fibrous capsule of the kidney by the convergence of several stellate veins, which drain the most superficial zone of the renal cortex and so are named from their surface appearance. Interlobular veins pass to the corticomedullary junction and also receive some ascending vasa recta before ending in arcuate veins (which accompany arcuate arteries), and anastomose with neighbouring veins. Arcuate veins drain into interlobar veins, which anastomose and form the renal vein.

The large renal veins lie anterior to the renal arteries and open into the inferior vena cava almost at right-angles (see Fig. 59.6 ; Fig. 72.16 ). The left renal vein is three times longer than the right (7.5 cm and 2.5 cm, respectively): the left kidney is the preferred side for live donor nephrectomy for this reason. However, the left renal vein can also be double, one vein passing posterior and the other anterior to the aorta before draining into the inferior vena cava, an arrangement that is sometimes referred to as persistence of the ‘renal collar’. The anterior vein can be absent, in which case there is a single retro-aortic left renal vein. These variations could negate any advantage of using the left kidney as a donor ( ). The left renal vein runs from its origin in the hilum of the kidney, posterior to the splenic vein and the body of the pancreas, and then across the anterior aspect of the abdominal aorta, just below the origin of the superior mesenteric artery. Anterior nutcracker syndrome, characterized by left renal vein hypertension secondary to compression of the vein between the ventral surface of the abdominal aorta and the dorsal surface of the superior mesenteric artery, has been associated with haematuria and varicocele in children ( ). The left gonadal vein enters the left renal vein from below, and the left suprarenal vein, usually receiving one of the left inferior phrenic veins, enters it above but nearer the midline (see Fig. 61.12 ). The left renal vein enters the inferior vena cava slightly superior to the right renal vein. The right renal vein lies posterior to the descending part of the duodenum and, sometimes, the head of the pancreas. It can be extremely short (less than 1 cm), such that safe nephrectomy can require excision of a cuff of the inferior vena cava.

The left renal vein may have to be ligated during surgery for aortic aneurysm because it has such a close relationship with the aorta; this seldom results in any harm to the kidney, provided that the ligature is placed medial to the draining gonadal and suprarenal veins, since these usually provide adequate collateral venous drainage. The right renal vein has no significant collateral drainage and cannot be ligated with impunity.

Innervation

Rami from the coeliac ganglion and plexus, aorticorenal ganglion, least thoracic splanchnic nerve, first lumbar splanchnic nerve and aortic plexus form a dense plexus of autonomic nerves around the renal artery (see Fig. 59.4 ).

Small ganglia occur in the renal plexus, the largest usually behind the origin of the renal artery. The plexus continues into the kidney around the arterial branches and supplies the vessels, renal glomeruli and, especially, the cortical renal tubules. Axons from plexuses around the arcuate arteries innervate juxtamedullary efferent arterioles and vasa recta, which control the blood flow between the renal cortex and medulla without affecting the glomerular circulation. Axons from the renal plexus contribute to the ureteric and gonadal plexuses.