bleeding as the splenic flexure of the colon is manipulated. There are also splenorenal ligamentous attachments between the spleen and Gerota’s fascia over the left upper pole kidney, which must be avoided or divided to prevent splenic injury during any left renal operation. The tail of the pancreas lies directly over the left upper pole kidney, and must be manipulated and reflected gently to avoid harm.

The Renal Vasculature

A thorough understanding of the renal vascular anatomy is essential to any surgical approach to the kidneys. The renal arteries and veins typically branch from the aorta and inferior vena cava, respectively, at the level of the second lumbar vertebral body, below the takeoff of the superior mesenteric artery (see Fig. 2.3). The right renal artery typically leaves the aorta at a slightly higher level than the left renal artery, and then must course with a slightly downward trajectory to reach the right kidney, which usually lies lower in the right retroperitoneum than the contralateral left kidney. The right renal artery passes posterior to the inferior vena cava and is considerably longer than the left renal artery. In rare cases, the right renal artery has been observed to cross anterior to the inferior

vena cava. The relatively short left renal artery tends to lie in a horizontal plane or even course slightly superiorly to reach the left kidney. Both renal arteries course somewhat posteriorly from the aorta due to the natural rotation of the kidneys (see Figs. 2.7 and 2.8). The main renal arteries bilaterally typically provide small arterial branches to the renal pelvis and proximal ureter, adrenal gland, renal capsule, and perinephric fat.

As they approach the renal sinus, the main renal arteries divide into four or more segmental renal arteries,with five branches most commonly described (see Fig. 2.9). The first and most constant segmental division is a posterior branch which usually exits the main renal artery outside the renal sinus and proceeds posterior to the renal pelvis to eventually supply a large posterior segment of the renal parenchyma. Four anterior segmental renal arterial branches can be described in most kidneys, proceeding from superior to inferior: the apical, upper, middle, and lower anterior segmental arteries, respectively, each supplying a corresponding segment of renal parenchyma (see Fig. 2.9). The lower anterior segmental artery may cross in close proximity to the ureteropelvic junction, where it may cause compression or, alternatively, be injured during surgery at this location. The posterior segmental artery may be injured during

Figure 2.10. intrarenal arterial anatomy (reprinted with permission from Kabalin1 copyright Elsevier 2002).

Segmental

artery

Inferior phrenic

Figure 2.11. Venous drainage of the left kidney,showing extensive venous collateral circulation (reprinted with permission from Kabalin1, copyright Elsevier 2002).

Although both main renal veins generally lie directly anterior to their associated main renal arteries, this relationship may vary, and significant separation may occur, especially as one moves more medially away from the renal sinus. While most commonly a single main renal artery and a single main renal vein supplying each kidney are observed, anatomic variations are common, occurring in 25–40% of kidneys. The most common variation is the presence of one or more supernumerary renal arteries, with up to five arteries to a single kidney described. These supernumerary arteries typically arise from the aorta, but may in unusual instances derive from the celiac, superior mesenteric, or iliac arteries.A supernumerary renal artery may enter through the renal sinus, or directly into the parenchyma of the upper or lower pole kidney. Multiple renal veins are a more unusual finding, with two right renal veins draining from the right renal hilum directly into the inferior vena cava the most common venous aberration. On the left side, the main renal vein may divide, sending one limb anterior and one limb posterior to the aorta, and in rare instances only the venous limb posterior to the aorta may be present.

The Renal Collecting System

Each renal medullary papilla extending into the renal sinus is cupped by a corresponding minor calyx of the renal collecting system. The number of papillae and minor calyces is highly variable, but seven to nine are commonly present. There are typically two longitudinal rows of papillae and calyces, roughly perpendicular to one another, extending anteriorly and posteriorly (see Fig. 2.8). Reaching centrally from the papillae, the minor calyces narrow, creating a neck or infundibulum, before joining with other minor calyces to form usually two or three major calyces, which in most cases then coalesce to form a single renal pelvis exiting the renal sinus (see Fig. 2.12).2 The renal pelvis lies posterior to the main renal vein and artery, respectively. The renal pelvis may be small and completely contained within the renal sinus, or may be quite voluminous and almost entirely extrarenal. The renal pelvis eventually narrows to join with the ureter at the ureteropelvic junction. The entire renal collecting system is one continuous structure from calyces to ureter, and the named anatomic segments, while useful for description, are somewhat artificial. The actual renal collecting system anatomy shows considerable variation between individuals, likened to fingerprints, and these anatomic descriptors may be more or less

Figure 2.12. renal collecting system (left kidney shown) (reprinted with permission from Kabalin1 copyright Elsevier 2002).