Renal morphometry

In adults, the left kidney is larger than the right, and this agrees with morphometric findings in fetal kidneys [1]. The right kidney has a mean length of 10.97 cm and 3.21 cm mean thickness at the hilum, in comparison to 11.21 cm and 3.37 cm, respectively, for the left kidney [2].

An interesting finding is that the superior pole has a greater width (mean, 6.48 cm) than the inferior pole (mean, 5.39 cm). Also, there is a statistically significant correlation between kidney length and an individual’s stature [2].

Position of the kidneys

Because the kidneys lie on the posterior abdominal wall, against the psoas major muscles, their longitudinal axis parallels the oblique course of the psoas (Figure 6.1). Moreover, since the psoas major muscle has a cone shape, the kidneys also are dorsally inclined on the longitudinal axis. Therefore, the superior poles are more medial and more posterior than the inferior poles (Figure 6.1). Also, because the hilar region is rotated anteriorly on the psoas muscle, the lateral borders of both kidneys are posteriorly positioned. This means the kidneys are angled 30–50° behind the frontal (coronal) plane (Figure 6.2) [3].

Perirenal coverings

The kidney surface is enclosed in a continuous covering of fibrous tissue, the renal capsule (“true renal capsule”). Each kidney within its capsule is surrounded by a mass of adipose tissue, lying between the peritoneum and the posterior abdominal wall (Figures 6.2 and 6.3). This perirenal fat is enclosed by the renal fascia (the so‐called fibrous renal fascia of Gerota). The renal fascia is enclosed anteriorly and posteriorly by another layer of adipose tissue, the pararenal fat, which varies in thickness (Figure 6.3).

The renal fascia is made up of a posterior layer (a well‐defined and strong structure) and an anterior layer (a more delicate structure, which tends to adhere to the peritoneum) (Figures 6.2 and 6.3). The anterior and posterior layers of the renal fascia (fascia of Gerota) subdivide the retroperitoneal space into three potential compartments: (i) the posterior pararenal space, which contains only fat; (ii) the intermediate perirenal space, which contains the suprarenal glands, kidneys, and proximal ureters, together with the perirenal fat; and (iii) the anterior pararenal space, which unlike the posterior and intermediate spaces, extends across the midline from one side of the abdomen to the other. This latter space contains the ascending and descending colon, the duodenal loop, and the pancreas [3] (Figure 6.4). Inferiorly, the layers of the renal fascia end weakly fused around the ureter (Figures 6.3 and 6.5). Superiorly, the two layers of the renal fascia fuse above the suprarenal gland and end fused with the infradiaphragmatic fascia (Figure 6.5). An additional fascial layer separates the suprarenal gland from the kidney (Figure 6.5). Laterally, the two layers of the renal fascia fuse behind the ascending and descending colons. Medially, the posterior fascial layer is fused with the fascia of the spine muscles. The anterior fascial layer merges into the connective tissue of the great vessels (aorta and inferior vena cava) (Figures 6.2 and 6.4).

These anatomic descriptions of the renal fascia show that the right and left perirenal spaces are potentially separated and, therefore, it is exceptional that a complication of an endourologic procedure, e.g. hematoma, urinoma, or perirenal abscess, involves the contralateral perirenal space [3].

Relationship of kidneys to the diaphragm, ribs, and pleura

The kidneys lie on the psoas and quadratus lumborum muscles. Usually, the left kidney is higher than the right kidney, with the posterior surface of the right kidney crossed by the 12th rib and the left kidney crossed by the 11th and 12th ribs (Figure 6.1). The posterior surface of the diaphragm attaches to the extremities of the 11th and 12th ribs (Figure 6.6). Close to the spine, the diaphragm is attached over the posterior abdominal muscles, and forms the medial and lateral arcuate ligaments on each side (Figure 6.6). In this way, the posterior aspect of the diaphragm (posterior leaves) arches in a dome above the superior pole of the kidneys, on each side. Therefore, when performing an intrarenal access by puncture, the endourologist may consider that the diaphragm is traversed by all intercostal punctures, and possibly by some punctures below the 12th rib (Figure 6.7). Also, it can be expected that the pleura is transversed without symptoms in most intercostal approaches [4].

Generally, the posterior reflection of the pleura extends inferiorly to the 12th rib; nevertheless, the lowermost lung edge lies above the 11th rib (at the 10th intercostal space) (Figure 6.7). Regardless of the degree of respiration (mid or full expiration), the risk of injury to the lung from a 10th intercostal percutaneous approach to the kidney is prohibitive [4]. Any intercostal puncture should be made in the lower half of the intercostal space, in order to avoid injury to the intercostal vessels above.

Relationship of kidneys to the liver and spleen

The liver on the right side and the spleen on the left may be posterolaterally positioned at the level of the suprahilar region of the kidney, because at this point these organs have their largest dimensions (Figure 6.8). Therefore, it should be remembered that a kidney puncture performed high in the abdomen will allow little space for the needle entrance [4]. If the intrarenal puncture is performed when the patient is in mid or full inspiration, the risk of injury to the liver and spleen is increased [4]. This knowledge is particularly important in patients with hepatomegaly or splenomegaly, in whom a computed tomography (CT) scan should be performed before puncturing the kidney.

Relationship of kidneys to the ascending and descending colons

The ascending colon runs from the ileocolic valve to the right colic flexure (hepatic flexure), where it passes into the transverse colon. The hepatic colic flexure (hepatic angle) lies anteriorly to the inferior portion of the right kidney. The descending colon extends inferiorly from the left colic flexure (splenic flexure) to the level of the iliac crest. The left colic flexure lies anterolateral to the left kidney.

It is important to consider the position of the retroperitoneal ascending and descending colons. Occasionally, in the course of a routine abdominal CT scan, the retroperitoneal colon has been observed to lie in a posterolateral or even a retrorenal position [5], and in these cases, there is a great risk of kidney injury with the intrarenal percutaneous approach. A retrorenal colon is more common in the area of the inferior poles of the kidneys (Figure 6.9). Retrorenal colon was found on CT scan in 1.9% of patients in the supine position, but 10% when the prone position (the more commonly adopted position for percutaneous access to the kidney) was assumed [5]. Therefore, with the patient in the prone position and before any invasive percutaneous renal procedure, retrorenal colon should be looked for, especially around the inferior poles of the kidney, using fluoroscopy [5].

Intrarenal vessels

Intrarenal arteries

Generally, the main renal artery divides into an anterior and a posterior branch after giving off the inferior suprarenal artery. Whereas the posterior branch (retropelvic artery) proceeds as the posterior segmental artery to supply the homonymous segment without further significant branching, the anterior branch of the renal artery provides three or four segmental arteries. The segmental arteries divide before entering the renal parenchyma into the interlobar arteries (infundibular arteries), which progress adjacent to the calyceal infundibula and the minor calyces, entering the renal columns between the renal pyramids (Figures 6.10 and 6.11) [6]. As the interlobar arteries progress, near the base of the pyramids, they give origin (usually by dichotomous division) to the arcuate arteries (Figures 6.10 and 6.11). The arcuate arteries give off the interlobular arteries, which run to the periphery, giving off the afferent arterioles of the glomeruli (Figure 6.11) [6].

Intrarenal veins

The intrarenal veins, unlike the arteries, do not have a segmental model. Moreover, in contrast to the arteries, there is free circulation throughout the venous system, with ample anastomoses between the veins. These anastomoses, therefore, prevent parenchymal congestion and ischemia in case of venous injury [7].

The small veins of the cortex, called stellate veins, drain into the interlobular veins that form a series of arches (Figure 6.12). Within the kidney substance, these arches are arranged in arcades, which lie mainly in the longitudinal axis. There are usually three systems of longitudinal anastomotic arcades and the anastomoses occur at different levels: between the stellate veins (more peripherally), between the arcuate veins (at the base of the pyramids), and between the interlobar (infundibular) veins (close to the renal sinus) (Figure 6.12). We have named these anastomoses as first order, second order, and third order, from periphery to center [7]. In early studies, we found three trunks (53.8%) and two trunks (28.8%) joining each other to form the main renal vein. Less frequently, we found four trunks (15.4%) and five trunks (1.9%) [7].

A detailed description of the kidney collecting system (the pelviocalyceal system), as well as the anatomic relationships between the intrarenal arteries and veins with the kidney collecting system, which are of utmost importance for endourology, is given below.

Anatomic classification

Recent advances in endourology have revived interest in collecting system anatomy, since a full understanding of such anatomy is necessary to perform reliable endourologic procedures as well as uroradiologic analysis [8–10]. We have proposed a pelviocalyceal classification, including all morphologic types of collecting systems, which we believe is helpful for standardizing patients and procedures [9]. This classification was derived from the analysis of 140 three‐dimensional (3D) polyester resin corrosion endocasts of the pelviocalyceal system (Figure 6.13), obtained from 70 fresh cadavers according to a technique described previously [8].

Basic intrarenal anatomy

The renal parenchyma basically consists of two kinds of tissue, the cortical tissue and medullar tissue. On a longitudinal section (Figure 6.14), the cortex forms the external layer of renal parenchyma. The renal medulla is formed by several inverted cones, surrounded by a layer of cortical tissue on all sides (except at the apices). As in longitudinal sections, a cone assumes the shape of a pyramid (Figure 6.14) and the established term for the medullar tissue is renal pyramid; the apex of this pyramid is termed the renal papilla. The layers of cortical tissue between adjacent pyramids are termed renal columns (cortical columns of Bertin; Figure 6.14) [8, 10].

The cortical tissue is made up of the glomeruli with proximal and distal convoluted tubules. The renal pyramids are made up of loops of Henle and collecting ducts; these ducts join to form the papillary ducts (about 20), which open at the papillary surface (area cribosa; Figure 6.15) and drain urine into the collecting system (into the fornix of a minor calyx).