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//VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

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 one-hundredth of total body weight. They branch laterally from the aorta just below the origin of the superior mesenteric artery (see Fig. 62.8; Fig. 74.10A). Both cross the corresponding crus of the diaphragm at right angles to the aorta. The right renal artery is longer and often 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 a little lower and passes behind the left renal vein, the body of the pancreas, and splenic vein. It may be crossed anteriorly by the inferior mesenteric vein.

  

Fig. 74.10  A, Axial multislice CT renal angiogram. B, Variations in the number and patterns of branching of the renal artery (percentages are approximate).
(B from Sobotta 2006.)

A single renal artery to each kidney is present in approximately 70% of individuals (Fig. 74.11). The arteries vary in their level of origin and in their calibre, obliquity and precise relations (Fig. 74.10B). In its extrarenal course each renal artery gives off one or more inferior suprarenal arteries, a branch to the ureter and branches which supply perinephric tissue, the renal capsule, and the pelvis. Near the renal hilum, each artery divides into an anterior and a posterior division, and these divide into segmental arteries supplying the renal vascular segments. Accessory renal arteries are common (30% of individuals), and usually arise from the aorta above or below (most commonly below) the main renal artery and follow it to the renal hilum. They are regarded as persistent embryonic lateral splanchnic arteries. Accessory vessels to the inferior pole cross anterior to the ureter and may, by obstructing the ureter, cause hydronephrosis. Rarely, accessory renal arteries arise from the coeliac or superior mesenteric arteries near the aortic bifurcation or from the common iliac arteries.

  

Fig. 74.11  Resin corrosion cast of human kidneys. Ureters, pelves and calyces are yellow; aorta, renal arteries and their branches are red.
(Prepared by the late DH Tompsett of the Royal College of Surgeons of England. By permission of the Museums of The Royal College of Surgeons.)

The subdivisions of the renal arteries are described sequentially as segmental, lobar, interlobar, arcuate and interlobular arteries and afferent and efferent glomerular arterioles (see Fig. 74.14).

  

Fig. 74.14  The major structures in the kidney cortex and medulla (left), the position of cortical and juxtamedullary nephrons (middle) and the major blood vessels (right).

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 have been identified (Fig. 74.12). The apical segment occupies the anteromedial region of the superior pole. The superior (anterior) segment includes the rest of the superior pole and the central anterosuperior region. The inferior segment encompasses the whole lower pole. The middle (anterior) segment lies between the anterior and inferior segments. The posterior segment includes the whole posterior region between the apical 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. Whatever pattern is present, it must be emphasized that vascular segments are supplied by virtual end arteries. In contrast, larger intrarenal veins have no segmental organization and anastomose freely.

  

Fig. 74.12  Segmental arterial anatomy of the right kidney: the posterior division branches near the hilum before the anterior division divides into the other segmental arteries.

Brödel (1911) 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 preferable. Knowledge of the vascular anatomy of the kidney is important when undertaking partial nephrectomy for renal cell cancers. In this surgery the branches of the renal artery are defined so that the surgeon may safely excise the renal substance containing 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 pyramid they subdivide into two or three interlobar arteries, extending towards the cortex around each pyramid. At the junction of the cortex and medulla, interlobar arteries dichotomize into arcuate arteries which diverge at right angles. As they arch between cortex and medulla, each divides further, ultimately supplying interlobular arteries which diverge radially into the cortex. The terminations of adjacent arcuate arteries do not anastomose but end in the 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. 74.14). Interlobular arteries ascend towards the superficial cortex or may branch occasionally en route. Some are more tortuous and recurve towards the 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 medulla, the intermediate pass horizontally, and the more superficial approach the renal surface obliquely before ending in a glomerulus (see Fig. 74.14). Efferent glomerular arterioles from most glomeruli (except at juxtamedullary and, sometimes, at intermediate cortical levels) soon divide to form a dense peritubular capillary plexus around the proximal and distal convoluted tubules: there are thus 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 medulla are relatively long, wide vessels, and contribute side branches to neighbouring capillary plexuses before entering the medulla, where each divides into 12–25 descending vasa recta. As their name suggests, these run straight to varying depths in the renal medulla, contributing side branches to a radially elongated capillary plexus (see Fig. 74.14) 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 ascending and descending vessels are grouped into vascular bundles, within which the external aspects of both types are closely apposed, bringing them close to the limbs of renal loops 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 with each other and adjacent ducts provides the structural basis for the countercurrent exchange and multiplier phenomena (see countercurrent exchange mechanism later in chapter (see Fig. 74.17)). These complex renal vascular patterns show regional specializations which are closely adapted to the spatial organization and functions of renal corpuscles, tubules and ducts (see below).

  

Fig. 74.17  The regional microstructure and principal activities of a kidney nephron and collecting duct. For clarity, a nephron of the long loop (juxtamedullary) type is shown.

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 renal capsule 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. The left is three times longer than the right (7.5 cm and 2.5 cm respectively) and for this reason, the left kidney is the preferred side for live donor nephrectomy. The left renal vein runs from its origin in the renal hilum, posterior to the splenic vein and the body of pancreas, and then across the anterior aspect of the aorta, just below the origin of the superior mesenteric artery. The left gonadal vein enters it from below and the left suprarenal vein, usually receiving one of the left inferior phrenic veins, enters it above but nearer the midline. The left renal vein enters the inferior vena cava a little superior and to the right. The right renal vein is behind the descending duodenum and sometimes the lateral part of the head of the pancreas. It can be extremely short (<1 cm) such that safe nephrectomy may require excision of a cuff of the inferior vena cava (Fig. 74.13).

  

Fig. 74.13  CT renal venogram. Acquired from a multislice CT examination and reconstructed as a 3D surface shaded reformat.

The left renal vein may be double, one vein passing posterior, the other anterior, to the aorta before joining the inferior vena cava. This is sometimes referred to as persistence of the ‘renal collar’. The anterior vein may be absent so that there is a single retroaortic left renal vein. The left renal vein may 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 to the right of 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.

Lymphatic drainage

Renal lymphatic vessels begin in three plexuses, around the renal tubules, under the renal capsule, and in the perirenal fat (the latter two connect freely). Collecting vessels from the intrarenal plexus form four or five trunks which follow the renal vein to end in the lateral aortic nodes; the subcapsular collecting vessels join them as they leave the hilum. The perirenal plexus drains directly into the same nodes.

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