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CHAPTER 74 – Kidney and ureter

KIDNEY

The kidneys excrete the end products of metabolism and excess water. These actions are essential for the control of concentrations of various substances in the body, maintaining electrolyte and water balance approximately constant in the tissue fluids. The kidneys also have endocrine functions, producing and releasing erythropoietin, which affects red blood cell formation; renin, which influences blood pressure; 1,25-di-hydroxycholecalciferol (the metabolically active form of vitamin D), which is involved in the control of calcium absorption and mineral metabolism; and various other soluble factors with metabolic actions.

In the fresh state, the kidneys are reddish-brown. They are situated posteriorly behind the peritoneum on each side of the vertebral column and are surrounded by adipose tissue. Superiorly they are level with the upper border of the 12th thoracic vertebra, inferiorly with the third lumbar vertebra. The right is usually slightly inferior to the left, reflecting its relationship to the liver. The left is a little longer and narrower than the right and lies nearer the median plane (Fig. 74.1). The long axis of each kidney is directed inferolaterally and the transverse axis posteromedially, which means that the anterior and posterior aspects usually described are in fact anterolateral and posteromedial. An appreciation of this orientation is important in percutaneous and endo-urologic renal surgery.

  

Fig. 74.1  Relationships of the kidneys and ureters in the male retroperitoneum.
(From Sobotta 2006.)

Each kidney is typically 11 cm in length, 6 cm in breadth and 3 cm in anteroposterior dimension. The left kidney may be 1.5 cm longer than the right; it is rare for the right kidney to be more than 1 cm longer than the left. The average weight is 150 g in men and 135 g in women. In thin individuals with a lax abdominal wall the lower pole of the lower right kidney may just be felt in full inspiration by bimanual lumbar examination, but this is unusual.

In the fetus and newborn, the kidney normally has 12 lobules. These are fused in adults to present a smooth surface although traces of lobulation may remain.

Absent and ectopic kidneys

A single absent kidney is seen in 1 in 1200 individuals and results from failure of metanephric blastema to join with a ureteric bud on the affected side. It has no clinical sequelae but may frequently be associated with absence of the ipsilateral vas deferens and/or epididymis and may be associated with other congenital anomalies including imperforate anus, cardiac valvular anomalies and oesophageal atresia. A single kidney often shows compensatory hypertrophy. The life expectancy of individuals with a single kidney is the same as those with two kidneys.

Failure of the kidney to ascend into the renal fossa in utero results in renal ectopia. Most commonly the kidney is found in the pelvis: this occurs in 1 in 2500 live births. Kidneys so placed often have associated malrotation anomalies, and may have marked fetal lobulation. Pelvic kidneys frequently become hydronephrotic as a result of an anterior placed ureter and an anomalous arterial supply. An associated pelviureteric junction obstruction is often present.

Very rarely, and despite the normal location of the ureteric orifices within the bladder, the two renal masses may be on the same side. This is termed crossed renal ectopia and usually the two renal masses are fused in such circumstances. A solitary crossed renal ectopia may be associated with skeletal and other genitourinary anomalies. A number of different anatomical patterns can result, all of which are extremely rare (Fig. 74.2).

  

Fig. 74.2  Crossed renal ectopia: diagram showing the possible arrangements of crossed ectopic kidneys.

Horseshoe kidney

Horseshoe kidneys are found in 1 in 400 individuals. A transverse bridge of renal tissue, the isthmus, which usually but not invariably contains functioning renal substance, connects the two renal masses. The isthmus lies between the inferior poles, most commonly anterior to the great vessels. The ureters curve anterior to the isthmus and often have a high insertion into the renal pelvis (Fig. 74.3A,B).

  

Fig. 74.3  A, Horseshoe kidney. Note the ureters pass anterior to the isthmus. Note also the relatively high insertion of the ureters into the renal pelvis. B, Late phase post-contrast axial CT scan showing a horseshoe kidney. Both lower poles are directed medially and fused by an isthmus (arrowed). Note that the renal pelves (seen here filled with contrast) are directed anteriorly.
(From Sobotta 2006.)

The blood supply to horseshoe kidneys is variable. One vessel to each moiety is seen in 30% of horseshoe kidneys, but multiple anomalous vessels are common; the isthmus may be supplied by a vessel directly from the aorta or from branches of the inferior mesenteric, common iliac or external iliac arteries. In view of this variable arterial anatomy, angiography or CT scanning with vascular reconstruction is very helpful when planning renal surgery on horseshoe kidneys. Horseshoe kidneys can have an associated congenital pelviureteric junction obstruction in up to 30% of cases. Anomalous vessels crossing the ureter and the abnormal course of the ureter as it passes over renal substance may also cause obstruction. Horseshoe kidneys have an increased incidence of stone disease, probably as a consequence of areas of inefficient drainage.

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