Each kidney is about 10-12 cm long, 5-6 cm wide and 3-3.5 cm thick. Its length approximates to the height of three and a half vertebral bodies.
The left kidney is usually larger by 2-3 cm in about 80% of patients. In everyone else, the size is equal or the right kidney is larger.
The kidneys lie in the retroperitoneum on the posterior abdominal wall, in the paravertebral gutters. They lie obliquely so that their upper poles are nearer the midline and more posterior than their lower poles.
They are not symmetrically located. The right kidney is lower than the left due to the bulk of the liver on the right side. The kidneys are also very mobile and may descend by up to 6 cm on standing or on inspiration, particularly in the elderly.
The development of the kidney is a good example of human development passing through various early phylogenetic stages. Vertebrate renal development follows three sets of development:
- Pronephros: Pronephros does not function – this marks the start of renal development at the fourth week of gestation
- Mesonephros: Mesonephros appears at the end of the fourth week, developing caudal to the degenerating pronephros. It contains a primary duct, the mesonephric duct, which opens into the urogenital sinus
- Metanephros: Elaborates into the final human form
Metanephros begins to develop at about the fifth week and is fully functional by about the ninth week of gestation. This ureteric bud grows into the metanephric mesoderm which condenses around it, forming the metanephric cap, and induces development of the kidney. As growth proceeds, the ureteric bud elongates and the kidney ascends out of the pelvis.
The ureteric bud forms the:
- Collecting ducts
- Renal pelvis and ureters
The metanephric mesoderm differentiates into:
- The renal parenchyma
- Proximal ductal system
Fusion anomalies occur when the kidneys are joined but the ureters still enter the bladder on each side.
With all fusion anomalies, there is an increased risk of:
- Pelviureteric junction (PUJ) obstruction
- Vesicoureteric reflux (VUR)
- Injury from anterior abdominal trauma
- Arterial anomalies
Horseshoe kidney is the most common fusion anomaly. It occurs in 1 in 400 (autopsy data) to 1 in 1000 people.
In this anomaly, renal parenchyma on each side of the vertebral column is usually joined at the lower poles, with an isthmus of renal parenchyma or fibrous tissue across the midline at the joined areas. Ureters course medially and anteriorly over this isthmus and generally drain well.
Obstruction is usually secondary to the high insertion of the ureter in the renal pelvis, rather than being secondary to the isthmus.
Crossed Fused Renal Ectopia
Crossed fused renal ectopia is the second most common fusion anomaly, estimated at 1 in 2000 people.
Here, the renal parenchyma (representing both kidneys) is on the same side of the vertebral column. One ureter crosses the midline and enters the bladder on the side opposite the kidneys. That is, although the kidney is ectopic, its corresponding ureter is orthotopic
During the long process of renal development, problems can arise at any stage.
Fetal lobulation may persist into adulthood. This may involve the entire kidney or just the middle and lower thirds. It is distinguished from parenchymal scarring by the position of the surface notches:
- Fetal lobulation: Notches are between the calyces
- Scarring: Notches are directly over the calyces
This is usually a minor anomaly of the renal axis. Normally, the kidney rotates medially as it ascends but it can over-rotate (the renal pelvis points more medially) or under-rotate (the pelvis points anteriorly or even laterally).
Unilateral renal agenesis is not uncommon (1 in 1500 births). It is characterised by:
- Usually being accompanied by ureteric agenesis
- Absence of the ipsilateral trigone and ureteric orifice
- Compensatory hypertrophy of the solitary kidney maintains normal renal function
Abnormal renal location results from a kidney that either fails to:
- Ascend from its origin in the true pelvis = pelvic kidney
- Descend from a superiorly ascended position = thoracic kidney (very rare)
There is an increased incidence of PUJ junction obstruction, VUR and multicystic renal dysplasia with a pelvic kidney.