As shown in Figure above , each kidney is divided into an outer cortex and an inner medulla. Renal columns of cortex (of Bertin) extend centrally and separate the medulla into pyramids:
- The apex of a pyramid, called the papilla, empties into a minor calyx
- Three or four minor calyces empty into a major calyx
- Two or three major calyces empty into the renal pelvis
- The renal pelvis is in continuity with the ureter via the renal hilum where it is surrounded by a narrow space, the renal sinus. This contains fat. The renal pelvis may be intra-renal or wholly extra-renal.
At the renal hilus, the renal vein, artery and pelvis of the ureter lie from anterior to posterior. A branch of the renal artery often passes posterior to the renal pelvis.
Functional Unit of Kidney
The functional unit of the kidney is called the nephron. This consists of a glomerulus within the cortex and a tubule within the medulla. The latter drains to a collecting duct which opens onto the surface of the renal papilla into a minor calyx.
Note the relatively poor blood supply to the papillary tips. This is believed to be an underlying factor in papillary necrosis.
Superiorly, the diaphragm separates the upper pole from the pleural costodiaphragmatic recess and the eleventh and twelfth ribs.
Inferiorly the kidney lies on the psoas, quadratus lumborum and transversus abdominis muscles from medial to lateral
The anterior relationships of the right kidney are:
- The second part of the duodenum crosses the hilum
- Jejunal loops inferomedial
- Hepatic flexure inferolateral
- The remaining surface is related to the visceral surface of the liver
The anterior relationships of the left kidney are:
- Spleen superolateral
- Pancreas and splenic vessels cross hilum and middle part
- Jejunal loops inferomedial
- Splenic flexure inferolateral
The superior relations of the kidneys are the adrenal glands. The left adrenal gland is more medially placed than the right.
The renal arteries are direct lateral branches of the abdominal aorta. Each arises from the aorta at L1/L2 level. The right renal artery is longer than the left and courses posterior to the inferior vena cava (IVC).
Each renal artery usually divides into three branches, one posterior and two anterior to the renal pelvis:
- The posterior branch supplies the posterior superior aspect
- The anterior branch (1) supplies the anterior superior aspect
- The anterior branch (2) supplies the entire inferior aspect
- The anterior branches are the dominant supply, supplying 60% of the kidney.
There is a theoretical avascular line where the anterior and posterior branches meet at the lateral aspect of the kidney. This is called Brödel’s line.
Variations in Arterial Supply
Renal arterial anomalies are very common. About 20-25% of people will have an accessory renal artery, most commonly a lower pole artery.
Arterial anomalies are particularly common when there is some abnormality of renal development such as a horseshoe kidney or pelvic kidney.
Venous tributaries unite to form a single renal vein which drains into the IVC. The left renal vein is longer than the right. It courses anterior to the aorta and receives the left adrenal and gonadal veins.
Variations in Venous Drainage
Venous anomalies are less common and a figure between 3-17% is quoted. Of common anomalies, the radiologist needs to be particularly aware of the retro-aortic left renal vein and the circum-aortic renal vein.
The kidney is surrounded by a fibrous tunic. Adipose tissue, termed the perinephric fat, surrounds the kidney and separates it from surrounding structures.
The kidney and perinephric fat are enclosed in a sheath of fibrous tissue – the renal fascia. The renal fascia has:
- An anterior layer/lamella termed Gerota’s fascia. Gerota’s fascia is continuous medially over the aorta with the corresponding lamella of the opposite side
- A posterior lamella termed the fascia of Zuckerkandl.Zuckerkandl’s fascia blends with the fascia on the quadratus lumborum and psoas muscle
Fusion of lamellae:
Lamellae are fused laterally, known as the lateral conal fascia. The lateral conal fascia is continuous with the fascia on the deep surface of the transversus abdominus muscle
Superiorly the two layers of renal fascia fuse and unite with the fascia on the deep surface of the diaphragm
Inferiorly the two layers remain separate. The anterior lamella blends with the sub-peritoneal fascia of the posterior abdominal wall and the posterior lamella becomes continuous with the fascia over the iliacus muscle
The renal fascia encloses the perirenal space. The adrenal gland lies within the space, separated from the kidney by fat. Inferiorly the two layers of renal fascia remain separate and hence the space is relatively open below. This allows for potential inferior extension of pathology (e.g. urine, blood and pus) within the space.
Anterior Pararenal Space
This is the space between the anterior lamella of renal fascia and posterior peritoneum. It is continuous across the midline. It contains:
- Ascending and descending colon
Posterior Pararenal Space
This is the space posterior to the renal fascia and anterior to the muscles of the posterior abdominal wall. It is:
- Limited medially by the attachment of the renal fascia to the psoas muscle
- Continuous laterally with the extra-peritoneal fatty tissue deep to the transversalis fascia
- Continuous inferiorly with fat anterior to the iliacus muscle