On its lateral side, the lower half of the kidney is covered by the left colic flexure and the beginning of the descending colon, and, on its medial side, by the parts of the intraperitoneal jejunum.
Posteriorly, the right and left kidneys are related to similar structures (Fig. 4.151). Superiorly is the diaphragm and inferior to this, moving in a medial to lateral direction, are the psoas major, quadratus lumborum, and transversus abdominis muscles.
The superior pole of the right kidney is anterior to rib XII, while the same region of the left kidney is anterior to ribs XI and XII. The pleural sacs and specifically the costodiaphragmatic recesses therefore extend posterior to the kidneys.
Also passing posterior to the kidneys are the subcostal vessels and nerves and the iliohypogastric and ilio-inguinal nerves.
The kidneys are enclosed in and associated with a unique arrangement of fascia and fat. Immediately outside the renal capsule, there is an accumulation of extraperitoneal fat—the perinephric fat (perirenal fat), which completely surrounds the kidney (Fig. 4.152). Enclosing the perinephric fat is a membranous condensation of the extraperitoneal fascia (the renal fascia). The suprarenal glands are also enclosed in this fascial compartment, usually separated from the kidneys by a thin septum. The renal fascia must be incised in any surgical approach to this organ.
At the lateral margins of each kidney, the anterior and posterior layers of the renal fascia fuse (Fig. 4.152). This fused layer may connect with the transversalis fascia on the lateral abdominal wall.
Above each suprarenal gland, the anterior and posterior layers of the renal fascia fuse and blend with the fascia that covers the diaphragm.
Medially, the anterior layer of the renal fascia continues over the vessels in the hilum and fuses with the connective tissue associated with the abdominal aorta and the inferior vena cava (Fig. 4.152). In some cases, the anterior layer may cross the midline to the opposite side and blend with its companion layer.
The posterior layer of the renal fascia passes medially between the kidney and the fascia covering the quadratus lumborum muscle to fuse with the fascia covering the psoas major muscle.
Inferiorly, the anterior and posterior layers of the renal fascia enclose the ureters.
In addition to perinephric fat and the renal fascia, a final layer of paranephric fat (pararenal fat) completes the fat and fascias associated with the kidney (Fig. 4.152). This fat accumulates posterior and posterolateral to each kidney.
Each kidney has a smooth anterior and posterior surface covered by a fibrous capsule, which is easily removable except during disease.
On the medial margin of each kidney is the hilum of the kidney, which is a deep vertical slit through which renal vessels, lymphatics, and nerves enter and leave the substance of the kidney (Fig. 4.153). Internally, the hilum is continuous with the renal sinus. Perinephric fat continues into the hilum and sinus and surrounds all structures.
Each kidney consists of an outer renal cortex and an inner renal medulla. The renal cortex is a continuous band of pale tissue that completely surrounds the renal medulla. Extensions of the renal cortex (the renal columns) project into the inner aspect of the kidney, dividing the renal medulla into discontinuous aggregations of triangular-shaped tissue (the renal pyramids).
The bases of the renal pyramids are directed outward, toward the renal cortex, while the apex of each renal pyramid projects inward, toward the renal sinus.
The apical projection (renal papilla) contains the openings of the papillary ducts draining the renal tubules and is surrounded by a minor calyx.