The kidneys are paired bean-shaped organs located retroperitoneally in the posterior abdominal region. They are found within the extraperitoneal connective tissue lateral to the spine.
In a spuine position, the kidneys extend from approximately T12 vertebra superiorly (superior pole) to the L3 vertebra inferiorly (inferior pole). The right kidney lies somewhat lower than the left one because of its relationship with the liver.
Both kidneys are similar in size and shape. However, the left kidney is longer and more slender than the right and lies closer to the midline. Each kidney has two smooth surfaces - anterior and posterior surfaces, which are covered by a fibrous capsule.
On the medial margin of each kidney is a deep vertical slit called the hilum of the kidney, which serves as an entry and exit point for renal vessels, lymphatics, and nerves. Internally, the hilum continues as the renal sinus. The perinephric fat continues into the hilum and sinus surrounding all of its structures.
Each kidney consists of an outer renal cortex and an inner medulla. Extensions of the renal cortex project internally and are called renal columns. These columns divide the renal medulla into triangular-shaped tissue called the renal pyramids. The bases of the renal pyramids are directed toward the renal cortex (outward), while the apex of each pyramid projects inward toward the renal sinus.
The apical projections of the renal pyramids are known as the renal papillae. They contain the openings of the papillary ducts that drain renal tubules and is surrounded by a minor calyx.
The minor calyces represent the proximal parts of the tubes that will eventually form the ureters. They receive urine from the papillary ducts. Several minor calyces form a major calyx in the renal sinus. Two or three major calyces fuse to form the renal pelvis, which is the funnel-shaped superior end of each ureter.
Renal fat and fascia
The kidneys are enclosed in a unique arrangement of fascia and fat. Immediately outside the renal capsule and extraperitoneal, the perinephric fat (extraperitoneal fat) completely surrounds the kidney.
The perinephric fat is enclosed by a membranous condensation of the extraperitoneal fascia - the renal fascia. The adrenal glands are usually also enclosed within this fascia and separated from the kidneys by a thin septum.
At the lateral margins of the kidneys, the anterior and posterior layers of the renal fascia fuse and connect with the transversalis fascia on the lateral part of the abdominal wall.
Above each adrenal gland, the anterior and posterior layers of the renal fascia also fuse and connect with the fascia covering the diaphragm.
Medially, the anterior layer of the renal fascia covers the vessels in the hilum and fuses with the connective tissue associated with the abdominal aorta and inferior vena cava. The posterior layer medially lies between the kidney and the fascia covering the quadratus lumborum muscle and fuses with the fascia covering the psoas major muscle.
Inferiorly, the anterior and posterior layers of the renal fascia enclose both ureters.
Above the renal fascia is the final layer of paranephric (pararenal) fat, which accumulates posteriorly and posterolaterally to each kidney.