Gross anatomy of the kidneys
The kidneys are paired bean-shaped organs that produce urine. They are located retroperitoneally in the posterior abdominal region.
They are found in the lumbar region within the extraperitoneal connective tissue lateral to the vertebral column.
The right kidney extends from approximately vertebra T12 superiorly to vertebra L3 inferiorly (in a supine position). 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.
They are approximately 10-12 cm long, 5-6 cm wide, and about 4 cm thick. Kidneys weigh around 110-200 grams.
Each kidney has two smooth surfaces, two poles, and two margins (borders).
The poles are known as the superior and inferior. The superior pole is wider than the inferior.
The surfaces are named the anterior and posterior surfaces, which are covered by a fibrous capsule. The anterior surface is more convex but posterior - flatter.
Between both surfaces are lateral (covex) and medial borders (concave).
On the medial margin of each kidney is a deep vertical slit called the hilum of the kidney, which serves as an entry point for renal artery and nerves and exit point for renal vein, lymphatic vessels, and ureter. There is a particular order in which those structures are situated.
The most superior structure is the renal vein, but under it is the artery, and under the artery lies the ureter (V - vein, A - artery, U - ureter).
Internally, the hilum continues as the renal sinus. Perinephric fat, which covers the fibrous capsule, continues into the hilum, and the sinus surrounds all of its structures.
The kidneys are enclosed in a unique arrangement of fascia and fat, and they are surrounded and protected by three layers (from innermost to outermost):
- Fibrous capsule
- Perirenal fat capsule (perinephric fat)
- Renal fascia
Immediately outside the fibrous 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 enclosed within the renal fascia and separated from the kidneys by a thin septum.
The renal fascia has two layers - the anterior and posterior.
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.
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. It ends up growing at the vertebral column.
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.