Gross anatomy of kidneys
The kidneys are paired bean-shaped abdominal organs that produce urine. They eliminate toxic metabolites with urine. Other essential functions of the kidneys include regulation of blood homeostasis and blood pressure.
The kidneys are located retroperitoneally in the posterior abdominal region. They are found in the lumbar region within the extraperitoneal connective tissue lateral to the spine.
In a supine position, the left kidney extends from the vertebra T11 superiorly to vertebra L2 inferiorly. The right kidney lies somewhat lower than the left one because of its relationship with the liver. It extends from approximately vertebra T12 superiorly to vertebra L3 inferiorly.
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.
In adults, the kidneys are approximately 4 to 5 inches (10 - 12 cm) long, 2 - 3 inches (5 - 7.5 cm) wide, and around 1 inch (2 - 2.5) cm thick. Kidneys each weigh around 1/4 to 1/2 pounds (110 - 200 grams).
Surfaces, poles, margins
Each kidney has two smooth surfaces, two poles, and two margins (borders).
The two poles of the kidney are known as the superior and inferior poles. The superior pole is wider than the inferior.
The two surfaces are named the anterior surface and posterior surface. They are covered by a fibrous capsule. The anterior surface is more convex but posterior - flatter.
Both surfaces are separated by margins (borders) - lateral (convex) and medial (concave).
Hilum of kidney
On the medial margin of each kidney is a deep vertical slit called the hilum of the kidney. It 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. You can use the abbreviation VAU for memorizing this order (V - vein, A - artery, U - ureter).
The hilum of the right kidney is located at the level of the L2 vertebra, while that of the left kidney - at the L1 vertebra.
Internally, the hilum continues as the renal sinus. Perinephric fat, which covers the fibrous capsule of each kidney, continues into the hilum, and the sinus surrounds all of its structures.
The kidneys are enclosed by a fibrous capsule, which is surrounded by a unique arrangement of fascia and fat, that create three layers (from innermost to outermost):
- Perinephric (perirenal) fat
- Renal fascia
- Paranephric (pararenal) fat
Immediately outside the fibrous capsule and extraperitoneally, the perinephric (perirenal) fat completely surrounds the kidney. The perinephric fat covering is enclosed by a membranous condensation of the extraperitoneal fascia known as 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.
Above each adrenal gland, the anterior and posterior layers of the renal fascia 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. 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.