Structures that lie behind the
peritoneum are termed "retroperitoneal". Organs that were once suspended within the abdominal cavity by
mesentery but migrated posterior to the peritoneum during the course of
embryogenesis to become retroperitoneal are considered to be secondarily retroperitoneal organs. • Primarily retroperitoneal, meaning the structures were retroperitoneal during the entirety of development: • urinary •
adrenal glands •
kidneys •
ureter • circulatory •
aorta •
inferior vena cava •
digestive •
anal canal • Secondarily retroperitoneal, meaning the structures initially were suspended in
mesentery and later migrated behind the peritoneum during development • the
duodenum, except for the proximal first segment, which is intraperitoneal • ascending and descending portions of the
colon (but not the transverse colon, sigmoid and the cecum) • pancreas, except for the tail, which is intraperitoneal
Subdivisions through posterior
abdominal wall, showing the relations of the capsule of the
kidney (pararenal fat labeled as paranephric body center left) ;Perirenal space It is also called the perinephric space. Bounded by the anterior and posterior leaves of the
renal fascia. It contains the following structures: •
Adrenal gland •
Kidney •
Renal vessels •
Perirenal fat (also "perirenal fat capsule", "perinephric fat, or "adipose capsule of the kidney") is external to the fibrous capsule of the kidney, and internal to the renal fascia (which separates it from the pararenal fat); connective tissue trabeculae extend through it to unite the fibrous capsule of the kidney, and the renal fascia. Perirenal fat is most abundant upon the posterior aspect, inferior pole and along the lateral margins of the kidney. It is situated posterior to the posterior aspect of
renal fascia, and anterior to the aponeuroses of the retrorenal muscles. It is plentiful in the
dihedral angle of the
iliopsoas muscle and the
quadratus lumborum muscle, filling the lumbar fossa posterior and inferior to the kidney. ==Clinical significance==