Peritoneal Cavity and Kidney Anatomy

Human Anatomy & Histology - Kidneys 2021-22 The self-study Kidney starts with a short repetition of the peritoneum and the peritoneal cavity, which were already introduced during self-study and anatomy class of the gastrointestinal tract. I. Peritoneal cavity The belly (abdomen) contains the abdominal or peritoneal cavity (also known as the peritoneal sac or greater sac, cavitas peritonealis). This is a so-called serous cavity, which means that it contains a small amount of serous liquid (similar to the pleural cavity and the pericardial cavity). All these cavities are formed by two membranous layers – a parietal layer (covering the walls of the cavity) and a visceral layer (covering the organs within the cavity). The abdominal cavity is surrounded by the parietal peritoneum, whereas the visceral peritoneum covers several abdominal organs. The transition between the parietal and the visceral layer is the mesentery which connects the intestine to the dorsal body wall (Figure 1). Figure 1 adapted from "Sobotta 1906: cross-section of abdomen with visceral and parietal peritoneum and mesentery" . This simplified scheme of a transverse section through a human body at the level of the kidneys shows the peritoneum as a continuous orange line. The parts that cover/line the inner body wall (dorsally, ventrally, laterally) are called parietal peritoneum (Latin paries = wall, parietis = of a wall), those surrounding the organs (such as the intestine) are called visceral peritoneum (viscera = inner organs of a body). This picture by Johannes Sobotta is in the Public Domain. Some organs are completely covered by the visceral peritoneum and are attached to the dorsal body wall by a mesentery – these are located intraperitoneally (such as parts of the small intestine, the jejunum and the ileum; Figure 2). Others are only partially covered by peritoneum or lie completely outside the peritoneal cavity, these are said to be located retroperitoneally (such as the duodenum, and parts of the large intestine. Intraperitoneal organs are completely covered by visceral peritoneum and are connected to the body wall via a mesentery (double-layer of peritoneum). This double-layer contains fat, connective tissue, blood vessels, lymphatics, and nerves. Stomach and the largest part of the small intestine are intraperitoneal organs. Retroperitoneal organs are partly covered by parietal peritoneum, but large parts are not in contact with peritoneum, examples are the pancreas, duodenum and the ascending and descending colon. Human Anatomy & Histology - Kidneys 2021-22 Figure 2 "Leiden - Drawing Scheme: Three locations of organs relative to the peritoneum” by Ron Slagter and O. Paul Gobée, LUMC, license: CC BY-NC-SA. On the left, the small intestine (ileum, jejunum) serves as a example for an intraperitoneal organ (completely surrounded by visceral peritoneum, with a mesentery). The middle panel shows a part of the large intestine (e.g. ascending colon) in a secondarily retroperitoneal position. The colon and its mesentery have become attached to the body wall. The right panel shows the position of a primarily retroperitoneal structure (behind the peritoneum, e.g. the kidneys, the aorta, the inferior vena Sometimes a distinction is made between primarily and secondarily retroperitoneal. Primarily retroperitoneal organs are those, which developed in the retroperitoneal space (examples are the aorta, the esophagus, the rectum). Secondarily retroperitoneal organs were once suspended with a mesentery in the peritoneal cavity but became attached to the retroperitoneal space during development – examples are the pancreas, the duodenum, the ascending and descending colon. In schematic drawings, the peritoneal cavity is often represented as a large cavity with a few organs surrounded by lots of space (Figure 1). The actual cavity is only a virtual space, since peritoneal and parietal layers of the peritoneum lie closely against each other, separated only by a little bit of serous liquid (similar to the situation of the pleura). The serous liquid enables easy sliding of the structures along each other (Figure 3). Figure 3 Adapted "Peritoneal lining (transverse section abdomen)" , license: CC BY-NC-SA This transverse section shows the peritoneum as a blue line. Spleen, stomach, liver are surrounded by visceral peritoneum, they are intraperitoneal organs, kidney, pancreas, aorta (Ao), inferior vena cava (IVC), adrenal glands and pancreas are in a retroperitoneal position. Be aware that the scheme represents a body that is lying with the feet towards you. The liver is on the right side of the body. The lesser sac is also known as the omental bursa (bursa omentalis). The lesser omentum is also called omentum minus.