Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Internal Organs
General Information on Internal Organs
Body Cavities

In humans, the following body cavities are distinguished: the pericardial cavity, which contains The Heart and the initial segments of the great vessels; the pleural cavity, surrounding the Lungs; the peritoneal, or Abdominal cavity, which lies caudal to the Diaphragm and encloses many Internal Organs; and the cavities surrounding the Testes (male Gonads).

The Peritoneum (peritoneum) is a thin serous membrane lining the abdominal cavity. The peritoneum covers both the walls of the cavity and the organs located within it. On its surface, the peritoneum is covered by a simple squamous epithelium—the mesothelium. Beneath it lies a layer of loose Connective Tissue with A large number of Collagen and elastic fibers, which provide strength and elasticity to the entire membrane. Depending on the functional load of a given area of the peritoneum, its thickness and The amount of connective tissue fibers vary.

The total surface area of the peritoneum is more than 2 m². The peritoneum is divided into two layers that are always continuous with each other. The layer of the peritoneum lining the walls of the cavity is called the parietal peritoneum, and the layer covering the organs is the visceral peritoneum. The latter is also referred to as the serous membrane of the organ, and the peritoneal cavity, like the pleural and pericardial cavities, is a serous cavity. The part of the peritoneum between its parietal and visceral layers forms ligaments, folds, and mesenteries. Thus, most abdominal organs are fixed to its posterior wall (Atl. Fig. 162-164).

Between the layers of the peritoneum, there is always a small slit-like cavity filled with serous fluid (serum). This fluid is secreted through the epithelium and facilitates the gliding of organs against one another. Serous cavities have no contact with the external environment. In males, the peritoneal cavity is closed; in females, it is open because the uterine tubes open into it. The mobility of organs in the abdominal cavity is facilitated by the pressure of the abdominal wall Muscles and the peristaltic contractions of the intestines.

Beneath the peritoneum, there is usually a rather substantial layer of adipose tissue. It is particularly abundant on the anterior abdominal wall below the umbilical fossa, while it is completely absent on the diaphragm.

With respect to the peritoneum, organs are positioned differently (Atl. Fig. 165). If the peritoneum covers an organ on all sides, its position is called intraperitoneal (The Stomach, jejunum and ileum, cecum with the vermiform Appendix, transverse colon, sigmoid colon and the beginning of the rectum, Spleen, Uterus, and fallopian tubes). If an organ is covered by the peritoneum on three sides, its position is called mesoperitoneal (the Liver, ascending and descending colon, middle part of the rectum, Urinary Bladder). Organs covered by the peritoneum on only one side are located retroperitoneally or extraperitoneally (the duodenum, Kidneys, Ureters, Pancreas, Prostate Gland, Vagina, lower part of the rectum).

Intraperitoneal organs are attached to the posterior body wall by means of mesenteries. They are sheets starting from the parietal peritoneum on the posterior body wall (the ROOT of the mesentery). The other end reaches the organ and envelops it. Blood and Lymphatic vessels, nerves, and Lymph Nodes run within the mesenteries.

On the anterior abdominal wall, the parietal peritoneum forms folds. These folds contain Blood Vessels or obliterated vessels and other tubular structures (ducts) that functioned during the prenatal period. For example, folds extending to the urinary bladder enclose the umbilical Arteries and the urachus. In the lateral umbilical folds, the epigastric arteries run toward the rectus abdominis muscles.

In addition to mesenteries and folds, the peritoneum forms ligaments that connect the body wall and the diaphragm to the internal organs. All ligaments are formed by double layers of the peritoneum. Thus, the falciform ligament runs to the liver from the anterior abdominal wall, and the coronary ligament runs from its posterior wall. The liver is anchored in the abdominal cavity by these ligaments. Ligaments also connect internal organs to each other: the liver to the lesser curvature of the stomach, the duodenum, and the right Kidney; the diaphragm to The Liver and the lesser curvature of the stomach, etc. Blood and Lymphatic vessels, nerves, ducts (Bile), and obliterated vessels that functioned in the fetus run within these ligaments.

The layers of the visceral peritoneum from the anterior and posterior surfaces of the stomach descend as a broad sheet and form the greater omentum. Reaching the pelvic inlet, both layers turn back, fuse with the descending layers, and ascend to the transverse colon. All four layers fuse with the anterior surface of the colon. Then, the ascending layers separate from the anterior ones and merge with the transverse mesocolon. Thus, the greater omentum is located behind the anterior abdominal wall and covers the intestines. The proximal part of the omentum is called the gastrocolic ligament; it anchors the transverse colon to the greater curvature of the stomach. In children, the peritoneal layers of the greater omentum are well-defined.

The lesser omentum is formed by the ligaments extending from the liver to the lesser curvature of the stomach and the duodenum.

The transverse mesocolon divides the abdominal cavity into two compartments. The upper compartment contains the stomach, liver, spleen, pancreas, and the upper PARTS OF THE duodenum. Between these organs are confined spaces or bursae (hepatic, omental). The lower compartment is occupied by the remaining parts of the Small Intestine and the Large Intestine. Here, the parietal peritoneum of the posterior wall becomes continuous with the root of the mesentery of the small intestine. It begins on the left at the level of the second lumbar vertebra, runs downward and to the right, and ends at the level of the sacroiliac joint (Atl. Fig. 164). The length of the mesenteric root is 17— 22 cm. It divides the abdominal cavity into the right and left mesenteric sinuses. The parietal peritoneum of the right sinus continues on the right into the visceral peritoneum of the ascending colon, on the left and below into the mesentery of the small intestine, and above into the transverse mesocolon. The parietal peritoneum of the left sinus continues on the left into the descending colon, on the right into the mesentery of the small intestine, above into the transverse mesocolon, and below into the pelvic peritoneum and the sigmoid mesocolon.