Detailed Anatomy of the Small and Large Intestine Segments
The Small Intestine: Duodenum, Jejunum, and Ileum
The small intestine consists of a fixed portion, the Duodenum, and the mobile portions: the Jejunum and the Ileum. The duodenum is largely retroperitoneal and is shaped like a ‘C’ or a left cavity handle. This handle is divided into four distinct parts:
- First Part (Superior/Subhepatic Portion): This is the only portion that is intraperitoneal. It is related to the liver and wraps around the lesser omentum, extending slightly posteriorly.
- Second Part (Descending Portion): This section receives two ducts: the primary pancreatic duct and the common bile duct (ductus choledochus) from the liver.
- Third Part (Horizontal Portion): This portion crosses the midline from right to left, typically at the level of the third or fourth lumbar vertebra.
- Fourth Part (Ascending Portion): This section rises slightly upward, communicating with the jejunum and ileum at the duodenojejunal angle.
Jejunum and Ileum: The Mobile Small Intestine
Six or seven feet of mobile small intestine (jejunum and ileum) are folded within the abdominal cavity. The small intestine ends at the level of the ileocecal angle in the right iliac fossa.
These six or seven feet are attached to the parietal peritoneum of the posterior wall by the mesentery. The root of the mesentery is detached from the posterior abdominal wall by a line extending from the duodenojejunal angle to the ileocecal angle, measuring approximately eight inches long. The peritoneum of the mesentery spreads out like a skirt, reaching the six or seven meters of the jejunum and ileum.
The last portion of the small intestine is the terminal ileum, which empties into the large intestine at the junction of the cecum and the ascending colon.
Anatomy and Segments of the Large Intestine
Cecum and Vermiform Appendix Location
The Cecum is like a cul-de-sac, where the vermiform appendix is usually located. The cecum and vermiform appendix are situated in the right iliac fossa. The cecum is intraperitoneal, receiving peritoneum from the jejunum and ileum.
Ascending Colon and the Hepatic Flexure
The Ascending Colon is retroperitoneal. It reaches the right upper quadrant and forms the right colic angle, which is related to the liver and is therefore also called the hepatic flexure. From here, the colon takes an approximately horizontal direction.
Transverse Colon and the Splenic Flexure
The Transverse Colon reaches the hypochondrium and changes direction, following down the left flank. The point where it changes direction is near the spleen, so the left corner is also called the splenic flexure. The transverse colon is the mobile part of the large intestine.
Descending Colon
The Descending Colon begins to fall from the splenic flexure. It is retroperitoneal. At the level of the anterior superior iliac spine, the descending colon transitions into the sigmoid colon.
Sigmoid Colon (Iliopelvic Colon)
The Sigmoid Colon, or Iliopelvic Colon, is described as an elongated S-shaped loop that encounters the external iliac fossa. It is intraperitoneal, suspended by the sigmoid mesocolon, which is shaped like an inverted V (^). The sigmoid colon is implemented in front of the sacrum and leads to the rectum.
The Rectum (Straight Intestine)
The upper portion of the Rectum is subperitoneal. The peritoneum folds forward to cover other organs, meaning the upper half is subperitoneal and the lower half is retroperitoneal.
Anal Sphincters: Voluntary and Involuntary
There are two anal sphincters: one voluntary and one involuntary. The involuntary sphincter muscle is located higher than the voluntary sphincter, which is situated externally.