(15) Abdomen III: Small and Large Intestines

SMALL INTESTINE—DUODENUM

Anatomical Structure and Parts

The duodenum is approximately $10\,\text{inches}$ long, extending from the pylorus of the stomach to the duodenojejunal junction (also known as the duodenojejunal flexure). It is organized into $4$ distinct parts:

• The First Part ($1\text{st}$ Part): Also referred to as the upper duodenal cap. This section is unique because it is free and suspended by the hepatoduodenal part of the lesser omentum.

• The Second Part ($2\text{nd}$ Part): Known as the descending part. It is fixed (retroperitoneal) and serves as the receiving site for the bile duct and the pancreatic duct. These two ducts merge at the hepatopancreatic ampulla, which subsequently opens into the duodenal lumen at the major duodenal papilla.

• The Third Part ($3\text{rd}$ Part): Known as the inferior or horizontal part. It is fixed (retroperitoneal) and crosses anteriorly across the aorta and the inferior vena cava (IVC).

• The Fourth Part ($4\text{th}$ Part): Known as the ascending part. It connects with the jejunum at the duodenojejunal junction.

The Major Duodenal Papilla

The major (greater) duodenal papilla is the specific opening for both the pancreatic and bile ducts. This opening is regulated by the sphincter of the hepatopancreatic ampulla, which controls the flow of digestive fluids.

Physiological Functions and Hormonal Regulation

The duodenum functions as a regulatory center, utilizing hormones released from its epithelium to manage the digestive environment:

• Secretin: This hormone is released when the duodenal pH levels drop too low (becoming too acidic). Secretin stimulates the secretion of water and bicarbonate into the duodenum. This neutralization is critical because enzymes such as pancreatic amylase and lipase require a higher pH to function at an optimal level.

• Cholecystokinin (CCK): This hormone is released in the presence of fatty acids. It performs two simultaneous actions to aid fat digestion and absorption: it stimulates the contraction of the gallbladder and causes the relaxation of the sphincter of the hepatopancreatic ampulla, allowing bile to enter the duodenum.

Neurovascular Supply and Clinical Conditions

Blood Supply and Drainage

• Arterial Supply: Primarily from the pancreaticoduodenal arteries, which originate from both the celiac trunk and the superior mesenteric artery.

• Venous Drainage: This generally follows the arterial pathways and empties into the portal venous system.

Innervation

• Nerve Supply: Derived from both sympathetic and parasympathetic nerves originating from the celiac and superior mesenteric plexuses.

Clinical Condition: Duodenal Ulcers

• Mechanism: These occur when acidic chyme from the stomach is squirted against the anterolateral wall of the first part of the duodenum.

• Frequency: Duodenal ulcers are documented as being more common than gastric ulcers.

SMALL INTESTINE—JEJUNUM AND ILEUM

General Anatomy and Attachment

The remainder of the small intestine is approximately $20\,\text{feet}$ long. It is divided into two sections:

• Jejunum: Comprises the proximal $\frac{2}{5}$ of the length.

• Ileum: Comprises the distal $\frac{3}{5}$ of the length.

This region is the primary site for the vast majority of digestion and nearly all absorption of ingested nutrients. It is attached to the posterior abdominal wall by the mesentery, a structure that provides a pathway for branches of the superior mesenteric artery, vein, and nerves to reach the intestine.

Neurovascular and Lymphatic Systems

• Blood Supply: Provided by the jejunal and ileal branches of the superior mesenteric artery. Drainage is handled by the superior mesenteric vein.

• Lymphatics: Vessels within the intestinal wall empty into plexuses, then into larger vessels, and eventually into the mesenteric group of lymph nodes. The final destination is the thoracic duct. This is specifically the route used for the transport of digested fats. Other nutrients are absorbed directly into the portal vein.

• Innervation: Nerves are derived from the vagus nerve (parasympathetic) and the greater splanchnic nerves (sympathetic) via the celiac ganglion and the nerve plexus surrounding the superior mesenteric artery.

Clinical Condition: Meckel's Diverticulum

A Meckel's diverticulum is an outpouching of the intestinal wall found in approximately $2\%\text{ of the population}$. It represents a persistence of the embryonic omphalomesenteric duct and is recognized as a common cause of intestinal bleeding.

LARGE INTESTINE

Primary Functions and Physical Characteristics

The primary roles of the large intestine are the absorption of water and electrolytes and the storage of undigested material until it is excreted as feces. It forms an anatomical arch surrounding the coils of the small intestine. It is distinguished from the small intestine by three unique features:

1. Teniae Coli: Three thickened bands of longitudinal muscle. Their contractions shorten the wall, facilitating peristalsis.

2. Haustra: Visible sacculations or pouches of the wall formed by the contraction of the teniae coli.

3. Epiploic Appendages: Small pouches of peritoneum filled with fat. These serve a protective and defensive role similar to the greater omentum.

Regional Anatomy

• Cecum: The portion located below the ileocecal junction. It is highly mobile despite lacking a mesentery.     * Vermiform Appendix: A narrow tube joining the cecum approximately $1\,\text{inch}$ below the ileocecal opening. It has its own short mesentery ($mesoappendix$).     * McBurney's Point: The consistent location of the base of the appendix, defined as $\frac{1}{3}$ of the distance from the right anterior superior iliac spine to the umbilicus.     * Ileocecal Valve: Located at the junction of the ileum and the cecum; it is only partially functional.

• Ascending Colon: Extends upward to the right colic flexure (hepatic flexure). It is retroperitoneal, covered by peritoneum on the front and sides.

• Transverse Colon: Extends from the right colic flexure to the left colic flexure (splenic flexure). It is suspended by the transverse mesocolon. The phrenicocolic ligament attaches the left colic flexure to the diaphragm.

• Descending Colon: Passes inferiorly from the left colic flexure to the pelvic brim. It is retroperitoneal.

• Sigmoid Colon: Extends from the iliac fossa to the level of the $3\text{rd}$ sacral vertebra. It is suspended by the sigmoid mesocolon and possesses significant freedom of movement.

Blood Supply of the Large Intestine

1. Ileocolic and Right Colic Arteries: Branches of the superior mesenteric artery supplying the cecum and ascending colon.

2. Middle Colic Artery: A branch of the superior mesenteric artery supplying the transverse colon.

3. Left Colic Artery: A branch of the inferior mesenteric artery supplying the descending colon.

4. Sigmoid Artery: A branch of the inferior mesenteric artery supplying the sigmoid colon.

5. Superior Rectal Artery: The terminal branch of the inferior mesenteric artery supplying the rectum.

6. Marginal Artery: An important anastomosis of the various colic arteries located along the margin of the large intestine.

Clinical Conditions of the Large Intestine

• Appendicitis: Inflammation typically caused by an obstruction of the appendix (often by fecal material). This results in pain and tenderness localized at McBurney's Point (defined as $\frac{1}{3}$ of the distance from the right anterior superior iliac spine to the umbilicus).

• Ulcerative Colitis: A chronic disease characterized by severe inflammation and ulceration of both the colon and the rectum.