Anatomy of the Small Intestine

Anatomy of the Small Intestine

• The small intestine is responsible for the majority of nutrient absorption and the completion of chemical digestion.
• Ingested nutrients spend at least 12 hours in the small intestine.
• It's a thin-walled tube, roughly 6 meters long.
• It extends from the stomach's pylorus to the large intestine's cecum.
• It occupies a significant portion of the abdominal cavity.

Structural Features of the Small Intestine Wall

• The mucosa (inner lining) has transverse folds called plicae circulares.
• These plicae have projections called villi.
• Both plicae and villi increase the surface area for absorption significantly.
• Each villus contains a lymphatic capillary (lacteal).

Digestion in the Small Intestine

• Most enzymatic digestion and absorption occur in the small intestine.
• Digestive enzymes and buffers are released by the pancreas, liver, and gallbladder.

Subdivisions of the Small Intestine

• The small intestine is divided into three sections: duodenum, jejunum, and ileum.

The Duodenum

• The first segment of the small intestine.
• Approximately 25 centimeters long.
• Starts at the pyloric sphincter.
• Contains major duodenal papilla.
• Receives digestive enzymes and bile.

Parts of the Duodenum

• Divided into four parts: superior, descending, horizontal, and ascending.
• Only the superior part's first 2 centimeters are intraperitoneal; the rest is retroperitoneal.
• Features a "C" shaped course around the head of the pancreas.
• The duodenum receives the main pancreatic duct and common bile duct. Sphincters control the entry of bile and pancreatic juice into duodenum.

The Jejunum

• The middle part of the small intestine.
• Approximately 2.5 meters.
• Constitutes about two-fifths of the small intestine.
• Plays a primary role in chemical digestion and nutrient absorption.

The Ileum

• The last segment of the small intestine.
• Approximately 3.6 meters (10.8 feet) in length.
• Forms roughly three-fifths of the small intestine.
• Terminates at the ileocecal valve (sphincter).

The Jejunum and Ileum

• Specialized for absorbing digested foods (nutrients), vitamins, and electrolytes.
• Coiled, attached to the posterior abdominal wall via the mesentery.
• The jejunum is slightly wider than the ileum and has deep circular folds called plicae circulares.
• The ileum has fewer circular folds but has lymphoid follicles (Peyer's patches).

Mesentery (Small Intestine)

• The mesentery attaches the coils of the jejunum and ileum to the posterior abdominal wall.
• The mesentery crosses the ascending and horizontal parts of the duodenum.
• Structures like the abdominal aorta, IVC, right ureter, right psoas major muscle, and right testicular (or ovarian) vessels are positioned near or pass through the root of the mesentery..

Arteries and Veins of the Small Intestine

• The superior mesenteric artery supplies blood to the jejunum and ileum.
• Arteries branch within the mesentery and form arcades (loops) that give rise to vasa rectae (straight arteries).
• Veins follow the arteries and drain into the superior mesenteric vein, which joins with the splenic vein to form the portal vein. The veins drain blood to the liver through the hepatic portal vein.
• The lymphatic vessels drain into the superior mesenteric lymph nodes and some ileum into ileocolic lymph nodes.

Jejunum and Ileum Innervation

• Parasympathetic innervation (vagus nerve) stimulates motility and secretion within myenteric and submucosal plexuses.
• Sympathetic innervation (splanchnic nerves) reduces motility and secretion.
• Innervation is insensitive to tactile stimuli except for distension, which leads to colic.

Junction of Bile Duct and Duodenum

• The (common) bile duct and pancreatic duct empty into the duodenum.
• The sphincter of Oddi (or hepatopancreatic sphincter) controls the flow of bile and pancreatic juices into the duodenum. Major duodenal papilla and minor duodenal papilla, are present at the interior surface of the duodenum.