Small Intestine Anatomy: Jejunum and Ileum

• The small intestine consists of the duodenum, jejunum, and ileum.
• It stretches from the duodenojejunal flexure to the ileocecal valve.
• The small intestine measures approximately 6-7 meters in length.
• The end point for the small intestine is the junction between the cecum and ascending colon.
• The jejunum comprises the proximal 2/5 of the small intestine and is located in the left upper quadrant.
• The ileum makes up the remaining 3/5 of the small intestine and resides in the right lower quadrant.
• A clear border does not exist between the jejunum and ileum, but there are distinctive characteristics.

Jejunum vs Ileum Characteristics:

• Color: The jejunum has a deeper red color, while the ileum presents a paler pink hue.
• Caliber: The jejunum spans 2-4 cm, while the ileum measures 2-3 cm.
• Wall: The jejunum has a thick and heavy wall, whereas the ileum features a thin and light wall.
• Vascularity: The jejunum has greater vascularity compared to the ileum.
• Vasa recta: The jejunum has long vasa recta, while the ileum has short ones.
• Arcades: The jejunum exhibits a few large loops, while the ileum contains many short loops.
• Fat in mesentery: The jejunum has less fat in the mesentery compared to the ileum.
• Circular folds (plicae circulares): The jejunum has large, tall, and closely packed circular folds, whereas the ileum's are low and sparse, and absent in the distal part.
• Lymphoid nodules (Peyer's patches): The jejunum has few lymphoid nodules, while the ileum has many.

Small Intestine Wall Structure:

• Mucous Layer: Forms circular folds and is covered by intestinal villi
• Submucous Layer: Loose connective tissue carrying vessels, lymphatics, and nerves
• Muscular layer: Contains external longitudinal and internal circular muscles
• Serous Layer: This is the peritoneum

Circular Folds

• Large crescentic folds of mucosa slow down the passage of contents.
• Large crescentic folds of mucosa increase the absorptive surface.
• Large circular folds are closely spaced distal to the major duodenal papilla and in the proximal half of the jejunum.
• These circular folds diminish from the midway along the ileum and disappear in the distal ileum.

Intestinal Villi

• Highly vascular processes that create a visible velvety texture.
• Large and numerous structures are found in the duodenum and jejunum.
• Smaller and fewer structures are found in the ileum.
• Intestinal villi serve to increase the surface area by about 8 times.

Lymphatic Follicles

• The scattered solitary lymphatic follicles are throughout the intestinal mucosa.
• Most of these are numerous in the distal ileum.
• Aggregated lymphoid follicles (Peyer's patches) contain 10-260 follicles.
• Aggregated lymphoid follicles (Peyer's patches) varies in length from 2 to 10 cm.
• The largest and most numerous structures are in the ileum.
• Present as occasional small circular structures in the distal jejunum and in the duodenum.

Mesentery

• It is the peritoneum that links the jejunum and ileum to the posterior abdominal wall.
• The mesentery has a length of approximately 15 cm.
• It extends from the duodenojejunal junction to the iliocolic junctional.
• Inside the mesentery is the jejunum and ileum.
• Also found inside the mesentery are the jejunal and ileal branches of the superior mesenteric vessels
• The mesentery's other components are autonomic nerves, lymph nodes and fatty tissue.
• Ascending aorta, Inferior vena cava, right ureter, right psoas major and right testicular or ovarian vessels cross the root of the mesentery.

Blood supply

• The jejunal and ileal arteries are branches of the superior mesenteric artery.
• They unite to form loops or arches called arterial arcades.
• Arterial arcades give rise to straight arteries called vasa recta.
• The jejunal and ileal veins drain into the superior mesenteric vein.

Lymph Vessels

• Lacteals are special lymph vessels of the small intestine.
• Lacteals drain into lymph plexus in the wall of the small intestine.
• From here they flow to the Chyle cistern, superior mesenteric lymph nodes, and mesenteric lymph nodes.

Nerve Supply

• Sympathetic innervation: T5-9, T10-11 (greater and lesser splanchnic nerves)
• Parasympathetic innervation: Vagus nerve
• Sensory (visceral afferent) fibers: The intestine is insensitive to pain stimuli (cut, burn) but is sensitive to distention (colic), leading to spasmodic abdominal pain.

Ileal Diverticulum

• Ileal Diverticulum also referred to as Diverticulum of Meckel.
• It projects from the border of the distal ileum.
• It is 5cm in length on average.
• Ileal Diverticulum represents the persistent proximal part of the vitelline (yolk) duct.