Comprehensive Study Notes on the Small Intestine, Mesentery Proper, and Superior Mesenteric Artery

The small intestine serves as the primary site for the digestion and absorption of nutrients.
It has a total length of approximately $6.25\,m$ .
The organ is anatomically divided into three distinct segments:
- 1. Duodenum
- 2. Jejunum
- 3. Ileum

Dimensions and Morphology:
- The duodenum measures about $25\,cm$ ( $0.25\,m$ ) in length.
- It is characterized by a C-shaped structure that wraps around the head of the pancreas.
Anatomical Divisions:
- Superior (D1): This is the first part and is the only segment categorized as intraperitoneal.
- Descending (D2): Contains the major and minor duodenal papillae; it is retroperitoneal.
- Horizontal (D3): Retroperitoneal.
- Ascending (D4): Retroperitoneal.
Glandular Features and Secretions:
- Brunner’s glands: These glands are found specifically in the first part (D1) of the duodenum. They produce a mucus that is rich in alkaline substances and bicarbonate. This mucus functions to protect the duodenal mucosal lining from the corrosive effects of acidic chyme entering from the stomach.
- The second part (D2) serves as the reception point for bile and pancreatic juices.

Jejunum:
- Its length is approximately $2.5\,m$ .
- It contains prominent plicae circulares, which are permanent circular folds located on the inner surface.
- It features numerous villi to allow for maximal nutrient absorption.
Ileum:
- Its length is approximately $3.5\,m$ .
- Anatomically, it possesses thinner walls compared to the jejunum.
- A defining feature is the presence of Peyer’s patches, which are clusters of lymphoid tissue.
- The distal end connects to the cecum through the ileocecal valve.

First Part (Superior - D1):
- Anterior Relations: Quadrate lobe of the liver and the gallbladder.
- Posterior Relations: Gastroduodenal artery, common bile duct, portal vein, and the inferior vena cava.
Second Part (Descending - D2):
- Position: Wraps around the head of the pancreas.
- Anterior Relations: Fundus of the gallbladder, right lobe of the liver, transverse colon, and coils of the jejunum.
- Posterior Relations: Right kidney, right renal vessels, inferior vena cava, and right psoas major muscle.
Third Part (Horizontal - D3):
- Anterior Relations: Superior mesenteric artery (SMA), superior mesenteric vein (SMV), coils of the jejunum, and the right ureter.
- Posterior Relations: Right psoas major muscle, inferior vena cava, and the abdominal aorta.
Fourth Part (Ascending - D4):
- Course: It rises to join the jejunum at the duodenojejunal flexure.
- Anterior Relations: Upper part of the mesentery root and coils of the jejunum.
- Posterior Relations: Left psoas major muscle, left sympathetic trunk, left renal vessels, and the inferior mesenteric vein.

Location: They exist as coils within the central and lower abdominal cavity.
General Positioning: The jejunum is situated primarily in the upper left quadrant, while the ileum is located in the lower right quadrant.
Anterior Relation: They are covered by the greater omentum and the anterior abdominal wall.
Posterior Relation: They rest upon the posterior abdominal wall, psoas muscles, ureters, and major vessels accessed via the mesentery root.
Terminal Relation: The distal ileum terminates at the ileocecal junction where it enters the cecum.

Definition and Structure:
- A fan-shaped, double-layered peritoneal fold.
- Facilitates the attachment of the jejunum and ileum to the posterior abdominal wall.
- Also referred to as the mesentery of the small intestine.
- Root: The root of the mesentery extends from the duodenojejunal flexure at the level of $L2$ to the ileocecal junction at the right sacroiliac joint.
Contents and Function:
- Contents: Includes blood vessels (specifically branches of the SMA), nerves, lymphatic vessels, and lymph nodes.
- Functions: Provides mobility to the intestines, prevents intestinal torsion, and acts as a conduit for various neurovascular structures.

Arterial Supply:
- The Superior Mesenteric Artery (SMA) provides nourishment to the midgut, which spans from the distal duodenum to the proximal two-thirds of the transverse colon.
- The proximal duodenum is supplied by the pancreaticoduodenal artery, which originates from the gastroduodenal artery.
Venous Drainage: Handled by the superior mesenteric vein (SMV).
Lymphatic Drainage: Lymph drains into the mesenteric lymph nodes.
Nerve Supply: Derived from both the sympathetic and parasympathetic nervous systems.
Clinical Relevance:
- Small Bowel Obstruction: Potential causes include adhesions, hernias, tumors, and intussusception.
- Volvulus: The twisting of the intestines on themselves or around the mesentery, which can lead to ischemia.
- Lymphadenopathy: Enlargement of lymph nodes due to infection or inflammation, common in conditions like Crohn’s disease.
- Other Conditions: Duodenal ulcers, peritonitis, and ileocaecal valve incompetence.

Origin and Course:
- Arises from the abdominal aorta at the level of $L1$ , approximately $1\,cm$ below the celiac trunk.
- It passes posterior to the pancreas and anterior to the third part of the duodenum (D3) before entering the mesentery proper.
Major Branches and Territory:
- Inferior Pancreaticoduodenal Artery: Supplies the pancreas and the third and fourth parts of the duodenum (D3/D4).
- Jejunal and Ileal Arteries: Consist of $15$ to $18$ arcades within the mesentery; the small terminal branches, the vasa recta, directly supply the intestinal walls.
- Ileocolic Artery: Supplies the terminal ileum, cecum, and the appendix.
- Right Colic Artery: Supplies the ascending colon.
- Middle Colic Artery: Supplies the transverse colon and forms an anastomosis with the left colic artery through the marginal artery of Drummond.

SMA Syndrome: Compression of the third part of the duodenum (D3) between the SMA and the aorta, often resulting in postprandial pain and vomiting.
Acute Mesenteric Ischemia: Life-threatening intestinal necrosis typically caused by a sudden SMA occlusion, such as an embolism.
Chronic Mesenteric Ischemia: Resulting from atherosclerosis, known as "intestinal angina," characterized by weight loss and postprandial pain.
Surgical Importance: The anastomotic networks, such as the marginal artery, are critical during bowel resection procedures.
Imaging and Trauma: CT angiography is the standard for diagnosing SMA malformations or occlusions. Erosion of the SMA can lead to pseudoaneurysm and severe hemorrhage.