Small inestine - structure, divisions, vascular supply, innervation, lymphatic drainage

After the food processing in stomach, it enters the intestine. Intestine is divided into 2 main parts known as large intestine and small intestine. Small intestine is essential in food digestion and nutrients reabsorption.

The small intestine is 4-6m long and is divided into 3 sections: duodenum, jejenum, illeum. Anteriorly, it is covered by greater omentum. Due to developmental reasons, duodenum contains intraperitoneal and (secondarily) retroperitoneal (secondarily) parts, whereas jejenum and ileum are only intraperitoneal.

Duodenum

The duodenum is made of 4 parts: superior, descending, horizontal, ascending with its length being 25-30cm.

4 parts:

• Superior duodenum (L1) - attachment site for hepatoduodenal ligament of lesser omentum (intraperitoneal - 2.5cm)
• Descending duodenum (L1-L3) - bile and pancreatic ducts empty here
• Horizontal duodenum (L3) - crosses inferior vena cava and aorta
• Ascending duodenum (L3-L2) - fastened by suspensory ligament (or ligament of Treitz) at duodenojejunal flexure, which attaches duodenum to right crus of the diaphragm

Superior duodenum

• Ampulla - connects pylorus of stomach to duodenum; intraperitoneal part and has no folds
• connected to liver via hepatoduodenal ligament
• Superior duodenal flexure - connects superior to descending duodenum

Descending duodenum

• Major duodenal papilla - opening of bile duct and major pancreatic duct
• Minor duodenal papilla - opening of accessory pancreatic duct
• lies around the head of pancreas

Horizontal duodenum

• Crosses inferior vena cava and aorta
• inferior to pancreas

ascending duodenum

• Duodenum ascends and is held by ligament of Treiz
• joins Jejunum at duodenojejunal flexure

Jejunum and Illeum

Ileum and Jejunum are suspended by mesentery, which is attached to posterior abdominall wall. Jejunum is the second part of small intestine and continues from the duodenojejunal flexure. There is no clear boarder between Jejunum and illeum, which is the last part of small intestine. The ileum then terminates at ideal orifice, which continues as the caecum of large intestine. However, they can be differentiated by the different features of their structures.

• Jejunum has a thicker wall compared to ileum, but is shorter
• Ileum has less villi and slowly disappears throughout
• Ileum contains Payer’s patches

Vascular supply

Arterial blood supply

• Duodenum is supplied by superior and inferior pancreaticoduodenal artery, in which proximal to major duodenal papilla, the duodenum is supplied by anterior and posterior pancreaticoduodenal arteries (branch of gastroduodenal artery); whereas distal to major duodenal papilla, the duodenum is supplied by inferior pancreaticoduodenal artery (branch of superior mesenteric artery)
• Jejunum supplied by Jejunal artery (branch of superior mesenteric artery
• illeum supplied by illeal artery (branch of superior mesenteric artery)

Venous drainage

• Drain into superior mesenteric vein then into the main tributaries of portal vein

Innervation

• Innervated by coeliac plexus and superior and inferior mesenteric plexus containing both sympathetic and parasympathetic fibers

Sympathetic innervation

• Sympathetic Nervous system inhibits the peristaltic movement and secretion of glands of the intestinal mucosa; activates muscle of ileocecal valve to prevent reflux
• T5-12
• Sympathetic fibers switched to postganglionic fibers in coeliac ganglia, superior and inferior mesenteric ganglia

Parasympathetic innervation

• Parasympathetic nervous system promotes the peristaltic movement and secretion of glands of the intestinal mucosa
• Vagus nerve
• Parasympathetic nerve fibers are preganglionic and only encounter postganglionic neurons in enteral nervous system within the intestinal wall

Lymphatic drainage

• Duodenum - pyloric, hepatic and superior mesenteric lymph node, which drains into coeliac lymph nodes
• jejunum and ileum - superior mesenteric lymph nodes then into coeliac lymph nodes

Bibiography

Netter’s Clinical Anatomy. Frank H. Netter. 4th Edition. ISBN 978-0-323-53188-7

Sobotta, Anatomy Textbook. Jens Waschke et al. English Edition with Latin Nomenclature. ISBN 978-0-7020-6760-0

Memorix Anatomy. Radovan Hudák et al. 1st edition. ISBN 978-80-906331-0-0