General anatomy (macro and micro) of intestinal tube

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; whereas large intestine extracts water from the chyme and excrete the digested product through the rectum.

Small intestine

Macro-anatomy of small intestine

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.

• 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

Jejunum and Illeum

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. However, they can be differentiated by the different features of their structures. The ileum then terminates at ideal orifice, which continues as the caecum of large intestine.

Micro-anatomy of small intestine

The small intestine is lined by circular folds, which is made of mucosa and submucosa. From the mucosa, there are small outgrowths known as villi, which are made of enterocytes, absorptive cells with simple columnar epithelium, and in between there are globes cells, helping to secrete mucus to lubricate the lining of small intestine. Specifically at the duodenum submucosa, Bruner glands can be found, which produces mucous that is rich in alkaline to protect the small intestine from gastric acid from stomach. Moreover, Ileum contains Peyers patches, which are lymphoid tissue underlying epithelial M cells.

Large intestine

The large intestine starts at the ileocecal junction and is made of caecum, colon and rectum. Most of caecum, appendix, transverse colon, sigmoid colon are inteaperitoneal; ascending colon, descending colon and proximal rectum are secondary retroperitoneal; distal parts of rectum and the anal canal is subperitoneal.

Macro-anatomy of large intestine

Caecum

• Begins at the illeocaecal junction, the orifice is marked by ideal papilla made of superior and inferior ileocecal lips that fuses at the orifice - prevents reflux back into the small intestine
• contains appendix vermiformis (6-10cm, mainly retrocecal) at blind end, 2-3cm below ileum junction. The base of this structure can be localised at the McBurney’s point and the tip at Lanz’s point.
• There are 7 possible positions for the appendix vermiformis, namely pre-ileal, post-ileal, sub-ideal, pelvic, sub-cecal, paracecal, retrocecal

Colon

The colon connects caecum and the anal cavity and mainly extracts water from faeces. It can be divided into 4 parts:

• Ascending colon - secondarily retroperitoneal and ascends on the right flank to reach right liver, where it bends into right colic flexure, also known as hepatic flexure
• Transverse colon - suspended by transverse mesocolon (mesentery that divides into supra and inframesocolic compartments), runs transversely and bends at left colic flexure, also known as splenic flexure)
• Descending colon - secondarily retroperitoneal and descends to join sigmoid colon
• sigmoid colon - suspended by sigmoid mesocolon (mesentery) and forms a loop of bowel that joins the rectum in pelvis region

Rectum

The rectum is the most distal part of large intestine and lies within the pelvis region and helps to store faeces.

• 1st part of rectum is infraperitoneal
• 2nd part of rectum is subperitoneum
• 3rd part of rectum is intraperitonum
• There are two flexures: sacral flexure and anorectal flexure

Micro-anatomy of large intestine

In order to differentiate large intestine and small intestine, there are specific structures within the large intestine that is not present within the small intestine.

• Taenia coli - 3 longitudinal bands of smooth muscle on colon and caecum
• Haustra - sacculations of colon
• Omental appendices - small fat accumulations on colon covered by visceral peritoneum

However, the rectum does not have taenie coli, haustra and Omental appendices

The Histological view of large intestine

• Lined by simple columnar epithelium
• contains microvilli for absorption (absorptive cells are known as colonocytes)
• Crypts of Lieberkuhn with globes cells and enteroendocrine glands
• Lamina propria is rich in lymphoid cells for the large bacterial population within the large intestine
• intraperitoneal parts of the colon are covered by serosa
• circular folds

Bibiography

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

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

Junqueira’s Basic Histology. Anthony L. Men’s her. 15th Edition. ISBN: 978-1-26-002618-4