Introduction
The large intestine also known as the large bowel, is the distalmost part of the gastrointestinal tract and of the digestive system in vertebrates.
Parts of Large Intestine
Large Intestine consists of the following parts-
- Caecum
- Vermiform appendix
- The colon – ascending, transverse, descending and sigmoid parts
- The rectum
- The anal canal.
Functions of the Large Intestine
Large intestine receives undigested food from the small intestine, absorbs water and electrolytes from chyme and passes faeces out of the gastrointestinal tract.
Structure of Large Intestine
The Large intestine averages 1.5m in length and 6.5cm in diameter. It is larger in diameter compared to the small intestine, hence the name.
The cecum
The cecum is the first part of the large intestine and is continuous with the ascending colon. It is a blind intestinal pouch approximately 7.5cm in both length and breadth. It is located in the right lower quadrant. The cecum lies within 2.5cm of the inguinal ligament and is almost entirely enveloped by peritoneum. The terminal part of the ileum enters the cecum obliquely and partly invaginates it.
The appendix
The vermiform appendix, also called appendix vermiformis or simply appendix is a blind intestinal diverticulum that contains masses of lymphoid tissue. It arises inferior to the ileocecal junction posteromedial to the cecum.
The appendix has a mesentery called meso-appendix. The meso-appendix attaches the proximal part of the appendix to the caecum.
The colon
The colon, for ease of description is divisible into four parts;
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
Ascending colon
This is the first part of the colon. It is about 15cm long and extends from the ileocecal junction to the right colic (hepatic) flexure. This flexure lies in relation to the right kidney and the inferior surface of the liver. The ascending colon is sacculated due to the presence of the three taeniae coli. The associated colon also bears small pouches of peritoneum which are distended with fat called appendices epiploicae.
Figure: The colon and associated structures
Transverse Colon
The transverse colon is about 45cm long and extends from the right colic/ hepatic flexure to the left colic/ splenic flexure. It lies anterior to the coils of jejunum and ileum.
The convexity of the greater curvature of the stomach lies in its concavity and both are connected by the gastro-colic omentum. The taenia coli continue from the ascending colon into the transverse colon. The appendices epipoicae are also present in the transverse colon, but here they are larger and more numerous compared to those in the ascending colon.
Descending colon
The descending colon is about 30cm long and extends from the splenic flexure to the pelvic brims about 5cm above the inguinal ligament. The three taenia coli in continuity with those of transverse colon lie one anteriorly and two posteriorly (medial and lateral). Appendices epiploicae are also numerous here.
The descending colon is a common site for diverticulosis.
Sigmoid colon
The sigmoid colon begins at the pelvic inlet as a continuation of the descending colon. It is characterised by its ‘S’ shape loop of about 40cm long. It extends from the iliac fossa of the S3 vertebra and joins the rectum.
Rectum
The rectum is the terminal part of the large intestinal. It is continuous with the sigmoid colon at the level of S3 vertebra. The rectum continues inferiorly as anal canal. The rectum is 12cm long. It ends at the level of the apex of the prostate in males or at the lower quarter of the vagina in females from where it continues as the anal canal.
Anal canal
The anal canal is 4cm long and is directed downwards and backwards from the rectum to end at the anal orifice. The mid-anal canal represents the junction between the parts that originate from the endoderm and the part that originate from the ectoderm.
The walls of the anal canal is formed by a complex arrangement of muscles that form the anal sphincters thus-
- The internal anal sphincter is formed by circularly arranged smooth muscles.
- The external anal sphincter is formed by skeletal muscles and closely attached to the skin of the anal orifice.
Blood supply to Large Intestine
The large intestine is supplied by branches of the small mesenteric and large mesenteric arteries.
The appendix and cecum are supplied by the appendicular artery (a branch of ileocolic artery).
Venous drainage
Venous blood from the large intestine is returned through the small and large mesenteric veins that in turn drain into the hepatic portal vein that enters the liver. Ileocolic vein (a tributary of Superior Mesenteric vein) drains both the cecum and appendix.
Lymphatic drainage
Lymphatic vessels from the cecum and appendix empty into lymph nodes in the meso-appendix and ileocolic lymph nodes lying along the ileocolic artery. Efferent lymphatics from these pass to superior mesenteric lymph nodes.
Nerves supply to large intestine
- Sympathetic innervation of the large intestine arises from superior mesenteric and inferior mesenteric plexuses as well as from the celiac plexus.
- Parasympathetic innervation arises from the paired pelvic splanchic and vagus nerve.
