The Large Intestine

Chapter objectives

After reading this chapter you should be able to:

• Describe the normal anatomy and physiology of the large intestine.

• Understand the physiology of absorption, secretion and motility in the large intestine.

• Identify the range of disorders of the large intestine.

• Relate the pathophysiology, diagnosis and treatment of a number of clinical disorders of the large intestine to nursing care.

ANATOMY AND PHYSIOLOGY

The last 150 cm of the digestive tract are the colon. It is a tube of approximately 4-6 cm in diameter and it extends from the ileo-caecal valve to the anus. The arrangement of the large intestine and its associated structures is shown in Figure 6.1.

The large intestine can be divided up into various regions: the caecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectum. The rectum is the portion beyond the sigmoid colon.

Consistent with other regions of the digestive tract, the wall of the large bowel is composed of four distinct layers: the mucosa, the submucosa, the muscularis and the serosa.

In the large intestine the outer longitudinal smooth muscle layer is arranged in three prominent bands, known as taeniea coli. These are shorter than in the small bowel, causing the mucosal lining to pucker and form small sacs called haustra. The size and shape of haustra vary with the state of contraction of the circular and longitudinal muscular layers. These muscular bands are shorter than the longitudinal muscle bands of the colon. Together these two features provide a sacculated appearance to the bowel. Auerbach's plexus is located between the circular and longitudinal muscle layers in the large bowel.

The large bowel has no villi, only the presence of projections. The luminal surface appears much smoother than that of the small bowel. One consequence

Ascending colon

Position of iieo-caecal valve

Hepatic flexure

Appendix

Caecum

Ileum

Ascending colon

Position of iieo-caecal valve

Hepatic flexure

Appendix

Caecum

Ileum

Taeniae coir

Descending colon

Sigmoid flexure

Splenlcflexure

Figure 6.1 Arrangement of the large intestine and its associated structures. Reproduced with permission from Hinchliff et al. (1996).

Taeniae coir

Descending colon

Sigmoid flexure

Splenlcflexure

Figure 6.1 Arrangement of the large intestine and its associated structures. Reproduced with permission from Hinchliff et al. (1996).

of this structural difference is that the surface area of the large intestine is only one-thirtieth of that in the small intestine.

At the distal end of the ileum, a flap known as the ileo-caecal valve acts as a physiological sphincter both to control the passage of intestinal contents from the small intestine to the large intestine and to prevent any reflux of bacteria from the colon back into the small bowel. The pressure of the ileo-caecal valve is about 20 mm Hg above normal colonic pressure in an average adult.

The caecum forms a blind-ended pouch that is wider in diameter than it is long and which sits below the junction of the small and large intestines. Attached to the end of the caecum is the vermiform appendix. The appendix is a thin, tubular finger-like projection that can range from 5 to 20 cm in length and is about 8 mm in diameter. It has no known digestive role in humans.

Immediately above the caecum is the ascending (right) colon, which passes up the right-hand side of the abdomen to the lower part of the liver, where it bends left at the hepatic flexure (right colic flexure). The transverse colon travels across the abdomen from right to left, from the hepatic flexure to the splenic flexure (left colic flexure). The descending colon runs down the left-hand side of the abdomen from the spleen to the iliac crest, where it forms the sigmoid colon.

The last portion of the large intestine is the rectum. It is about 15 cm long in adults. The distal portion of the rectum forms the anal canal, beginning where the rectum narrows markedly. The surface of the upper anal canal exhibits a number of folds, known as anal (or rectal) folds. Depressions between anal columns are known as anal sinuses. These sinuses end abruptly at the lower end of the columns, in a region known as the dentate line, where there are small crescent-shaped folds of mucosa around the wall. These folds form the anal valve. The anal canal is surrounded by sphincter muscles that control the release of faecal material. The internal anal sphincter is a thickening of the circular layer of the muscularis externa. The external sphincter is composed of skeletal muscle. The arrangement of the anal sphincters is shown in Figure 6.2.

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