The rectum continues from the sigmoid colon to the anal canal and has a thick muscular layer. It follows the curvature of the sacrum and is firmly attached to it by connective tissue. The rectum ends about 5 cm below the tip of the coccyx, at the beginning of the anal canal.

The last 2 to 3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal anal sphincter at the superior end of the anal canal. This sphincter is under involuntary control. There is an external anal sphincter at the inferior end of the anal canal. This sphincter is composed of skeletal muscle and is under voluntary control.

Anal canal is the final segment of the large intestine.

Internal anal sphincter involuntary smooth muscle sphincter in the anal canal.

External anal sphincter are voluntary skeletal muscle sphincters in the anal canal.

Anal Canal

Finally, food residue reaches the last part of the large intestine, the anal canal, which is located in the perineum, completely outside of the abdominopelvic cavity. This 3.8–5 cm (1.5–2 in) long structure opens to the exterior of the body at the anus. The anal canal includes two sphincters. The internal anal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control. Except when defecating, both usually remain closed.

Histology

There are several notable differences between the walls of the large and small intestines (Figure 23.22). For example, few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria.

Figure 23.22 Histology of the large Intestine (a) The histologies of the large intestine and small intestine (not shown) are adapted for the digestive functions of each organ. (b) This micrograph shows the colon’s simple columnar epithelium and goblet cells. LM x 464. (credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Anatomy

Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages (Figure 23.23). The teniae coli are three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end. Tonic contractions of the teniae coli bunch up the colon into a succession of pouches called haustra (singular = haustrum), which are responsible for the wrinkled appearance of the colon. Attached to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of these is unknown. Although the rectum and anal canal have neither teniae coli nor haustra, they do have well-developed layers of muscularis that create the strong contractions needed for defecation.

Figure 23.23 Teniae Coli, Haustra, and Epiploic Appendages

The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. The anal canal’s mucous membrane is organized into longitudinal folds, each called an anal column, which house a grid of arteries and veins. Two superficial venous plexuses are found in the anal canal: one within the anal columns and one at the anus.

Depressions between the anal columns, each called an anal sinus, secrete mucus that facilitates defecation. The pectinate line (or dentate line) is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin. The mucosa above this line is fairly insensitive, whereas the area below is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated by somatic sensory fibers.

Rectal valve is one of three transverse folds in the rectum where feces is separated from flatus

Rectum is part of the large intestine between the sigmoid colon and anal canal

Rectum

Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. Even though rectum is Latin for "straight," this structure follows the curved contour of the sacrum and has three lateral bends that create a trio of internal transverse folds called the rectal valves. These valves help separate the feces from gas to prevent the simultaneous passage of feces and gas.

Large Intestine

The large intestine includes the cecum, colon, and rectum.

Absorption, Feces Formation, and Defecation

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces ("stool"). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Of every 500 mL (17 ounces) of food residue that enters the cecum each day, about 150 mL (5 ounces) become feces.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva's maneuver, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate.If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can be caused by the ingestion of foodborne pathogens. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to three or four per week.

The entire colon is about 5 feet (150 cm) long, and is divided into five major segments. The rectum is the last anatomic segment before the anus.

The ascending and descending colon are supported by peritoneal folds called mesentery.

The right colon consists of the cecum, ascending colon, hepatic flexure and the right half of the transverse colon. The left colon consists of the left half of the transverse colon, splenic flexure, descending colon, and sigmoid.

Parts of Colon and Rectum

• Cecum (proximal right colon)
  6 x 9 cm pouch covered with peritoneum
• Appendix
  A vermiform (wormlike) diverticulum located in the lower cecum
• Ascending colon
  20-25 cm long, located behind the peritoneum
• Hepatic flexure
  Lies under right lobe of liver
• Transverse colon
  Lies anterior in abdomen, attached to gastrocolic ligament
• Splenic flexure
  Near tail of pancreas and spleen
• Descending colon
  10-15 cm long, located behind the peritoneum
• Sigmoid colon
  Loop extending distally from border of left posterior major psoas muscle
• Rectosigmoid segment
  Between 10 and 15 cm from anal verge
• Rectum
  12 cm long; upper third covered by peritoneum; no peritoneum on lower third which is also called the rectal ampulla. About 10 cm of the rectum lies below the lower edge of the peritoneum (below the peritoneal reflection), outside the peritoneal cavity
• Anal canal
  Most distal 4-5 cm to anal verge

What is the lower GI tract?

The lower gastrointestinal (GI) tract is the last part of the digestive tract. The lower GI tract consists of the large intestine and the anus.

The large intestine absorbs water and changes the waste products of the digestive process from liquid into formed stool. The large intestine includes

• the appendix, a finger-shaped pouch attached to the cecum
• the cecum, the first part of the large intestine, which is connected to the end of the small intestine
• the colon, which has four sections: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon
• the rectum, the lower end of the large intestine leading to the anus

The anus is a 1-inch opening at the end of your digestive tract through which stool leaves your body. The anus includes the sphincter muscles—muscles that open and close and allow you to control bowel movements.

What are anatomic problems of the lower GI tract?

Anatomic problems of the lower GI tract are problems related to the structure of the lower GI tract. The parts of the lower GI tract may be in the wrong place, may not be shaped normally, or may not connect normally to other parts of the body.

Some anatomic problems of the lower GI tract arise when the lower GI tract does not develop normally before birth. These are called birth defects and include

• anorectal malformations
• colonic atresia and stenosis
• malrotation

Other anatomic problems, which might be present at birth or develop later, include

• intussusception
• colonic and anorectal fistulas
• rectal prolapse
• colonic volvulus