Cecum and Appendix

The cecum is the first part of the large intestine. It is a blind, pouch-like structure located below the ileocecal valve in the right iliac region. It is approximately 6 cm long and receives material from the ileum while continuing the absorption of water and salts. 

Rectum

The rectum begins at the level of the third sacral vertebra as a continuation of the sigmoid colon and ends at the anal canal. It is approximately 12 cm long and follows the curvature of the sacrum and coccyx rather than being straight. It has two anteroposterior curves: an upper curve with its convexity directed backward and a lower curve with its convexity directed forward.

The rectum contains a dilated region called the rectal ampulla, which serves as a temporary storage site for feces. Unlike the colon, the rectum lacks sacculations. Its mucosa contains several permanent transverse folds known as Houston’s valves or rectal valves, which help support fecal material and separate feces from gas.

The upper portion of the rectum is partially covered by peritoneum, while the lower portion lacks a peritoneal covering. In females, the rectum lies in relation to the uterus and vagina, whereas in males it is related to the bladder, seminal vesicles, and prostate gland.

The muscular wall of the rectum is strong. The circular muscle layer thickens near the anus to form the internal anal sphincter, which is composed of smooth muscle and functions involuntarily.

Anal Canal and Anus

The anal canal is the final portion of the large intestine. It begins at the apex of the prostate in males and extends to the anus. It is approximately 2.5-5 cm long and lies entirely outside the abdominopelvic cavity.

The upper half of the anal canal contains longitudinal folds called rectal columns (columns of Morgagni), separated by grooves known as rectal sinuses. Small anal valves connect the lower ends of these columns.

Histology of the Large Intestine

The wall of the large intestine differs from that of the small intestine. It lacks villi and circular folds and contains relatively few enzyme-secreting cells. The mucosa is lined mainly by simple columnar epithelium composed of absorptive enterocytes and numerous goblet cells. Goblet cells produce mucus that lubricates feces and protects the intestinal wall from acids and gases produced by intestinal bacteria. Enterocytes absorb water, salts, and vitamins produced by the intestinal microbiota.

A distinguishing feature of the large intestine is the presence of three longitudinal bands of smooth muscle known as taeniae coli rather than a continuous outer longitudinal muscle layer. The large intestine also contains sacculations and appendices epiploicae, which help distinguish it from the small intestine.

The large intestine has three unique features: the teniae coli, haustra, and epiploic appendages. The teniae coli are three bands of smooth muscle that form the longitudinal muscle layer of the large intestine, except at its terminal end. Their continuous contractions create pouch-like sacs called haustra, which give the colon its wrinkled appearance. Attached to the teniae coli are small fat-filled sacs of visceral peritoneum called epiploic appendages, although their function is unknown. Unlike the colon, the rectum and anal canal do not have teniae coli or haustra, but they have well-developed muscular layers that produce the strong contractions needed for defecation.

The mucosa of the anal canal is lined with stratified squamous epithelium, which protects it from abrasion as feces pass through. Its mucous membrane forms longitudinal folds called anal columns, which contain blood vessels. Between these columns are anal sinuses, which secrete mucus to lubricate the passage of feces. The pectinate (dentate) line marks the junction between the hindgut and the external skin. The mucosa above this line is relatively insensitive because it is supplied by visceral sensory nerves, whereas the area below is highly sensitive due to its somatic sensory nerve supply.

The large intestine contains trillions of beneficial bacteria known as the bacterial flora. Most of these bacteria are harmless and help with chemical digestion, absorption, and the production of vitamins such as biotin, pantothenic acid, and vitamin K. Some also contribute to immune function. The intestinal mucosa prevents these bacteria from invading body tissues through immune mechanisms involving dendritic cells, T cells, and IgA antibodies.

The main digestive function of the large intestine is the absorption of water from the remaining chyme. The residue usually remains in the large intestine for 12-24 hours, allowing most of the remaining water to be absorbed before the waste is eliminated.

Mechanical Digestion

Mechanical digestion begins when chyme passes from the ileum into the cecum through the ileocecal sphincter. When the cecum becomes distended with chyme, contractions of the sphincter increase and movements of the colon begin.

The large intestine uses three types of movements. Haustral contractions are slow segmentation movements that occur about every 30 minutes and last approximately 1 minute. These contractions move the intestinal contents from one haustrum to the next while mixing them to improve water absorption. Peristalsis also occurs but is slower than in the upper parts of the digestive tract. Mass movements are powerful waves that usually occur three or four times a day, often during or shortly after eating. They rapidly push fecal material toward the rectum. These movements are stimulated by the gastrocolic reflex, which is triggered by stomach distension and digestion in the small intestine. Dietary fiber softens the stool and increases the strength of colonic contractions, improving movement through the colon.

Chemical Digestion

The large intestine does not produce digestive enzymes. Instead, chemical digestion is carried out entirely by bacteria in the colon. These bacteria ferment the remaining carbohydrates, producing gases such as hydrogen, carbon dioxide, and methane. These gases form flatus, and excessive gas production results in flatulence. Foods rich in indigestible carbohydrates, such as beans, increase gas production.

Absorption, Feces Formation, and Defecation

Most of the remaining water in the intestinal contents is absorbed in the large intestine, changing the liquid residue into semisolid feces. Feces consist of undigested food, unabsorbed substances, bacteria, dead epithelial cells, inorganic salts, and enough water to allow easy passage. Of approximately 500 mL of material entering the cecum each day, about 150 mL becomes feces.

Defecation begins when mass movements force feces into the rectum, stretching the rectal wall and triggering the defecation reflex. This parasympathetic reflex contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The brain then determines whether the external anal sphincter will be voluntarily relaxed to allow defecation or kept closed temporarily. If defecation is delayed, the rectum relaxes until another mass movement occurs.

During defecation, a person can assist the process by performing the Valsalva maneuver, which increases intra-abdominal pressure by contracting the diaphragm and abdominal muscles while closing the glottis.

If feces remain in the large intestine for too long, more water is absorbed, making the stool harder and leading to constipation. If intestinal contents move too quickly, insufficient water is absorbed, resulting in diarrhea. Bowel movement frequency varies widely between individuals, ranging from two or three times per day to three or four times per week.