The Large Intestine

Objectives

When you have completed this section, you should be able to

• describe the gross anatomy of the large intestine;

• contrast the mucosa of the colon with that of the small intestine;

• state the physiological significance of intestinal bacteria;

• discuss the types of contractions that occur in the colon; and

• explain the neurological control of defecation.

The large intestine (fig. 25.30) receives about 500 mL of indigestible food residue per day, reduces it to about 150 mL of feces by absorbing water and salts, and eliminates the feces by defecation.

Gross Anatomy

The large intestine measures about 1.5 m (5 ft) long and 6.5 cm (2.5 in.) in diameter in the cadaver. It begins with the cecum,30 a blind pouch in the lower right abdominal quadrant inferior to the ileocecal valve. Attached to the lower end of the cecum is the appendix, a blind tube 2 to 7 cm long. The appendix is densely populated with lymphocytes and is a significant source of immune cells.

The colon is that part of the large intestine between the ileocecal junction and anal canal. It is divided into the ascending, transverse, descending, and sigmoid regions. The ascending colon begins at the ileocecal valve and passes up the right side of the abdominal cavity. It makes

30cec = blind a 90° turn at the right colic (hepatic) flexure, near the right lobe of the liver, and becomes the transverse colon. This passes horizontally across the upper abdominal cavity and turns 90° downward at the left colic (splenic) flexure near the spleen. Here it becomes the descending colon, which passes down the left side of the abdominal cavity. Ascending, transverse, and descending colons thus form a squarish, three-sided frame around the small intestine.

The pelvic cavity is narrower than the abdominal cavity, so at the pelvic inlet the colon turns medially and downward, forming a roughly S-shaped portion called the sigmoid31 colon. (Visual examination of this region is performed with an instrument called a sigmoidoscope.) In the pelvic cavity, the large intestine straightens and forms the rectum.32 The rectum has three internal transverse folds called rectal valves that enable it to retain feces while passing gas.

The final 3 cm of the large intestine is the anal canal (fig. 25.30fc), which passes through the levator ani muscle of the pelvic floor and terminates at the anus. Here, the mucosa forms longitudinal ridges called anal columns with depressions between them called anal sinuses. As feces pass through the canal, they press against the sinuses and cause them to exude extra mucus and lubricate the canal during defecation. Large hemorrhoidal veins form superficial plexuses in the anal columns and around the orifice. Unlike veins in the extremities, they lack valves and are particularly subject to distension and venous pooling. Hemorrhoids are permanently distended veins that protrude into the anal canal or form bulges distal to the anus.

The muscularis externa of the colon is unusual in that its longitudinal fibers do not encircle the colon but are divided into three ribbonlike strips called the teniae coli (TEE-nee-ee CO-lye). The muscle tone of the teniae coli contracts the colon lengthwise and causes its wall to form pouches called haustra33 (HAW-stra; singular, haustrum). In the rectum and anal canal, however, the longitudinal muscle forms a continuous sheet and haustra are absent. The anus, like the urethra, is regulated by two sphincters—an internal anal sphincter composed of smooth muscle of the muscularis externa and an external anal sphincter composed of skeletal muscle of the pelvic diaphragm.

The ascending and descending colon are retroperi-toneal, whereas the transverse and sigmoid colon are covered with serosa and anchored to the dorsal abdominal wall by the mesocolon. The serosa of these regions often has epiploic34 appendages, clublike fatty pouches of peritoneum of unknown function.

32rect = straight

33haustr = to draw

34epiploic = pertaining to an omentum

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Chapter 25 The Digestive System 975

Right colic flexure

Transverse colon

Superior mesenteric artery

Haustrum

Ascending colon

Ileocecal valve

Ileum-

Cecum Appendix

Rectum

Anal canal -

Right colic flexure

Transverse colon

Superior mesenteric artery

Haustrum

Ascending colon

Anal canal -

Tenia Coli

Mesocolon

Left colic flexure

Epiploic appendages

Descending colon

Tenia coli

Sigmoid colon

Mesocolon

Left colic flexure

Epiploic appendages

Descending colon

Tenia coli

Sigmoid colon

External anal sphincter

Epiploic Appendages Large Intestine

Figure 25.30 The Large Intestine. (a) Gross anatomy. (b) Detail of the anal canal.

Which anal sphincter is controlled by the autonomic nervous system? Which is controlled by the somatic nervous system?

Figure 25.30 The Large Intestine. (a) Gross anatomy. (b) Detail of the anal canal.

Which anal sphincter is controlled by the autonomic nervous system? Which is controlled by the somatic nervous system?

Saladin: Anatomy & I 25. The Digestive System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

976 Part Four Regulation and Maintenance

Microscopic Anatomy

The mucosa of the large intestine has a simple columnar epithelium in all regions except the lower half of the anal canal, where it has a nonkeratinized stratified squamous epithelium. The latter provides more resistance to the abrasion caused by the passage of feces. There are no circular folds or villi in the large intestine, but there are intestinal crypts. They are deeper than in the small intestine and have a greater density of goblet cells; mucus is their only significant secretion.

The anatomical features of the digestive tract from the large intestine to the anus are summarized in table 25.5.

Bacterial Flora and Intestinal Gas

The large intestine is densely populated with several species of bacteria collectively called the bacterial flora.35 They ferment cellulose, other undigested carbohydrates, and fats and synthesize B vitamins and vitamin K, which are absorbed by the colon. This vitamin K is especially important because the diet alone usually does not provide enough to ensure adequate blood clotting.

The average person expels about 500 mL of flatus (gas) per day. Most of this is swallowed air that has worked its way through the digestive tract, but the bacterial flora add to it. Painful cramping can result when undigested nutrients pass into the colon and furnish an abnormal substrate for bacterial action—for example, in lactose intolerance. Flatus is composed of nitrogen (N2), carbon dioxide (CO2), hydrogen (H2), methane (CH4), hydrogen sulfide (H2S), and two amines: indole and skatole. Indole, skatole,

35flora = flowers, plants

Table 25.5

Anatomical Checklist of the Digestive System from the Large Intestine Through the Anus

Cecum

Anal canal

Appendix

Internal anal sphincter

Ascending colon

External anal sphincter

Right colic flexure

Anal columns

Transverse colon

Anal sinuses

Left colic flexure

Hemorrhoidal veins

Descending colon

Teniae coli

Sigmoid colon

Haustra

Rectum

Epiploic appendages

Rectal valves

Mesocolon

and H2S produce the odor of flatus and feces, whereas the others are odorless. The hydrogen gas is combustible and has been known to explode in surgery that used electrical cauterization.

Absorption and Motility

The large intestine takes about 12 to 24 hours to reduce the residue of a meal to feces. It does not chemically change the residue but reabsorbs water and electrolytes (especially NaCl) from it. The feces consist of about 75% water and 25% solids. The solids are about 30% bacteria, 30% undigested dietary fiber, 10% to 20% fat, and smaller amounts of protein, sloughed epithelial cells, salts, mucus, and other digestive secretions. The fat is not from the diet but from broken-down epithelial cells and bacteria.

The most common type of colonic motility is a type of segmentation called haustral contractions, which occur about every 30 minutes. Distension of a haustrum with feces stimulates it to contract. This churns and mixes the residue, promotes water and salt absorption, and passes the residue distally to another haustrum. Stronger contractions called mass movements occur one to three times a day, last about 15 minutes, and move residue for several centimeters at a time. They are often triggered by the gas-trocolic and duodenocolic reflexes, in which filling of the stomach and duodenum stimulates motility of the colon. Mass movements occur especially in the transverse to sig-moid colon, and often within an hour after breakfast.

Defecation

Stretching of the rectum stimulates the defecation reflexes, which account for the urge to defecate that is often felt soon after a meal. The predictability of this response is useful in house-training pets and toilet-training children. In the intrinsic defecation reflex, stretch signals travel by the myenteric nerve plexus to the muscularis of the descending and sigmoid colons and the rectum. This triggers a peristaltic wave that drives feces downward, and it relaxes the internal anal sphincter. Defecation occurs only if the external anal sphincter is voluntarily relaxed at the same time.

The intrinsic reflex is relatively weak and usually requires the cooperative action of a stronger parasympa-thetic defecation reflex involving the spinal cord. As shown in figure 25.31, this reflex involves three processes: (1) Stretching of the rectum sends signals to the sacral segments of the spinal cord. (2) The spinal cord returns signals by way of the parasympathetic fibers in the pelvic nerves to the rectum, thus intensifying rectal peristalsis. (3) At the same time, parasympathetic fibers relax the internal anal sphincter to allow the passage of feces.

The external anal sphincter, being composed of skeletal muscle, is under voluntary control, enabling us to

Saladin: Anatomy & I 25. The Digestive System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition limit defecation to appropriate circumstances (step 4 in fig. 25.31). As involuntary contractions of the rectum push the feces downward, voluntary contractions of the levator ani pull the anal canal upward and allow the feces to fall away. Defecation is also aided by the Valsalva maneuver, which increases abdominal pressure, compresses the rectum, and squeezes the feces from it. This maneuver can also initiate the defecation reflex by forcing feces from the descending colon into the rectum.

If the defecation urge is suppressed, contractions cease in a few minutes and the rectum relaxes. The defecation reflexes reoccur a few hours later or when another mass movement propels more feces into the rectum.

Chapter 25 The Digestive System 977

The effects of aging on the digestive system are discussed on page 1112. Table 25.6 lists and describes some common digestive disorders.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

26. How does the mucosa of the large intestine differ from that of the small intestine? How does the muscularis externa differ?

27. Name and briefly describe two types of contractions that occur in the colon and nowhere else in the alimentary canal.

28. Describe the reflexes that cause defecation in an infant. Describe the additional neural controls that function following toilet-training.

Insight 25.5 Medical History

Autonomic Colon Function

Figure 25.31 Neural Control of Defecation. (1) Filling of the rectum with feces stimulates stretch receptors, which transmit impulses to the spinal cord. (2) A spinal reflex stimulates contractions of the rectum and relaxation of the internal anal sphincter. (3) Defecation normally does not occur unless voluntary impulses relax the external anal sphincter.

Figure 25.31 Neural Control of Defecation. (1) Filling of the rectum with feces stimulates stretch receptors, which transmit impulses to the spinal cord. (2) A spinal reflex stimulates contractions of the rectum and relaxation of the internal anal sphincter. (3) Defecation normally does not occur unless voluntary impulses relax the external anal sphincter.

The Man with a Hole in His Stomach

Perhaps the most famous episode in the history of digestive physiology began in 1822 on Mackinac Island in the strait between Lake Michigan and Lake Huron. Alexis St. Martin, a 19-year-old fur trapper, was standing outside a trading post when he was accidentally hit by a shotgun blast from 3 feet away. An Army doctor stationed at Fort Mackinac, William Beaumont (1785-1853), was summoned to examine St. Martin. As Beaumont later wrote, "a portion of the lung as large as a turkey's egg" protruded through St. Martin's lacerated and burnt flesh. Below that was a portion of the stomach with a puncture in it "large enough to receive my forefinger." Beaumont did his best to pick out bone fragments and dress the wound, though he did not expect St. Martin to survive.

Surprisingly, St. Martin lived. Over a period of months the wound extruded pieces of bone, cartilage, gunshot, and gun wadding. As the wound healed, a fistula (hole) remained in the stomach, so large that Beaumont had to cover it with a compress to prevent food from coming out. A fold of tissue later grew over the fistula, but it was easily opened. A year later, St. Martin was still feeble. Town authorities decided they could no longer support him on public funds and wanted to ship him 2,000 miles to his home in Canada. Beaumont, however, was imbued with a passionate sense of destiny. Very little was known about digestion, and he saw the accident as a unique opportunity to learn. He took St. Martin in at his personal expense and performed 238 experiments on him over several years. Beaumont had never attended medical school and had little idea how scientists work, yet he proved to be an astute experimenter. Under crude frontier conditions and with almost no equipment, he discovered many of the basic facts of gastric physiology discussed in this chapter.

"I can look directly into the cavity of the stomach, observe its motion, and almost see the process of digestion," Beaumont wrote. "I can pour in water with a funnel and put in food with a spoon, and draw them out again with a siphon." He put pieces of meat on a string into the stomach and removed them hourly for examination. He sent vials of gastric juice to the leading chemists of America and Europe, who could do little but report that it contained hydrochloric acid. He proved that digestion required HCl and could even occur outside the stomach, but he found that HCl alone did not digest meat; gastric juice must contain some other digestive ingredient. Theodor Schwann, one of the founders of the cell theory (see chapter 3), identified that ingredient

Saladin: Anatomy & I 25. The Digestive System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

978 Part Four Regulation and Maintenance

Table 25.6 Some Digestive System Diseases

Acute pancreatitis Severe pancreatic inflammation perhaps caused by trauma leading to leakage of pancreatic enzymes into parenchyma, where they digest tissue and cause inflammation and hemorrhage. Inflammation of the appendix, with swelling, pain, and sometimes gangrene, perforation, and peritonitis. Accumulation of serous fluid in the peritoneal cavity, often causing extreme distension of the abdomen. Most often caused by cirrhosis of the liver (see chapter 26) and frequently associated with alcoholism. The diseased liver "weeps" fluid into the abdomen. About 25% of people who develop ascites as a consequence of cirrhosis die within one year. Digestive system is subject to cancer especially of the esophagus, stomach, colon, liver, and pancreas, with colon and pancreatic cancer being among the leading causes of cancer death in the United States. Inflammation of small and large intestines, similar to ulcerative colitis. Produces granular lesions and fibrosis of intestine, diarrhea, and lower abdominal pain. Often hereditary. Presence of inflamed herniations (outpocketings, diverticula) of the colon, associated especially with low-fiber diets. Diverticula may rupture, leading to peritonitis. Difficulty swallowing. Can result from esophageal obstructions (tumors, constrictions) or impaired peristalsis (due to neuromuscular disorders).

Protrusion of part of the stomach into the thoracic cavity, where the negative thoracic pressure may cause it to balloon. Often causes gastroesophageal reflux (especially when a person is supine) and esophagitis (inflammation of the esophagus). Chronic inflammation resulting in ulceration of the large intestine, especially the sigmoid colon and rectum. Tends to be hereditary but exact causes are not well known. Disorders described elsewhere

Impacted molars 944 Lactose intolerance 970 Peptic ulcer 956 Periodontal disease 946

Appendicitis Ascites

Cancers

Crohn disease

Diverticulitis

Dysphagia

Hiatal hernia

Ulcerative colitis

Constipation 973 Dental caries 946 Diarrhea 973 Gallstones 962 Gastroesophageal reflux 948

Gingivitis 946 Hemorrhoids 974 Hepatic cirrhosis 1012 Hepatitis 1012

as pepsin. Beaumont also demonstrated that gastric juice is secreted only in response to food; it did not accumulate between meals as previously thought. He disproved the idea that hunger is caused by the walls of the empty stomach rubbing against each other.

For his part, St. Martin felt helpless and humiliated by Beaumont's experiments. His fellow trappers taunted him as "the man with a hole in his stomach," and he longed to return to hunting and trapping in the wilderness. He had a wife and daughter in Canada whom he rarely got to see, and he ran away repeatedly to join them. He was once gone for 4 years before his poverty and physical disability made him yield to Beaumont's financial enticement to come back. Beaumont despised St. Martin for his drunkenness and profanity and was quite insensitive to St. Martin's embarrassment and discomfort over the experiments. Yet St. Martin's temper enabled Beaumont to make the first direct observations of the relationship between emotion and digestion. When St. Martin was particularly distressed, Beaumont noted little digestion occurring—as we now know, the sympathetic nervous system inhibits digestive activity.

Beaumont published a book in 1833 that laid the foundation for modern gastric physiology and dietetics. It was enthusiastically received by the medical community and had no equal until Russian physiologist Ivan Pavlov (1849-1936) performed his celebrated experiments on digestion in animals. Building on the methods pioneered by Beaumont, Pavlov received the 1904 Nobel Prize for Physiology or Medicine.

In 1853, Beaumont slipped on some ice, suffered a blow to the base of his skull, and died a few weeks later. St. Martin continued to tour medical schools and submit to experiments by other physiologists, whose conclusions were often less correct than Beaumont's. Some, for example, attributed chemical digestion to lactic acid instead of hydrochloric acid. St. Martin lived in wretched poverty in a tiny shack with his wife and several children and died 28 years after Beaumont. By then he was senile and believed he had been to Paris, where Beaumont had often promised to take him.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Relaxation Audio Sounds Babbling Brook

Relaxation Audio Sounds Babbling Brook

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds from the Babbling Brooks.

Get My Free MP3 Audio


Responses

  • gloriana
    What to expect when ileocecal valve is removed?
    6 years ago
  • almaz
    Which of the following occurs in the large intestine by the work of anaerobic bacteria?
    6 years ago
  • Samppa
    What is the part of the large intestine between the ileocecal junction and the hepatic flexure?
    4 years ago
  • Gaudenzio
    What part of the colon contracts in defecation?
    12 months ago

Post a comment