12.8: Activities of the Large Intestine

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The large intestine hosts a diverse community of microbes that assist with digestion and vitamin production while the organ itself absorbs water, compacts waste, and prepares feces for elimination.

Master this section and you'll be able to:

• Describe the gut microbiome and explain how intestinal microorganisms contribute to the well-being of their host. 
• Explain the processes of digestion and absorption and feces formation in the large intestine.

Gut Microbiome: Overview

The term gut microbiome refers to the enormous community of microorganisms living in your digestive tract. Most are bacteria, but the community also includes viruses, fungi, archaea, and tiny single-celled eukaryotes. Together, these microbes assist with digestion, produce certain vitamins, support immune function, and help protect the body from harmful pathogens — tasks your body cannot perform alone.

Composition and Location

Most microbes that enter the digestive tract are destroyed earlier in digestion by lysozyme, defensins, stomach acid, or protein-digesting enzymes. However, trillions of microorganisms, collectively called the gut microbiota (historically called the “bacterial flora”), survive and thrive in the large intestine. More than 700 species have been identified. Most are nonpathogenic commensal organisms that stay safely within the gut lumen and provide important benefits to the host. In the image you can see Escherichia coli (E. coli for short), a common inhabitant of the human gut.

Roles in Digestion and Absorption

These microbes are active participants in digestion. Bacteria break down undigested polysaccharides — dietary fiber — and metabolize them into short-chain fatty acids that are absorbed by passive diffusion and provide several important benefits once they are inside our body. Because the bacterial metabolism produces acids, the large intestine secretes bicarbonate to help neutralize the acidity. 

Several bacterial species also synthesize vitamins that humans cannot make on their own, including biotin (B7), pantothenic acid (B5), and vitamin K. Although this bacterial vitamin supply provides only a modest portion of the daily requirement, it becomes especially important when dietary intake is low or inconsistent.
Antibiotic treatment can disrupt this process because antibiotics often kill helpful microbes along with harmful ones, which may lead to deficiencies if the gut flora is suppressed for long periods.

Immune Support and Barrier Protection

Some members of the gut microbiota also strengthen immune responses and help maintain a stable intestinal environment. However, it is essential that these microbes stay inside the intestinal lumen. The large intestine maintains a tightly regulated barrier system — including epithelial cells sealed by tight junctions, a protective mucus layer, and continuous immune surveillance — that keeps the microbiota from crossing into deeper tissues.
 

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Absorption and Feces Formation

Absorption and feces formation in the large intestine center on reclaiming water and electrolytes from the remaining chyme while compacting indigestible material into solid waste. The material gradually becomes feces that can be stored and later eliminated.

Digestive Functions of the Large Intestine

The material that enters the large intestine is what remains of chyme after most digestion and absorption have already occurred. It contains very few nutrients other than water. As this residue moves slowly through the large intestine, usually over 12 to 24 hours, much of the remaining water is reabsorbed.

Because the large intestine mainly absorbs water and does not play a major role in nutrient absorption, it can be completely removed without severely disrupting digestion. In some patients with severe inflammatory bowel disease, surgeons remove the large intestine in a procedure called a colectomy. If possible, they form a new internal fecal pouch from a segment of the small intestine and connect it to the anus so that defecation can still occur through the normal route. If this is not feasible, an ileostomy is created by bringing the distal ileum out through the abdominal wall. In that case, the watery intestinal contents empty into a bag-like adhesive appliance attached to the skin.

Mechanical Digestion

Mechanical digestion in the large intestine begins when chyme passes from the ileum into the cecum. This movement is regulated by the ileocecal valve (sphincter). After a meal, peristalsis in the ileum pushes chyme through the sphincter. When the cecum becomes distended, the sphincter tightens to prevent material from moving backward into the small intestine.

Once chyme reaches the cecum, the colon begins its characteristic movements: the haustral contractions. These slow, segmenting movements occur mainly in the transverse and descending colon. When a haustrum becomes stretched by chyme, its circular muscle contracts and pushes the residue into the next haustrum. These contractions occur about every 30 minutes and last roughly 1 minute. They also mix the residue, which helps with water absorption.

Peristalsis also occurs in the large intestine, but it is slower than in the small intestine. These waves help gradually move the residue forward along the colon.

A third and more powerful type of motion is the mass movement. These forceful contractions begin in the transverse colon and quickly drive the contents toward the rectum. Mass movements typically occur three or four times each day, usually during a meal or shortly after. Stretching of the stomach and the presence of nutrients in the small intestine trigger the gastrocolic reflex, which increases motility in the colon and helps initiate these strong waves.

Adequate fiber intake helps the colon perform all these functions effectively. Fiber softens the stool and enhances the strength of colonic contractions, improving motility and reducing the risk of constipation.

Chemical Digestion

Although the glands of the large intestine secrete mucus, they do not release digestive enzymes. As a result, any chemical digestion that occurs in the large intestine is carried out entirely by the bacteria living in the colon. Through a process called saccharolytic fermentation, these bacteria break down some of the remaining carbohydrates. This activity produces hydrogen, carbon dioxide, and methane gases, which together form flatus (intestinal gas). Flatulence refers to excessive flatus. On average, the colon generates up to 1500 mL of flatus per day, and even more when foods high in indigestible sugars and complex carbohydrates, such as beans and soluble dietary fiber, are consumed.
 

Water Absorption and Formation of Feces

The small intestine absorbs nearly 90 percent of the water you ingest, whether it comes from liquids or from solid food. The large intestine absorbs most of what remains, gradually converting liquid chyme into semisolid feces. Feces contain undigested food residues, unabsorbed nutrients, millions of bacteria, shed epithelial cells from the mucosa, inorganic salts, waste products, and enough water to allow smooth passage out of the body. Of the approximately 500 mL (about 17 ounces) of residue that enters the cecum each day, only about 150 mL (5 ounces) leave the body as feces.
 

The Defecation Reflex

Defecation is a coordinated blend of involuntary reflexes and voluntary control that safely and efficiently clears waste from the digestive system. It begins when powerful mass movements sweep fecal material into the rectum, stretching its walls and activating the defecation reflex. This spinal cord–mediated, parasympathetic reflex contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and briefly tightens the external anal sphincter. As feces enter the anal canal, sensory signals reach the brain, giving you the choice to either relax the external anal sphincter to proceed or keep it contracted to delay the process.

If the urge is postponed, the rectum gradually relaxes and some material may even be moved back into the colon by reverse peristalsis, where additional water is absorbed. This reduces pressure for the moment, but the next mass movement will trigger the reflex again. Delaying defecation for too long causes the stool to harden as more water is absorbed, which can lead to constipation. On the other hand, if fecal material moves too quickly through the intestines, not enough water is absorbed, resulting in diarrhea. Everything from hydration and diet to stress levels influences how often these reflexes occur, which is why normal bowel habits vary widely among healthy individuals.

When you decide to defecate, voluntary effort works together with the reflex. The abdominal muscles contract to increase intra-abdominal pressure and the perineal wall lowers, reducing the anorectal angle from about 90 degrees to nearly straight. The external anal sphincter relaxes, and coordinated contractions of the rectum, internal anal sphincter, and puborectalis muscle help propel feces out of the body. These final adjustments essentially pull the anal canal up and over the passing stool, completing one of the most fundamental yet finely tuned processes your body carries out each day.

Figure \(\PageIndex{1}\): Neural Control of Defecation. The spinal cord coordinates the defecation reflex. Parasympathetic pathways stimulate contraction of the sigmoid colon and rectum and relax the internal anal sphincter. At the same time, higher brain centers provide voluntary control over the external anal sphincter, allowing you to delay or permit defecation.  

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