23.5 The Small and Large Intestines

Anatomy and Functions of the Small and Large Intestines

The small and large intestines complete the digestive process that begins in the mouth and stomach. The small intestine handles most chemical digestion and nutrient absorption, while the large intestine reclaims water, houses trillions of beneficial bacteria, and forms feces for elimination.

Small vs. Large Intestine Comparison

The small intestine is actually the longer of the two, despite its name. It's called "small" because of its narrower diameter.

• Small intestine: approximately 6 meters long, narrow in diameter, coiled within the abdominal cavity and suspended by the mesentery (a fan-shaped fold of peritoneum that anchors the intestines and carries blood vessels to them). It's divided into three regions:
  • Duodenum: the first and shortest segment, where most chemical digestion occurs
  • Jejunum: the middle segment, the primary site of nutrient absorption
  • Ileum: the longest segment, absorbs remaining nutrients (especially bile salts and vitamin B12)
• Large intestine: approximately 1.5 meters long but wider in diameter, forming a frame around the small intestine. Its regions, in order, are:
  • Cecum → ascending colon (right side) → transverse colon (crosses upper abdomen) → descending colon (left side) → sigmoid colon → rectum → anal canal

Anatomy for Nutrient Absorption

The small intestine has an enormous surface area, roughly the size of a tennis court. Three structural modifications make this possible, each building on the last:

1. Circular folds (plicae circulares): permanent folds in the intestinal wall that force chyme to spiral as it moves through, slowing transit and increasing contact time with the absorptive surface.

2. Villi: finger-like projections that cover the circular folds. Each villus is lined with enterocytes (absorptive epithelial cells) and contains:

   • A dense capillary network that absorbs monosaccharides, amino acids, and other water-soluble nutrients into the blood
   • A lacteal, a lymphatic capillary that absorbs lipids (reassembled as triglycerides) and transports them into the lymphatic system rather than directly into the blood

3. Microvilli: microscopic projections on the apical surface of each enterocyte, collectively called the brush border. They contain membrane-bound digestive enzymes and dramatically increase the absorptive surface area.

Think of it this way: circular folds increase surface area at the organ level, villi at the tissue level, and microvilli at the cellular level.

Digestive Processes in the Small Intestine

Once acidic chyme enters the duodenum from the stomach, it mixes with three major sources of digestive secretions:

1. Pancreatic juice delivers enzymes that break down all three macronutrient classes:

   • Trypsin and chymotrypsin digest proteins into smaller peptides
   • Pancreatic lipase digests triglycerides into fatty acids and monoglycerides
   • Pancreatic amylase continues starch digestion into maltose and other disaccharides

2. Bile (produced by the liver, stored in the gallbladder) emulsifies fats, breaking large fat globules into smaller droplets. This increases the surface area available for lipase to work on. Bile doesn't chemically digest fat; it just makes fat accessible to enzymes.

3. Brush border enzymes on the microvilli of enterocytes complete the final steps of digestion:

   • Peptidases break small peptides into individual amino acids
   • Sucrase, lactase, and maltase split disaccharides into monosaccharides (e.g., maltase converts maltose into two glucose molecules)

Two types of motility keep things moving and mixing:

• Segmentation: back-and-forth contractions that churn chyme and press it against the intestinal wall for maximum absorption
• Peristalsis: wave-like contractions that propel contents toward the large intestine

Key Features of the Large Intestine

The large intestine has several distinctive structural features you won't find in the small intestine:

• Haustra: pouch-like sacculations in the colon wall that allow expansion and temporary storage of contents
• Teniae coli: three bands of longitudinal smooth muscle running along the colon's length. Their tonic contraction is what creates the haustra and drives haustral churning, a slow mixing movement that helps absorb water
• Epiploic (omental) appendages: small, fat-filled pouches hanging off the colon's external surface. Their exact function is unclear, though they may provide cushioning or serve a minor immune role

Other important structures:

• Cecum: a blind-ended pouch where the small intestine empties into the large intestine. The appendix projects from the cecum and contains lymphoid tissue. It likely plays a role in gut immunity and may serve as a reservoir for beneficial bacteria to repopulate the colon after illness.
• Rectum: the final straight portion that stores feces before elimination
• Anal canal: the terminal segment, controlled by two sphincters:
  • Internal anal sphincter: smooth muscle, involuntary control
  • External anal sphincter: skeletal muscle, voluntary control (this is what gives you conscious control over defecation)

Role of Gut Bacteria

The large intestine harbors an estimated 100 trillion microorganisms, collectively called the gut microbiota (the full genetic catalog of these organisms is the microbiome). These bacteria perform several functions the human body cannot do on its own:

• Ferment indigestible carbohydrates (dietary fiber, resistant starch) to produce short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate. Butyrate is especially important because it's the primary energy source for colonocytes (the epithelial cells lining the colon). SCFAs also reduce inflammation and improve insulin sensitivity.
• Synthesize vitamins the host needs, including vitamin K (essential for blood clotting), biotin, and folate.
• Compete with pathogens for nutrients and attachment sites, preventing harmful bacteria from gaining a foothold.
• Stimulate gut-associated lymphoid tissue (GALT), helping to train and regulate immune responses throughout the body.

Journey of Food Waste Through the Large Intestine

By the time material reaches the large intestine, most nutrients have already been absorbed. What remains is mostly water, electrolytes, indigestible fiber, and bacteria.

1. The ileocecal valve controls passage of chyme from the ileum into the cecum, preventing backflow.
2. In the cecum, chyme mixes with resident gut bacteria, which begin fermenting undigested material.
3. Haustral churning slowly moves contents through the ascending, transverse, and descending colon. During this transit, the colon absorbs water, electrolytes (sodium, potassium), and SCFAs, gradually solidifying the contents into feces.
4. Feces collect in the sigmoid colon for temporary storage before passing into the rectum.
5. Distension of the rectal walls triggers the defecation reflex: stretch receptors send signals via the spinal cord that cause the internal anal sphincter to relax involuntarily.
6. The external anal sphincter remains under voluntary control. If conditions are appropriate, you relax it consciously, and feces are expelled through the anal canal.

Key Functions of the Intestines