23.5 The Small and Large Intestines

The small and large intestines are vital players in digestion and absorption. The small intestine, a long, narrow tube, breaks down nutrients and absorbs them through specialized structures. The large intestine, shorter and wider, focuses on water absorption and houses beneficial gut bacteria.

These organs work together to complete the digestive process. The small intestine's extensive surface area maximizes nutrient absorption, while the large intestine forms feces and supports gut health. Understanding their anatomy and functions is crucial for grasping the complexities of the digestive system.

Anatomy and Functions of the Small and Large Intestines

Small vs large intestine comparison

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• Small intestine consists of a longer, narrower tube approximately 6 meters long divided into duodenum, jejunum, and ileum located in the abdominal cavity coiled and suspended by the mesentery
• Large intestine consists of a shorter, wider tube approximately 1.5 meters long divided into cecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal forming a frame around the small intestine ascending on the right, crossing the upper abdomen, and descending on the left

Anatomy for nutrient absorption

• Circular folds (plicae circulares) increase surface area and slow the passage of chyme in the small intestine
• Villi, finger-like projections, further increase surface area for absorption in the small intestine
  • Enterocytes (absorptive cells) line the villi and have microvilli forming the brush border
• Microvilli, microscopic projections on enterocytes, contain digestive enzymes and greatly increase surface area in the small intestine
• Extensive capillary network within each villus allows efficient nutrient absorption into the bloodstream
• Lacteals, lymphatic capillaries in villi, absorb fatty acids (triglycerides) and transport them to the lymphatic system

Digestive processes in small intestine

1. Chyme from the stomach mixes with digestive secretions from the pancreas, liver (bile), and intestinal glands in the small intestine
2. Pancreatic enzymes (trypsin, chymotrypsin, lipase, amylase) break down proteins, fats, and carbohydrates
3. Bile from the liver emulsifies fats increasing surface area for enzymatic action by lipase
4. Brush border enzymes (peptidases, sucrase, lactase, maltase) complete the digestion of nutrients on the surface of enterocytes
5. Peristalsis and segmentation mix the chyme and bring it into contact with the intestinal wall for absorption

Key features of large intestine

• Haustra, sacculations in the colon wall, allow for expansion and storage of feces
• Teniae coli, three longitudinal muscle bands, contract to create haustral churning mixing contents
• Epiploic appendages, small fat-filled pouches on the colon's surface, function is unclear but may serve as a protective cushion
• Cecum, a pouch at the beginning of the large intestine, connects to the appendix
  • Appendix, a small, finger-like projection, contains lymphoid tissue and may have a role in gut immunity and maintaining healthy gut bacteria
• Rectum, the final straight portion of the large intestine, stores feces before defecation
• Anal canal, the terminal segment, has internal (involuntary) and external (voluntary) sphincters for fecal continence

Role of gut bacteria

• Gut microbiota (also known as intestinal flora), primarily in the large intestine, consists of trillions of bacteria (100 trillion)
• Ferment undigested carbohydrates (fiber, resistant starch) to produce short-chain fatty acids (SCFAs)
  • SCFAs (butyrate, propionate, acetate) provide energy for colonocytes and have various health benefits (reduce inflammation, improve insulin sensitivity)
• Synthesize certain vitamins (vitamin K, biotin, folate) for the host
• Compete with pathogenic bacteria, preventing their overgrowth and maintaining gut health
• Stimulate the development and function of the gut-associated lymphoid tissue (GALT) helping to regulate immune responses
• Form the microbiome, which plays a crucial role in digestion, immunity, and overall health

Journey of food waste

1. Ileocecal valve controls the passage of chyme from the ileum to the cecum
2. Chyme enters the cecum and mixes with gut bacteria
3. Haustral churning moves the contents through the ascending, transverse, and descending colon

• Water, electrolytes (sodium, potassium), and SCFAs are absorbed, solidifying the contents into feces

1. Feces enter the sigmoid colon, where they are stored until passing into the rectum
2. Distension of the rectal walls stimulates the defecation reflex
3. Internal anal sphincter relaxes involuntarily, while the external anal sphincter is voluntarily controlled
4. Feces are expelled through the anal canal during defecation

Key functions of the intestines

• Digestion: Breaking down complex nutrients into simpler forms for absorption
• Absorption: Uptake of nutrients, water, and electrolytes through the intestinal wall
• Motility: Movement of intestinal contents through peristalsis and segmentation
• Secretion: Release of enzymes, mucus, and other substances to aid in digestion and protection
• Storage: Temporary holding of feces in the large intestine before elimination