Digestive System, Part 2

Small Intestine

The small intestine is the primary site for chemical digestion and nutrient absorption in the gastrointestinal (GI) tract. It extends from the pyloric sphincter of the stomach to the ileocecal sphincter at the junction with the large intestine.

• Regions: The small intestine is divided into three regions: duodenum, jejunum, and ileum.

• Circular folds (plicae circulares): Permanent ridges in the mucosa and submucosa increase surface area for absorption.

• Function: Completes chemical digestion and absorbs nutrients into the bloodstream.

Small Intestine: Histology

The wall of the small intestine is specialized for absorption and secretion, with several distinct layers and cell types.

• Mucosal layer: Folded into villi, which increase surface area. Each villus contains a lacteal (lymphatic capillary) and blood capillaries.

• Microvilli: Apical surface of absorptive cells (enterocytes) is further folded into microvilli, forming the brush border where final digestion occurs.

• Submucosal layer: Contains duodenal glands that secrete mucus to neutralize chyme and numerous MALT (mucosa-associated lymphoid tissue), especially Peyer's patches in the ileum.

• Secretory cells of mucosa:

  • Absorptive cells (enterocytes): Absorb nutrients.

  • Goblet cells: Secrete mucus.

  • Enteroendocrine cells: Secrete hormones such as secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP).

Intestinal Motility

Movement of contents through the intestines is essential for digestion and absorption.

• Segmentation: Circular smooth muscle contractions mix chyme and increase contact with absorptive surfaces ("intestinal churning").

• Peristalsis: Alternating contractions of circular and longitudinal muscles propel contents forward.

• Large intestine: Uses haustral churning and mass movements (strong peristaltic waves, 3-4 times/day) to mix and propel fecal material.

The Large Intestine

Large Intestine

The large intestine completes absorption of water, produces certain vitamins, forms feces, and expels them from the body. It extends from the ileocecal sphincter to the anus.

• Functions:

  • Complete absorption of water

  • Produce certain vitamins (e.g., vitamin K, some B vitamins by bacteria)

  • Form and expel feces

• Subdivisions: Cecum, colon, rectum, anal canal

• Appendix: Inferior to cecum

• Colon regions: Ascending, transverse, descending, sigmoid

Large Intestine: Histology

The large intestine has specialized structures for its absorptive and secretory functions.

• Mucosa: Contains numerous goblet cells for mucus secretion; lacks circular folds and villi; contains absorptive cells called colonocytes.

• Muscularis layer: Longitudinal muscle is concentrated into three bands called taeniae coli, which contract to form pouches called haustra.

Defecation Reflex

The process of eliminating feces from the body is regulated by the defecation reflex.

• Feces: Composed of undigested food, bacteria and their products, salts, water, and epithelial cells.

• Defecation reflex:

  • Stimulated by stretch receptors in the sigmoid colon.

  • Elimination aided by voluntary contractions of the diaphragm and abdominal muscles.

  • Relaxation of the external anal sphincter allows elimination.

Accessory Organs of the Lower GI Tract

Pancreas

The pancreas is a gland located posterior to the stomach, with both endocrine and exocrine functions.

• Endocrine (1%): Islets of Langerhans secrete hormones (e.g., insulin, glucagon).

• Exocrine (99%): Acinar cells secrete digestive enzymes into the duodenum.

• Major enzymes:

  • Pancreatic amylase (starch digestion)

  • Trypsin, chymotrypsin, carboxypeptidase (protein digestion)

  • Pancreatic lipase (fat digestion)

  • Ribonuclease, deoxyribonuclease (nucleic acid digestion)

Liver & Gall Bladder

The liver and gall bladder are essential accessory organs for digestion and metabolism.

• Liver:

  • Largest internal organ; divided into right and left lobes.

  • Produces bile (emulsifies fats, aids in waste excretion).

  • Primary metabolic organ: processes nutrients, detoxifies blood.

• Gall bladder:

  • Inferior and slightly posterior to the liver.

  • Receives, stores, and releases excess bile from the liver.

Liver: Histology & Blood Supply

The liver is organized into functional units called lobules, each containing hepatocytes arranged around a central vein.

• Hepatocytes: Liver cells that process nutrients and produce bile.

• Lobules: Hexagonal units with a central vein; blood flows from portal triads (branches of hepatic artery, portal vein, and bile duct) toward the central vein.

• Bile: Leaves lobules via bile canaliculi and ducts.

Nutrient Digestion and Absorption

Digestion and Absorption of Nutrients

Digestion breaks down food into absorbable units, which are then transported into the blood or lymph.

• Carbohydrates: Digested into monosaccharides (e.g., glucose, fructose).

• Proteins: Digested into amino acids.

• Lipids: Digested into fatty acids and monoglycerides.

Absorption Pathways

• Blood capillaries: Absorb monosaccharides, amino acids, short-chain fatty acids, water, electrolytes, and various vitamins.

• Lacteals (lymphatic capillaries): Absorb chylomicrons (lipid-protein complexes formed from long-chain fatty acids and monoglycerides).

Summary Table: Digestive Enzymes and Their Functions

Key Equations

• General formula for carbohydrate digestion:

• General formula for protein digestion:

• General formula for lipid digestion:

Additional info: The notes have been expanded to include definitions, examples, and a summary table for digestive enzymes, as well as key equations for digestion processes.