What is the physiology of the gastrointestinal (GI) system from the oral cavity to the descending colon?

Physiology of the Gastrointestinal System from Oral Cavity to Descending Colon

The gastrointestinal system functions as an integrated digestive pathway that begins with mechanical and chemical breakdown of food in the oral cavity and continues through a series of specialized compartments that progressively break down, absorb, and process nutrients while eliminating waste products.

Oral Cavity

• Initial digestion begins with mechanical breakdown through mastication (chewing)
• Salivary secretions (approximately 0.5 liters daily) 1 contain:
  • Salivary amylase: Begins carbohydrate digestion
  • Lingual lipase: Limited initial fat digestion
  • Mucus: Lubricates food bolus
  • Antibacterial compounds: Provide first-line immune defense

Esophagus

• Functions primarily for transport via peristaltic contractions
• Lower esophageal sphincter prevents gastric reflux
• No significant digestive or absorptive function

Stomach

• Produces approximately 2.0 liters of gastric juice daily 1
• Secretes hydrochloric acid (HCl) to:
  • Create acidic environment (pH 1.5-3.5)
  • Activate pepsinogen to pepsin
  • Kill ingested microorganisms
• Gastric enzymes include:
  • Pepsin: Begins protein digestion
  • Gastric lipase: Limited fat digestion, more important during pathological conditions 1
• Mechanical churning creates chyme
• Regulated emptying through pyloric sphincter

Small Intestine

Duodenum

• Receives 1.5 liters of pancreatico-biliary secretions 1
• Neutralizes acidic chyme with bicarbonate
• Bile from gallbladder emulsifies fats
• Pancreatic enzymes include:
  • Amylase: Continues carbohydrate digestion
  • Lipase: Fat digestion
  • Proteases (trypsin, chymotrypsin): Protein digestion
• The duodenum is the primary site where digestive products activate multiple postprandial neuro-humoral mechanisms controlling motor and secretory responses 1

Jejunum

• Primary site for nutrient absorption
• Rich in villi and microvilli (brush border) to maximize surface area
• Absorbs:
  • Monosaccharides (glucose, fructose, galactose)
  • Amino acids and dipeptides
  • Fatty acids and monoglycerides
  • Most vitamins and minerals
• Up to 70-80% of nutrients are absorbed by the duodeno-jejunal junction 1
• Jejunal mucosa is "leaky" with rapid sodium fluxes across it 1

Ileum

• Specialized for absorption of:
  • Vitamin B12 (with intrinsic factor)
  • Bile salts (enterohepatic circulation)
  • Fat-soluble vitamins (A, D, E, K)
• Vitamin B12 and fat malabsorption occurs when more than 60-100 cm of terminal ileum is resected 1
• Contains Peyer's patches (immune function)
• Ileal brake mechanism slows gastric emptying and intestinal transit when nutrients reach the ileum 1

Colon

Ascending Colon

• Receives approximately 1-1.5 liters of fluid daily from small intestine
• Primary functions:
  • Water and electrolyte absorption
  • Bacterial fermentation of undigested carbohydrates
  • Production of short-chain fatty acids
• Houses most of the gut microbiota (approximately 10^14 microorganisms)

Transverse Colon

• Continues water absorption
• Further bacterial fermentation
• Formation of semi-solid stool

Descending Colon

• Final water absorption
• Storage of fecal matter before defecation
• Defecation is a complex process with four phases: basal, pre-expulsive, expulsive and end phase 2

Regulatory Mechanisms

• Neural regulation:

  • Enteric nervous system ("gut brain")
  • Autonomic nervous system (sympathetic and parasympathetic)
  • Vagal-cholinergic reflexes are particularly important for pancreatic secretion 1

• Hormonal regulation:

  • Gastrin: Stimulates gastric acid secretion
  • Cholecystokinin (CCK): Stimulates pancreatic enzyme secretion and gallbladder contraction
  • Secretin: Stimulates pancreatic bicarbonate secretion
  • Ghrelin: Stimulates hunger
  • Peptide YY (PYY) and Glucagon-like peptide-1 (GLP-1): Slow gastric emptying and intestinal transit 1

Integrated Functions

• Digestion: Mechanical and enzymatic breakdown of food
• Absorption: Transfer of nutrients from lumen to bloodstream
• Secretion: Production of digestive enzymes and hormones
• Motility: Coordinated movement of food through the tract
• Immune function: Barrier against pathogens
• Microbiome interaction: Symbiotic relationship with gut bacteria

Adaptive Processes

• Intestinal adaptation attempts to restore total gut absorption after intestinal resection through:
  • Structural adaptation (increasing absorptive area)
  • Functional adaptation (slowing gastrointestinal transit) 1
• Patients with short bowel syndrome often develop hyperphagia (increased food intake) as a compensatory mechanism 1

Clinical Considerations

• Gut health is supported by:
  • Diet rich in fiber from fruits, vegetables, whole grains, and legumes
  • Probiotic foods that introduce beneficial bacteria
  • Adequate hydration (at least 8 cups of water daily)
  • Limited processed foods 3
• The gut-brain connection significantly impacts digestive function, with chronic stress potentially disrupting normal GI physiology 4

Understanding this complex system is essential for addressing common GI disorders, which rank among the most prevalent health conditions worldwide 4.