4.3 The Function of Digestive Organs

The digestive system is a complex network of organs working together to break down food, absorb nutrients, and eliminate waste. From the mouth to the anus, each organ plays a crucial role in transforming what we eat into fuel for our bodies.

As we age, our digestive system undergoes changes that can impact nutrient absorption and overall health. Understanding these changes is vital for maintaining proper nutrition and preventing digestive disorders in older adults.

Digestive System Anatomy and Physiology

Roles of digestive organs

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• Mouth
  • Mechanical digestion: chewing food into smaller pieces increases surface area for enzymatic action
  • Chemical digestion: salivary amylase begins breakdown of carbohydrates (starch) into simpler sugars
• Esophagus
  • Peristalsis: rhythmic muscular contractions propel food from the mouth to the stomach (bolus)
• Stomach
  • Mechanical digestion: churning and mixing food with gastric secretions forms chyme
  • Chemical digestion: pepsin and hydrochloric acid break down proteins into peptides
• Small intestine
  • Duodenum: receives chyme from the stomach and secretions from the pancreas (enzymes) and liver (bile)
  • Jejunum and ileum: majority of nutrient absorption occurs through villi and microvilli
  • Brush border enzymes: complete digestion of carbohydrates (maltase, sucrase, lactase) and proteins (peptidases)
• Pancreas
  • Exocrine function: secretes digestive enzymes (lipase, amylase, trypsin) into the duodenum
  • Endocrine function: releases insulin and glucagon to regulate blood sugar levels
• Liver
  • Produces bile: emulsifies fats into smaller droplets (micelles) for easier digestion by lipase
  • Metabolic functions: nutrient processing, storage (glycogen, vitamins), and detoxification
• Gallbladder
  • Stores and concentrates bile from the liver
  • Releases bile into the duodenum when stimulated by cholecystokinin (CCK) in response to fat
• Large intestine
  • Absorption of water and electrolytes (sodium, potassium) to form solid feces
  • Fermentation of undigested fibers by gut bacteria produces short-chain fatty acids (butyrate)
  • Formation and storage of feces in the descending colon, sigmoid colon, and rectum
• Rectum and anus
  • Storage and elimination of feces through defecation reflex triggered by rectal distension

Digestive System Functions and Regulation

• Digestion: breakdown of food into smaller molecules for absorption
• Absorption: uptake of nutrients from the digestive tract into the bloodstream
• Secretion: release of enzymes, acids, and other substances to aid in digestion
• Motility: movement of food through the digestive tract (e.g., peristalsis)
• Enteric nervous system: network of neurons in the gut wall that regulates digestive functions
• Gastrointestinal hormones: chemical messengers that coordinate digestive processes
• Microbiome: community of microorganisms in the gut that contribute to digestion and overall health

Common digestive disorders

• Gastroesophageal reflux disease (GERD)
  • Weakened lower esophageal sphincter allows stomach acid to reflux into the esophagus
  • Causes heartburn, chest pain, and potential esophageal damage (erosions, strictures, Barrett's esophagus)
• Peptic ulcers
  • Erosion of the stomach (gastric ulcer) or duodenal lining (duodenal ulcer) due to H. pylori infection or NSAIDs
  • Leads to abdominal pain, bleeding, and potential perforation or obstruction
• Inflammatory bowel disease (IBD)
  • Crohn's disease: chronic inflammation throughout the GI tract, most commonly in the ileum and colon
  • Ulcerative colitis: chronic inflammation limited to the colon and rectum
  • Both cause diarrhea, abdominal pain, malnutrition, and increased risk of colorectal cancer
• Celiac disease
  • Autoimmune disorder triggered by gluten consumption in genetically susceptible individuals
  • Damages the small intestinal villi, leading to malabsorption and nutrient deficiencies (iron, calcium, vitamin D)
• Diverticulosis and diverticulitis
  • Diverticulosis: formation of pouches (diverticula) in the colon wall, often asymptomatic
  • Diverticulitis: inflammation and infection of diverticula causing abdominal pain, fever, and potential complications (perforation, abscess, fistula)

Aging and digestive function

• Oral health
  • Tooth loss and periodontal disease can impair chewing and food intake, leading to nutritional deficiencies
  • Decreased salivary flow (xerostomia) affects taste, chewing, and swallowing, increasing risk of dental caries and infections
• Esophageal changes
  • Weakened peristalsis and lower esophageal sphincter can lead to dysphagia (difficulty swallowing) and GERD
• Gastric changes
  • Decreased gastric acid secretion (hypochlorhydria) impairs vitamin B12 absorption, increasing risk of pernicious anemia
  • Delayed gastric emptying can cause early satiety and reduced appetite, contributing to unintentional weight loss
• Intestinal changes
  • Reduced intestinal motility can lead to constipation and fecal impaction
  • Decreased brush border enzyme activity and absorptive surface area may impair nutrient absorption (calcium, vitamin D, B vitamins)
• Pancreatic and hepatic changes
  • Reduced pancreatic enzyme secretion can affect digestion of fats, proteins, and carbohydrates
  • Decreased hepatic function may impair nutrient metabolism, drug clearance, and increase susceptibility to drug-nutrient interactions
• Microbiome alterations
  • Changes in gut bacteria composition (dysbiosis) may influence nutrient absorption, immune function, and risk of infections (C. difficile)
• Polypharmacy
  • Multiple medications can interact with nutrients (calcium, iron, vitamin D) and affect digestive function (constipation, diarrhea, nausea)
• Psychosocial factors
  • Social isolation, depression, and cognitive decline can impact dietary intake, food choices, and overall nutritional status