🫀Anatomy and Physiology II Unit 7 – Digestive System: Processes & Absorption

Key Structures and Organs

• Gastrointestinal tract consists of a continuous tube from mouth to anus (esophagus, stomach, small intestine, large intestine)
• Accessory organs aid in digestion but are not part of the GI tract (liver, gallbladder, pancreas)
  • Liver produces bile that emulsifies fats and neutralizes stomach acid
  • Gallbladder stores and concentrates bile before releasing it into the small intestine
  • Pancreas secretes digestive enzymes and bicarbonate to neutralize stomach acid
• Layers of the GI tract include mucosa, submucosa, muscularis externa, and serosa
• Mucosa contains epithelial cells, goblet cells that secrete mucus, and enteroendocrine cells that release hormones
• Muscularis externa has smooth muscle layers responsible for peristalsis and segmentation

Digestive Process Overview

• Digestion involves ingestion, mechanical and chemical breakdown, absorption, and elimination
• Mechanical digestion physically breaks down food into smaller pieces (chewing, churning in stomach)
• Chemical digestion uses enzymes to break down macromolecules into absorbable units
  • Carbohydrates are broken down into monosaccharides (glucose, fructose, galactose)
  • Proteins are broken down into amino acids
  • Lipids are broken down into fatty acids and monoglycerides
• Absorption occurs primarily in the small intestine, where nutrients are transported into the bloodstream
• Elimination removes undigested material and waste products from the body through defecation

Mouth to Stomach: Initial Breakdown

• Mastication (chewing) mechanically breaks down food and mixes it with saliva
• Saliva contains salivary amylase, which begins the chemical digestion of carbohydrates
• Bolus (moistened food mass) is swallowed and passes through the esophagus via peristalsis
• Lower esophageal sphincter relaxes to allow the bolus to enter the stomach
• Stomach churns and mixes food with gastric secretions (hydrochloric acid, pepsin, lipase)
  • Hydrochloric acid denatures proteins and activates pepsinogen to pepsin
  • Pepsin begins the chemical digestion of proteins into smaller polypeptides
  • Lipase starts the breakdown of triglycerides into fatty acids and diglycerides
• Stomach mucosa is protected from acid and enzymes by a layer of mucus and bicarbonate

Small Intestine: Major Absorption Site

• Chyme (semi-liquid mixture of partially digested food) enters the duodenum from the stomach
• Pancreatic enzymes (amylase, trypsin, chymotrypsin, lipase) and bile are released into the duodenum
  • Pancreatic amylase continues carbohydrate digestion into disaccharides
  • Trypsin and chymotrypsin break down polypeptides into smaller peptides
  • Pancreatic lipase further digests lipids into fatty acids and monoglycerides
• Brush border enzymes on intestinal villi complete digestion (maltase, sucrase, lactase, peptidases)
• Absorption occurs through intestinal villi and microvilli, which increase surface area
  • Monosaccharides and amino acids are absorbed via active transport
  • Fatty acids and monoglycerides are absorbed via passive diffusion and form chylomicrons
• Absorbed nutrients enter the bloodstream (amino acids, monosaccharides) or lymphatic system (chylomicrons)

Large Intestine: Final Stages

• Undigested material enters the large intestine (cecum, colon, rectum)
• Primary functions include water and electrolyte absorption, storage of feces, and microbial fermentation
• Gut microbiota ferment undigested fibers, producing short-chain fatty acids and vitamins (K, B12)
• Feces consist of undigested food, bacteria, mucus, and shed epithelial cells
• Defecation reflex is triggered by rectal distension, resulting in peristalsis and relaxation of the anal sphincters

Enzymes and Chemical Processes

• Enzymes are specific to their substrates and have optimal pH ranges
  • Salivary amylase: pH 6.8, breaks down starch into maltose
  • Pepsin: pH 1.5-2.5, breaks down proteins into polypeptides
  • Pancreatic enzymes: pH 7-8, continue digestion of carbohydrates, proteins, and lipids
  • Brush border enzymes: pH 7-8, complete digestion of disaccharides and peptides
• Bile salts emulsify fats, increasing surface area for lipase action
• Hydrolysis reactions break down macromolecules by adding water (carbohydrates, proteins, lipids)
• Denaturation of proteins by acid and pepsin unfolds their structure, making them more accessible to enzymes

Nutrient Absorption Mechanisms

• Simple diffusion: Passive movement of molecules down their concentration gradient (short-chain fatty acids, water)
• Facilitated diffusion: Passive movement through carrier proteins (fructose)
• Active transport: Movement against concentration gradient using ATP (glucose, amino acids)
  • Sodium-glucose cotransporter (SGLT1) couples glucose absorption with sodium
• Endocytosis: Uptake of larger molecules or particles (vitamin B12, cholesterol)
• Tight junctions between epithelial cells prevent paracellular leakage and maintain selective permeability
• Absorbed nutrients are transported via the hepatic portal vein to the liver for processing and distribution

Common Digestive Disorders

• Gastroesophageal reflux disease (GERD): Backflow of stomach acid into the esophagus, causing heartburn and damage
• Peptic ulcers: Open sores in the stomach or duodenum, often caused by Helicobacter pylori or NSAIDs
• Inflammatory bowel diseases (Crohn's disease, ulcerative colitis): Chronic inflammation of the GI tract
  • Crohn's disease can affect any part of the GI tract, often the ileum and colon
  • Ulcerative colitis is limited to the colon and rectum
• Celiac disease: Autoimmune disorder triggered by gluten, leading to damage of the small intestine villi
• Lactose intolerance: Inability to digest lactose due to lack of lactase enzyme, causing bloating and diarrhea
• Irritable bowel syndrome (IBS): Functional disorder characterized by abdominal pain, bloating, and altered bowel habits