Digestion: Mechanical and Chemical Processes

Detailed Notes

18.1 Overview: Mechanical and Chemical Digestion

The digestive system performs two interdependent types of digestion, operating simultaneously from the oral cavity through the small intestine.

Mechanical digestion is the physical reduction of food into smaller fragments. It does not change the chemical bonds within nutrients; instead, it dramatically increases the surface area available for enzymatic attack. Processes include mastication (chewing), churning (stomach mixing), segmentation (small intestine mixing), and peristalsis (propulsive waves). Mechanical digestion prepares food for the second phase.

Chemical digestion uses hydrolysis reactions — water is used to split chemical bonds — catalyzed by specific enzymes at each stage of the GI tract. Each macromolecule class is broken down to its most basic absorbable form:

18.2 Oral Cavity: Where Digestion Begins

Digestion starts the moment food enters the mouth, where mechanical and chemical processes combine to form a bolus — a soft, swallowable mass.

Mechanical Digestion: Mastication

Mastication (chewing) is the first and most voluntary form of mechanical digestion. The teeth — incisors (cutting), canines (tearing), and premolars/molars (grinding) — reduce food into smaller pieces. The tongue manipulates food, pushing it between the teeth while mixing it with saliva. The resulting bolus is shaped by the tongue and pushed posteriorly into the oropharynx to initiate swallowing (deglutition).

Mastication increases the surface area of food particles by orders of magnitude, exposing more chemical bonds to enzymatic attack.

Chemical Digestion in the Mouth

Two enzymes in saliva begin chemical digestion, though their contributions are minor relative to later stages:

1. Salivary amylase (produced by salivary glands — parotid, submandibular, sublingual): Hydrolyzes starch (a polysaccharide of α-1,4 linked glucose) into smaller fragments — maltose (a disaccharide) and dextrins. Because food spends only seconds to minutes in the mouth, salivary amylase contributes only about 5% of total starch digestion. It is inactivated by gastric acid once the bolus reaches the stomach, though some activity persists in the center of the bolus until acid penetrates.

1. Lingual lipase (secreted by lingual serous glands of the tongue): Begins limited hydrolysis of triglycerides into fatty acids and diglycerides. Like salivary amylase, its contribution is small; the acid-stable gastric lipase takes over in the stomach.

Saliva also contains:

• Water and mucus: Lubricates food particles and aids bolus formation
• Lysozyme and IgA antibodies: Provide innate immune defense against oral microbes
• Electrolytes (bicarbonate, phosphate): Maintain salivary pH near 6.5–7.0

18.3 Esophagus: Peristaltic Propulsion

The esophagus does not perform chemical digestion or absorption. Its sole role is mechanical — a muscular conduit that propels the bolus from the pharynx to the stomach in about 9–10 seconds.

Peristalsis is a coordinated wave of contraction and relaxation of smooth muscle in the esophageal wall, controlled by the enteric nervous system and modulated by vagal input. There are two types:

1. Primary peristalsis: Initiated by the act of swallowing. The swallowing center in the medulla oblongata sends signals via the vagus nerve (CN X) to the esophageal musculature. A contraction wave starts at the upper esophageal sphincter (UES) and travels down to the lower esophageal sphincter (LES), which relaxes to allow the bolus into the stomach, then closes to prevent reflux.

1. Secondary peristalsis: Initiated by distension (stretching) of the esophageal wall by residual food. If primary peristalsis fails to clear the bolus entirely, local stretch receptors activate the enteric nervous system to generate a new peristaltic wave without requiring a swallow. This is a local reflex.

18.4 Stomach: Mechanical and Chemical Digestion

The stomach is a muscular, J-shaped organ that serves as a temporary reservoir where food is converted into a semi-liquid paste called chyme. It performs both extensive mechanical digestion and the first major phase of protein digestion.

Mechanical Digestion in the Stomach

Receptive Relaxation: As the bolus enters the stomach, the smooth muscle of the gastric fundus and body relaxes to accommodate the incoming volume without a significant rise in intragastric pressure. This is mediated by the vagus nerve and local reflexes.

Mixing Waves: Once food is present, gentle peristaltic mixing waves begin in the body and propagate toward the pylorus. These waves:

• Start as weak ripples in the upper body
• Intensify as they approach the pyloric antrum
• Mix food with gastric secretions to form chyme
• Become increasingly forceful — the pyloric sphincter remains mostly closed; each wave squirts only a few milliliters of chyme into the duodenum while most is retropulsed back into the stomach for further mixing. This is called retropulsion.

Gastric Emptying: The pyloric sphincter controls the rate at which chyme enters the duodenum. Emptying is regulated by:

• Gastric factors: Degree of distension (stretch) — more volume stimulates emptying via vagovagal reflexes and gastrin release
• Duodenal factors: The presence of acid, fat, hypertonic solutions, and distension in the duodenum triggers the enterogastric reflex and the release of CCK (cholecystokinin), secretin, and GIP (gastric inhibitory peptide) — all of which slow gastric emptying to prevent overwhelming the duodenum's processing capacity

A typical meal takes 2–4 hours to empty from the stomach.

Chemical Digestion in the Stomach

Hydrochloric Acid (HCl):

• Secreted by parietal cells in the gastric glands of the fundus and body
• Lowers gastric pH to 1.5–2.0
• Functions: denatures proteins (unfolds tertiary/quaternary structure, exposing peptide bonds), converts pepsinogen to pepsin (by cleaving an inhibitory peptide at low pH), kills most ingested pathogens, and provides the optimal acidic pH for pepsin activity
• HCl secretion is stimulated by acetylcholine (vagus), gastrin (G cells), and histamine (ECL cells)
• The proton pump (H⁺/K⁺-ATPase) in the parietal cell apical membrane is the final common pathway for acid secretion

Pepsin:

• Secreted by chief cells as the inactive zymogen pepsinogen
• In the acidic gastric lumen, pepsinogen auto-catalytically cleaves to form active pepsin (optimal pH ~2.0)
• Pepsin is an endopeptidase — it cleaves internal peptide bonds, preferentially near aromatic amino acids (tyrosine, phenylalanine, tryptophan)
• Breaks large proteins into smaller polypeptides and some free amino acids
• Pepsin is irreversibly inactivated above pH 5.0, which occurs when chyme mixes with alkaline pancreatic juice in the duodenum

Gastric Lipase:

• Secreted by chief cells; acid-stable (unlike salivary amylase, which is denatured)
• Hydrolyzes triglycerides, particularly those with short- and medium-chain fatty acids
• Accounts for about 10–30% of total triglyceride digestion; more important in neonates (milk fat digestion)
• Does not require bile salts for activity

Intrinsic Factor:

• Also secreted by parietal cells
• A glycoprotein essential for vitamin B₁₂ (cobalamin) absorption in the terminal ileum
• Binds B₁₂ in the stomach and protects it from degradation until the complex is recognized by cubilin receptors in the ileum
• Pernicious anemia results from autoimmune destruction of parietal cells, causing intrinsic factor deficiency and consequent B₁₂ malabsorption

Summary: Gastric Secretions

18.5 Small Intestine: The Primary Site of Digestion and Absorption

The small intestine is where chemical digestion is completed and virtually all nutrient absorption occurs. It is structurally adapted for this role: circular folds (plicae circulares), villi, and microvilli — together providing a surface area of approximately 200–250 m².

Mechanical Digestion in the Small Intestine

Segmentation: The dominant mixing pattern in the fed state. Localized, rhythmic contractions of circular smooth muscle divide and re-divide the intestinal contents, sloshing them back and forth without net forward propulsion. Segmentation:

• Mixes chyme with pancreatic juice, bile, and intestinal secretions
• Brings digested nutrients into contact with the absorptive epithelium
• Is controlled by the basic electrical rhythm (BER) of smooth muscle pacemaker cells — the interstitial cells of Cajal (ICC) — which set the frequency of segmentation (about 12 contractions/min in the duodenum, decreasing distally)

Peristalsis: Occurs less frequently than segmentation but provides net aboral (downstream) propulsion. A contraction ring forms and moves distally, pushing chyme forward. Peristalsis is coordinated by the myenteric plexus (Auerbach's plexus) of the enteric nervous system.

Migrating Motor Complex (MMC): A distinct pattern of electromechanical activity that occurs between meals (in the fasting state), approximately every 90–120 minutes. The MMC consists of four phases and serves as a "housekeeper" — sweeping residual indigestible material, desquamated cells, and bacteria from the small intestine into the large intestine. The hormone motilin (secreted cyclically by M cells of the duodenum) initiates the MMC. Disruption of the MMC may contribute to small intestinal bacterial overgrowth (SIBO).

Chemical Digestion in the Small Intestine — Carbohydrates

Carbohydrate digestion in the small intestine is completed in two stages:

1. Pancreatic amylase (from pancreatic acinar cells, delivered via the pancreatic duct): Continues the work salivary amylase began. It hydrolyzes starch and glycogen at α-1,4 glycosidic bonds, producing oligosaccharides (maltotriose, α-dextrins) and the disaccharide maltose. Pancreatic amylase cannot cleave α-1,6 branch points or terminal α-1,4 bonds.

1. Brush border enzymes (embedded in the microvillus membrane of enterocytes) complete the job:
   • Maltase: Maltose → glucose + glucose
   • Sucrase: Sucrose → glucose + fructose
   • Lactase: Lactose → glucose + galactose
   • α-Dextrinase (isomaltase): Cleaves α-1,6 glycosidic bonds at branch points of dextrins

The resulting monosaccharides (glucose, fructose, galactose) are then absorbed across the enterocyte membrane via specific transporters (SGLT1 for glucose/galactose, GLUT5 for fructose).

Chemical Digestion in the Small Intestine — Proteins

Protein digestion involves an elegant safety mechanism: the pancreas releases proteases as inactive proenzymes (zymogens) to prevent autodigestion of pancreatic tissue.

Activation Cascade:

1. The duodenal mucosa releases enteropeptidase (enterokinase) — a brush border enzyme
2. Enteropeptidase cleaves trypsinogen (pancreatic zymogen) → active trypsin
3. Trypsin then activates other zymogens:
   • Chymotrypsinogen → chymotrypsin
   • Procarboxypeptidase → carboxypeptidase
   • Proelastase → elastase
   • More trypsinogen → trypsin (positive feedback amplification)

Proteolytic Enzymes and Their Specificities:

The end products are free amino acids, dipeptides, and tripeptides, all of which are absorbed across the enterocyte membrane.

Chemical Digestion in the Small Intestine — Lipids

Lipid digestion is unique because lipids are hydrophobic and tend to coalesce into large droplets in the aqueous environment of the intestinal lumen. The digestive strategy has three coordinated components: emulsification, enzymatic hydrolysis, and micelle formation.

1. Emulsification by Bile Salts: Bile salts (synthesized from cholesterol in the liver) have both hydrophobic and hydrophilic regions, making them amphipathic. They coat large fat globules in the duodenum and, through the mechanical action of segmentation, break them into much smaller emulsion droplets (~1 µm diameter). This massively increases the surface area for lipase action.

1. Enzymatic Hydrolysis by Pancreatic Lipase and Colipase: Pancreatic lipase is the workhorse of lipid digestion. It hydrolyzes triglycerides at the sn-1 and sn-3 positions, yielding 2-monoglycerides and free fatty acids. However, pancreatic lipase is inhibited by bile salts covering the emulsion droplet surface. Colipase (also secreted by the pancreas as procolipase and activated by trypsin) solves this problem: it anchors pancreatic lipase to the emulsion droplet surface, displacing bile salts and allowing lipase to access the triglycerides.

1. Micelle Formation: The products of lipid digestion — monoglycerides, free fatty acids, cholesterol, and fat-soluble vitamins — are poorly soluble in water. Bile salts assemble these into micelles (4–8 nm diameter), with hydrophobic cores sequestering the lipid products and hydrophilic exteriors facing the aqueous lumen. Micelles transport lipid digestion products across the unstirred water layer to the enterocyte surface for absorption.

Chemical Digestion in the Small Intestine — Nucleic Acids

Nucleic acids (DNA and RNA) from dietary sources are broken down in two stages:

1. Pancreatic Nucleases:
   • Ribonuclease (RNase) and Deoxyribonuclease (DNase) from pancreatic acinar cells
   • Hydrolyze RNA and DNA into nucleotides (short chains of a few nucleotides each)

1. Brush Border Enzymes:
   • Nucleosidases (nucleotidases): Remove phosphate groups, converting nucleotides into nucleosides
   • Phosphatases: Further hydrolyze nucleosides into free nitrogenous bases, pentose sugars, and phosphate

The final products — nucleosides, free bases, and sugars — are absorbed across the enterocyte membrane.

18.6 Pancreatic Juice: The Complete Digestive Cocktail

The pancreas is both an endocrine organ (islets of Langerhans — insulin, glucagon) and an exocrine organ (acinar and duct cells — digestive secretions). The exocrine pancreas produces 1.2–1.5 liters of pancreatic juice per day.

Acinar Cells: Secrete the digestive enzymes:

• Proteases (as zymogens): Trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase
• Pancreatic α-amylase: Starch → oligosaccharides
• Pancreatic lipase: Triglycerides → monoglycerides + free fatty acids
• Colipase (as procolipase): Anchors lipase to lipid droplets
• Nucleases: Ribonuclease, deoxyribonuclease
• Phospholipase A₂ (as proenzyme): Hydrolyzes phospholipids

Duct Cells: Secrete a bicarbonate-rich (HCO₃⁻) alkaline fluid:

• Neutralizes acidic chyme entering the duodenum from the stomach (raises pH from ~2.0 to ~7.0–8.0)
• Provides the optimal pH for pancreatic and intestinal enzymes, which function at neutral to slightly alkaline pH
• Secretin (released by duodenal S cells in response to low pH) is the primary hormonal stimulus for duct cell HCO₃⁻ secretion

Regulation of Pancreatic Secretion:

18.7 Bile: Emulsification and Excretion

Bile is both a digestive secretion and an excretory route for waste products. The liver produces 500–1000 mL of bile per day.

Synthesis and Storage:

• Hepatocytes synthesize bile continuously
• Bile flows through the hepatic ducts into the common hepatic duct
• Between meals, the hepatopancreatic sphincter (sphincter of Oddi) is contracted, forcing bile to back up through the cystic duct into the gallbladder
• The gallbladder concentrates bile 5–10 fold by absorbing water and electrolytes
• After a meal, CCK causes gallbladder contraction and sphincter of Oddi relaxation, delivering concentrated bile into the duodenum

Composition of Bile:

Enterohepatic Circulation: Bile salts are valuable molecules — the body synthesizes only ~0.2–0.6 g/day to replace fecal losses, yet secretes 20–30 g/day into the duodenum. This is possible because 95% of bile salts are reabsorbed in the terminal ileum via specific sodium-dependent bile acid transporters, returned to the liver via the portal vein, and re-secreted into bile. This recycling loop is the enterohepatic circulation. Bile salts may cycle 2–3 times per meal and 6–8 times per day.

18.8 Large Intestine: Fermentation and Water Absorption

The large intestine (colon) does not perform significant chemical digestion — no digestive enzymes are secreted by the colonic mucosa. Its functions are:

1. Water and Electrolyte Absorption: The colon absorbs approximately 1.0–1.5 L of water per day, converting the ~1.5 L of chyme residue entering from the ileum into ~100–200 mL of formed feces. Sodium is actively absorbed (via ENaC channels and Na⁺/K⁺-ATPase), and water follows osmotically. Potassium and chloride are also absorbed or secreted as needed.

1. Bacterial Fermentation: The colon houses an enormous population of gut flora (the gut microbiome — ~10¹¹–10¹² bacteria/mL). These commensal bacteria ferment indigestible carbohydrates (dietary fiber, resistant starch, undigested oligosaccharides), producing:
   • Short-chain fatty acids (SCFAs): Acetate, propionate, and butyrate — absorbed by colonocytes as an energy source
   • Gases: Hydrogen, carbon dioxide, and methane (in ~30% of people) — responsible for flatus
   • Vitamins: Vitamin K (essential for synthesis of clotting factors II, VII, IX, X) and several B vitamins (biotin, B₁₂, folate, thiamine — though absorption of bacterially produced B₁₂ is minimal as it occurs downstream of the ileal B₁₂ receptor)

1. Feces Formation: Feces consist of ~75% water and ~25% solids. The solids include:
   • Undigested fiber (cellulose, lignin)
   • Bacteria (~30% of solid mass)
   • Inorganic material
   • Small amounts of fat and protein

Haustral contractions and mass movements (strong peristaltic waves occurring 1–3 times per day, often after meals — the gastrocolic reflex) propel contents toward the rectum for defecation.

18.9 Summary Table: Regional Digestion

18.10 Clinical Correlations

Lactose Intolerance: Deficiency of the brush border enzyme lactase, which hydrolyzes the disaccharide lactose into glucose and galactose. Undigested lactose remains in the intestinal lumen, drawing water by osmosis → osmotic diarrhea. Colonic bacteria ferment the lactose → production of gas (H₂, CO₂) and SCFAs → bloating, cramping, flatulence. Prevalence varies by ethnicity: >90% in some East Asian populations, ~10–15% in Northern Europeans. Management: dietary lactose restriction, lactase enzyme supplements.

Pancreatitis: Inflammation of the pancreas, most commonly caused by gallstones (obstructing the pancreatic duct) and chronic alcohol use. In acute pancreatitis, premature activation of trypsinogen within the pancreas leads to autodigestion of pancreatic tissue, causing severe epigastric pain radiating to the back, elevated serum amylase and lipase, and potentially systemic complications (SIRS, ARDS, multi-organ failure). Chronic pancreatitis leads to progressive destruction of both exocrine (malabsorption, steatorrhea) and endocrine (diabetes mellitus) pancreatic tissue.

Gallstones (Cholelithiasis): Precipitation of bile components — most commonly cholesterol stones (>80% in Western populations) — in the gallbladder. Risk factors: female sex (estrogen increases cholesterol secretion into bile), obesity, rapid weight loss, age >40, certain ethnicities (e.g., Pima Native Americans). When a stone obstructs the cystic duct, the gallbladder cannot empty (CCK stimulates contraction against an obstructed outflow), causing biliary colic — severe right upper quadrant pain, often postprandial (especially after fatty meals). If the stone lodges in the common bile duct, obstructive jaundice results from bilirubin backing up into the bloodstream.

Cystic Fibrosis (CF): An autosomal recessive disorder caused by mutations in the CFTR gene (cystic fibrosis transmembrane conductance regulator), which encodes a chloride channel. In the pancreas, defective Cl⁻ secretion → thickened, dehydrated pancreatic secretions → plugging of pancreatic ducts → failure of pancreatic enzymes to reach the duodenum (pancreatic insufficiency). Consequences: malabsorption of fats and fat-soluble vitamins (A, D, E, K), steatorrhea (fatty, foul-smelling stools), and failure to thrive in children despite adequate caloric intake. Management: pancreatic enzyme replacement therapy (PERT) taken with meals.

Celiac Disease: An autoimmune disorder triggered by ingestion of gluten (a protein found in wheat, barley, and rye) in genetically susceptible individuals (HLA-DQ2 or HLA-DQ8). The immune response targets the enzyme tissue transglutaminase (tTG) in the small intestinal mucosa, leading to villous atrophy (flattening of intestinal villi), crypt hyperplasia, and lymphocytic infiltration. Consequences: loss of absorptive surface area → malabsorption of multiple nutrients, iron-deficiency anemia, diarrhea, weight loss, and dermatitis herpetiformis (a pruritic skin rash). Diagnosis: serology (anti-tTG IgA), confirmed by duodenal biopsy. Treatment: lifelong strict gluten-free diet.

Gastroesophageal Reflux Disease (GERD): Chronic reflux of gastric contents (acid and pepsin) into the esophagus due to dysfunction of the lower esophageal sphincter (LES). The esophageal mucosa lacks the protective mucus-bicarbonate barrier of the stomach and is vulnerable to acid injury → erosive esophagitis, heartburn (retrosternal burning), regurgitation, and in chronic cases, Barrett's esophagus (metaplasia of distal esophageal epithelium from squamous to columnar — a premalignant condition increasing risk of esophageal adenocarcinoma). Contributing factors: hiatal hernia, obesity, pregnancy, certain foods (fatty meals, caffeine, alcohol, chocolate), and medications that reduce LES tone.