Chatper 16. Digestive System
Recommended Article: 【Biology】 Table of Contents for Biology
1. Key Steps in Food Processing
2. Step 1: Oral Cavity, Pharynx, Esophagus
7. Regulation of Appetite Hormones
1. Key Steps in Food Processing
⑴ Four main steps
① Ingestion: The act of eating
○ Suspension feeders: Animals that eat suspended particles in liquids (e.g., humpback whales)
○ Substrate feeders: Animals that eat their environment (substrate) (e.g., caterpillars, maggots)
○ Fluid feeders: Animals that feed on bodily fluids (e.g., mosquitoes)
○ Bulk feeders: Animals that consume relatively large food pieces (e.g., African rock python)
② Digestion: The process of breaking down food into small molecules that can be absorbed by the body
○ Peristalsis (mechanical digestion): The process by which the smooth muscles of the digestive tract wall contract rhythmically to create waves that propel food forward.
○ Segmentation (mechanical digestion): In the case of larger food particles, rhythmic contractions of smooth muscle break the food into smaller pieces.
○ Chemical digestion: The process of breaking down polymers into monomers through enzymatic hydrolysis.
③ Absorption: The process of absorbing amino acids, monosaccharides, etc.
④ Excretion and Elimination: The process of expelling undigested materials from the digestive tract
⑤ Why polymers can’t be used directly
○ Cellular absorption: Polymers can’t pass through cell membranes
○ Species specificity
○ Polymers in animals’ bodies don’t match those in food
○ However, all organisms use the same monomers to build their own polymers
○ Immune response
⑵ Classification of Digestion by Digestive Compartment
① Digestive compartments: Digestion occurs only within specialized compartments, reducing the risk of self-digestion.
② Intracellular digestion
○ Phagosome: Simplest digestive compartment, where digestion occurs after fusion with lysosomes
○ Endocytosis, Phagocytosis
○ Sponge animals: Digest food via intracellular digestion
③ Extracellular digestion
○ Occurs in compartments continuously connected to the external environment
○ Can digest larger food particles compared to intracellular digestion
○ Gastrovascular cavity
○ Performs digestion and distribution of nutrients to the entire body
○ Observed in platyhelminthes.
○ Hydra: Digestive enzymes are secreted from the gland cells of the gastrodermis lining the gastrovascular cavity.
○ Undigested materials exit through a single opening that serves as both mouth and anus
○ Complete digestive tract, alimentary canal
○ Food moves in one direction
○ Allows step-by-step digestion and absorption
○ Can eat new food before previously eaten food is completely digested
⑶ Tissue Layers of Vertebrate Digestive Tract
① Structure: Mucosa, Submucosa, Submucosal Plexus, Two Layers of Smooth Muscle Cells, Serosa
② The digestive tract generally maintains the longitudinal muscles contracted and the circular muscles relaxed, thereby increasing the surface area of the canal wall.
③ Internal circular muscle: Segmentation, prevents backflow
④ External longitudinal muscle: Peristalsis
Figure 1. Tissue Layers of Vertebrate Digestive Tract
2. Step 1: Oral Cavity, Pharynx, Esophagus
⑴ Teeth: Mastication (mechanical digestion)
① Carnivores: Well-developed sharp incisors and canines, pointed small premolars and molars for breaking and cutting food
② Herbivores: Broad, ridged teeth with wide surfaces for grinding tough plant material.
③ Omnivores: Possess a less specialized dentition.
④ Human teeth: Enamel layer, dentin layer, and pulp cavity.
⑵ Saliva
① Secreted by salivary glands even before eating through reflexes or conditioning
② Salivary Glands
○ Secretion of saliva from acinar cells
○ The salivary glands consist of three pairs in total: the sublingual glands, the parotid glands, and the submandibular glands.
○ Pathway of salivary secretion: Food → Taste receptors on the tongue → Sensory nerves → Medulla oblongata → Motor nerves → Salivary secretion
○ After saliva is produced in the acinar cells, it passes through the salivary ducts, during which ions are secreted and reabsorbed by ductal cells as saliva is released.
Figure 2. The composition of saliva depending on the rate of salivary secretion
③ Components of Saliva
○ Provides moisture to food
○ Mucin: Acts as a lubricant, making food slippery for easy swallowing
○ Bicarbonate ions (HCO3-): Neutralizes oral acidity (buffer) → Prevents tooth decay
○ Lysozyme
○ Breaks β 1→4 linkage of peptidoglycan
○ Peptidoglycan is a major component of bacterial cell walls
○ Amylase
○ Converts polysaccharides (no taste) → maltose (sweet taste)
○ Irregular hydrolysis of glycosidic bonds in starch, breaking it down into maltose and dextrins.
○ Fatty acids: Makes saliva slightly acidic, and inhibits bacterial growth
○ Lingual lipase
○ In newborns, since digestive enzymes are still immature, fat can be broken down by lingual lipase.
○ IgA: Antibodies that remove parasites, etc.
④ Regulation of Saliva Secretion
○ Regulated by the medulla oblongata, the autonomic nervous center.
○ Sympathetic nervous system inhibits saliva secretion, and parasympathetic nervous system stimulates it.
○ The sympathetic nervous system inhibits saliva secretion, resulting in increased viscosity of saliva.
⑶ Process of Swallowing Food
① 1st. When food is not being swallowed: Contraction of esophageal sphincters, elevation of epiglottis → Opening of airway, closure of esophagus
② 2nd. Bolus of food reaches the pharynx, triggering the swallowing reflex (deglutition reflex)
○ Larynx: Upper part of the respiratory tract
○ As the larynx moves upward, it pushes the epiglottis backward → the epiglottis closes over the glottis, the entrance to the airway (closing the airway).
○ Relaxation of esophageal sphincter → Opening of esophagus
○ Controlled by the medulla
③ 3rd. Food enters the esophagus, the larynx moves downward, and the airway opens.
○ Uvula rises to prevent food from going backward
④ 4th. Waves of muscle contraction (peristalsis) move the bolus down the esophagus to reach the stomach.
⑷ Esophagus: Peristalsis, Segmentation, Mucus (lubrication)
① Upper Esophageal Sphincter: Striated muscle, involved in deglutition reflex
② Lower Esophageal Sphincter (LES, Cardiac Sphincter): Smooth muscle, involved in peristalsis
○ Located below the diaphragm, regulated under the same intra-abdominal pressure as the stomach.
○ Prevents food from refluxing into the esophagus.
○ Normally remains closed, but when stimulated by food, the LES reflexively relaxes and opens.
○ In late pregnancy, increased intra-abdominal pressure from the upward displacement of the abdominal cavity (toward the thoracic cavity) can compress the stomach, occasionally causing the LES to open.
○ Heartburn: impaired control of the LES → reflux of gastric juice → esophageal damage → ulcer
○ Vomiting
○ Controlled by the medulla oblongata (vomiting center).
○ Serves as an instinctive protective function against toxic substances.
○ Vomiting reflex is triggered by gastric or intestinal distension, stimulation of intestinal walls, chemical receptors in the brain, or head rotation.
○ During vomiting, the gastric outlet closes while the inlet loosens, causing food to flow backward.
○ Vomiting not only damages the esophagus but also raises gastric pH, impairing proper digestive function.
3. Step 2: Stomach
⑴ Structure
Figure 3. Structure of the Stomach
① Muscular pouch-shaped organ
② Cardiac portion (cardia) connects to the esophagus, and pyloric portion (pylorus) connects to the duodenum
③ Ways stomach wall cells protect themselves from proteolytic enzyme pepsin and strong acid hydrochloric acid
○ Formation of a protective barrier by mucus
○ Epithelial cells of stomach wall form tight junctions to protect the interior of the stomach wall
○ Rapid cell division of basal cells helps repair damaged cells, regeneration every 3 days
○ Secretion of inactive hydrolases (e.g., pepsinogen, trypsinogen, chymotrypsinogen)
④ Structure of the stomach muscle: mucosa – submucosa – circular muscle – longitudinal muscle – serosa
○ Submucosa: Contains blood vessels such as veins and arteries; main habitat of Helicobacter pylori
○ Serosa: Connected to the peritoneum through the mesentery
○ Vagus nerve: Extrinsic nerve that regulates the enteric nervous plexus
⑵ Functions: Storage (2 ~ 3 L), Digestion, Defense
① Residence Time: Carbohydrates ＜ Proteins ＜ Lipids
② Prolonged residence time of lipids inhibits digestion
③ Absorption: Alcohol
⑶ Mechanical Digestion: Peristalsis, Mixing Movement
① Peristalsis: Cajal cells generate spontaneous pacemaker activity, promoting smooth muscle movement.
② As stomach expands due to food, mechanical digestion is enhanced
⑷ Chemical Digestion (2 L/day): Protein digestion by gastric juice
① Gastric Juice Secretion: Food → Stimulation of stomach wall → Gastrin secretion → Blood → Stomach gland stimulation → Gastric juice secretion
② Pepsinogen
○ Endopeptidase: Hydrolyzes peptide bonds within proteins to break them down into polypeptides.
○ Optimal pH for pepsinogen is 2.
○ Pepsinogen secretion from chief cells, activated into pepsin by hydrochloric acid.
○ Pepsin reactivates pepsinogen (auto-catalysis, positive feedback).
○ Pepsin recognizes N-termini of Phe, Trp, Tyr to hydrolyze peptide bonds on the amino group side.
③ Hydrochloric Acid (HCl)
○ Function: Disrupts tertiary structure of proteins for digestion.
○ Parietal cell: The H⁺-K⁺-ATPase on the parietal cell membrane actively transports H⁺ and passively transports Cl⁻, resulting in the secretion of hydrochloric acid.
○ Cl-/HCO3- cotransport: Secondary active transport, Cl- imported from blood vessel, HCO3- exported to blood vessel.
○ H+/Cl- cotransport: Primary active transport, secretion towards stomach.
○ Result: Stomach interior becomes acidic, blood pH increases.
○ Stomach acid maintained at pH 1 ~ 3 → Tissue breakdown, pathogen removal.
○ Active function: Activates pepsinogen to pepsin, prorennin to rennin.
○ Alkaline tide: A phenomenon in which animals such as crocodiles and snakes experience an increase of 0.5 to 1.0 in blood pH after feeding, due to excessive acid secretion; considered a cause of postprandial drowsiness.
○ The higher the protein content in food, the greater the buffering effect against acid, thereby stimulating hydrochloric acid secretion.
④ Mucus
○ Mucin (glycoprotein), cells, salt, water mixture secreted by mucous cells.
○ Function: Protects stomach wall, and provides lubrication.
○ Stomach epithelial cells regenerate every 3 days to protect the stomach wall.
○ Gastric ulcer
○ Ammonia secreted by Helicobacter pylori helps survival of themselves in stomach acid.
○ Ammonia additionally disrupts mucin function, leading to gastric ulcers.
⑤ Gastrin
○ When the pyloric region of the stomach is stimulated physically or chemically (by proteins or peptides), the G cells located in the pylorus secrete gastrin.
○ Gastrin, histamine, and the parasympathetic nervous system: stimulate gastric juice secretion.
⑥ Somatostatin
○ Function: Inhibits excessive gastric juice secretion.
○ D cells secrete somatostatin.
○ Somatostatin has various other functions like GH inhibition, TSH inhibition, link.
⑦ Prorennin: Rennin coagulates casein, the protein in milk, making it easier to digest; present in the gastric juice of infants.
⑧ Mucin: Secreted by mucous cells, and protects stomach wall composed of proteins from hydrochloric acid and pepsin.
⑨ Gastric Juice (Chyme): Acidic fluid formed by mixing partially digested food with gastric juice, which passes through pylorus to duodenum.
⑸ Gastric Motility
① Smooth muscle contracts and relaxes continuously, forming acidic chyme.
② Pyloric sphincter muscle: Located between stomach and duodenum, and controls pyloric reflex.
○ When closed: duodenal distension, increased acidity, increased fat, hypertonic solution → increased enterogastrone → long reflex, short reflex → inhibition of pyloric sphincter relaxation → maximization of digestive efficiency
○ When opened: When the contents of the duodenum become alkaline (digestion in the duodenum is completed), the pylorus reflexively opens
○ Regulation by enterogastrone (a collective term for small intestinal hormones)
○ Long reflex: Increased sympathetic activity, decreased parasympathetic activity
○ Short reflex: Involves the enteric nervous plexus
⑹ Three phases of gastric acid secretion control: Cephalic and intestinal parts of stomach are involved.
Figure 4. Regulation of Gastric Acid Secretion, 3 Stages
① The first active stage
○ 1st. Stimulus: Amino acids in food stimulate G cells.
○ 2nd. G cells secrete gastrin.
○ 3rd. Gastrin travels via blood to stimulate wall cells in stomach.
○ 4th. Wall cells secrete HCl to lower stomach pH.
② The second active stage
○ 1st. Medulla oblongata stimulates vagus nerve.
○ 2nd. Vagus nerve (parasympathetic) secretes acetylcholine to stimulate G cells and enterochromaffin-like cells.
○ 3rd. Enterochromaffin-like cells secrete histamine to stimulate wall cells.
○ 4th. Wall cells secrete HCl to lower stomach pH.
③ Inhibition stage
○ 1st. If stomach pH becomes too low, D cells are stimulated.
○ 2nd. D cells secrete somatostatin into blood.
○ 3rd. Somatostatin travels via blood to stimulate wall cells.
○ 4th. Wall cells reduce gastrin secretion.
⑺ Helicobacter pylori
① Survives in very acidic conditions by secreting mucin that neutralizes acid.
② Since mucin cannot cover the stomach wall where this bacterium adheres, the wall becomes damaged and may develop into a gastric ulcer.
⑻ Stomach of ruminants
① Rumen: Symbiotic intestinal microorganisms synthesize cellulase.
② Reticulum: Symbiotic intestinal microorganisms synthesize cellulase.
③ Omasum: Absorbs water.
④ Abomasum: Carries out digestive functions.
4. Stage 3: Small Intestine
⑴ Small Intestine
① Place where the hydrolysis of food polymers and absorption of nutrients mainly occur
② Peristalsis, segmentation, chemical digestion are all performed
○ Peristalsis: Carried out by Cajal cells, as in the stomach.
③ Villi of the small intestine
Figure. 5. Villi of the Small Intestine
○ Mucosal folds: The small intestine remains in a partially contracted state due to the longitudinal muscle, creating folds on the mucosal surface (×3)
○ Villi: Magnification of each fold reveals finger-like projections called villi (×10)
○ Microvilli: Located on the epithelial cells of the villi
○ Various hydrolytic enzymes are anchored and secreted at the brush border
○ On each villus, strands of the glycocalyx project outward (×20), with a total surface area of about 300 ㎡
○ Intestinal villi projections: Maximize the absorptive surface area (×600)
④ Duodenum (25 cm)
○ Intestinal juice: secreted from the intestinal glands in the small intestinal wall → finally breaks down carbohydrates and proteins into their basic units for absorption in the small intestine.
○ Enterokinase: present in the epithelial cell membrane of the small intestine
○ Trypsin: secreted from the pancreas as trypsinogen; activated to trypsin by enterokinase
○ Chymotrypsin: secreted from the pancreas as chymotrypsinogen; activated to chymotrypsin by trypsin
○ Carboxypeptidase: secreted from the pancreas as pro-carboxypeptidase; activated to carboxypeptidase by chymotrypsin
○ Maltase: maltose → glucose + glucose
○ Sucrase: sucrose → glucose + fructose
○ Lactase: lactose → glucose + galactose
○ (Di)aminopeptidase, dipeptidase, carboxypeptidase: polypeptides, tripeptides, dipeptides → amino acids
○ Nuclease: DNA, RNA → nucleotides
○ Mucus: Maintains flexibility of the contents in the lumen (lubricant function)
○ The site where acidic chyme mixes with digestive enzymes secreted from the pancreas, liver, gallbladder, and the glands of the small intestine itself, allowing digestion to occur.
⑤ Jejunum and ileum (260 cm): absorption of nutrients and water
○ Jejunum: secretes N-peptidase
○ N-peptidase: secreted in an active form from the beginning; an exopeptidase that recognizes and cleaves the amino terminus of proteins
○ About 90% of water is absorbed in the small intestine
⑵ Pancreas
① Location: Large gland right below the stomach, having both endocrine and exocrine parts (connected to common bile duct, 1.5 L/day)
② Pancreatic juice secretion: Acidic food in the stomach → stimulation of the duodenal wall → secretion of secretin → enters the bloodstream → stimulates pancreatic cells → pancreatic juice secretion
○ Secretin: Hormone that promotes pancreatic juice secretion
③ Digestive enzymes
○ Amylase: Carbohydrate digestion enzyme
○ Endopeptidase: Proteolytic enzyme that hydrolyzes internal peptide bonds, e.g., trypsin, chymotrypsin
○ Exopeptidase: Proteolytic enzyme that hydrolyzes terminal peptide bonds, e.g., carboxypeptidase
○ N-peptidase secreted from the jejunum is also an exopeptidase
○ Trypsin: In the duodenum, enterokinase activates trypsinogen into trypsin
○ Chymotrypsin: In the duodenum, trypsin activates chymotrypsinogen into chymotrypsin
○ Carboxypeptidase: In the duodenum, chymotrypsin activates pro-carboxypeptidase into carboxypeptidase
○ Fat-hydrolyzing enzyme: lipase
○ Nucleic acid–hydrolyzing enzymes: RNase, DNase
○ Elastase: Enzyme that breaks down elastin
④ Bicarbonate ions (HCO3-): Secreted by ductal cells, it neutralizes the pH of chyme together with bile.
○ Protection against gastric acid damage: Duodenal ulcers are 10 times more common than gastric ulcers
○ Enzymes in the small intestine function optimally at neutral or slightly alkaline pH
○ Transport of bicarbonate ions
○ Primary Active Transport: Sodium-potassium pump establishes a sodium ion concentration gradient
○ Secondary Active Transport: When hydrogen ions move from the secretory cells of the small intestine toward the blood, sodium ions move along
○ Secondary Active Transport: Chloride ions move along with sodium ions
○ Bicarbonate ions are transported by facilitated diffusion through a chloride ion cotransporter.
**Figure 6. Movement of Bicarbonate Ions
⑶ Liver
① Structure: Hepatocytes are arranged in a hexagonal pattern.
Figure 7. 3 Zones of the Pancreas
○ Zone 1. Periportal Zone: High oxygen saturation, gluconeogenesis, cholesterol synthesis, urea synthesis, beta oxidation, albumin expression
○ Zone 2. Intermediate Zone (Midlobular Zone)
○ Zone 3. Pericentral (Centrilobular) Zone: Glycolysis, lipid synthesis, glutamine synthesis, bile juice synthesis, detoxification by cytochrome P450, β-catenin/Wnt signaling
② Composition
○ Type 1. Hepatocytes: 60-80 %
○ Type 2. Non-Parenchymal Cells: 20-40%
○ Endothelial Cells: About 50%
○ Kupffer Cells: 20%
○ Gallbladder Cells: 5%
○ Hepatic Stellate Cells: 8%
○ Lymphocytes: 25%
○ Natural Killer (NK) Cells: 31%
○ B Cells: 6%
○ Dendritic Cells (DC): < 1%
○ T Cells: 63%
○ CD4+ T Cells
○ CD8+ T Cells
○ Type I NKT Cells
○ CD1d-dependent NKT Cells
○ TCRγδ T Cells
③ Exocrine function: Secretion of bile and bicarbonate.
○ Bile: Stored in the gallbladder
○ Synthesized from cholesterol (0.5 L/day)
○ Yellow-green liquid
○ Delivered to the gallbladder through the hepatic duct, then released into the duodenum via the common bile duct.
○ The sphincter of the common bile duct relaxes during absorption and contracts after absorption.
○ Food stimulates the duodenal wall → secretion of cholecystokinin → enters the bloodstream → stimulates the gallbladder → secretion of bile
○ Main components: water, bile salts, bile acids, cholesterol, bile pigments
○ Characteristics of bile: Contains no digestive enzymes, aids in fat digestion, prevents food putrefaction, alkaline in nature.
○ About 85% of bile salts are reabsorbed in the ileum, and about 10% are reabsorbed in the colon.
○ Bicarbonate ions: Neutralizes the hydrochloric acid that enters the small intestine from the stomach
④ Endocrine
○ Secretion of Insulin-like Growth Factor-1 (IGF-1) promotes cell division
○ Activation of Vitamin D
○ Angiotensinogen secretion: Regulates reabsorption of sodium ions in the kidneys
○ Cytokine secretion
⑤ Blood Sugar Regulation
○ When blood sugar is low: Gluconeogenesis
○ When blood sugar is high: Glycogen synthesis
⑥ Lipid Metabolism: β oxidation, synthesis of lipoproteins, cholesterol synthesis
⑦ Protein Metabolism
○ Urea Synthesis
○ Synthesis of Plasma Proteins
○ Blood clotting-related proteins: Fibrinogen, Heparin (anticoagulant), Prothrombin
○ Albumin, inorganic salts, fatty acids, etc.: Serve various transport functions and help regulate plasma osmotic pressure.
○ Hormone-binding Proteins
○ Deamination Reaction
⑧ Excretion function
○ Excretion: Disposal of old red blood cells, secretion of bilirubin (bile pigment), breakdown of organic compounds created and absorbed, elimination of metals through bile
○ During the fetal period, it also takes part in red blood cell production.
⑨ Elimination function
○ Solid waste is expelled into the intestines through bile canaliculi
⑩ Detoxification: Mainly occurs in smooth endoplasmic reticulum
○ Drug removal
○ Removal of hydrogen peroxide by catalase
○ Ornithine Cycle: Converts ammonia to urea
○ Example: Cytochrome P450 (particularly abundant in smooth endoplasmic reticulum)
⑪ Blood coagulation: synthesizes heparin (prevents blood clotting) and produces blood coagulation factors such as prothrombin and fibrinogen.
⑫ Heat production function: Releases a large amount of heat through active metabolism, contributing indirectly to body temperature regulation.
⑬ Liver-Related Disorders
○ Fatty Liver: MASLD (metabolic disease), Non-alcoholic steatohepatitis (NASH, MASH) etc.
○ Diagnosis 1. Biopsy / histology
○ Diagnosis 2. Imaging: CT, PDFF (MRI proton density fat fraction)
○ Diagnosis 3. International Classification of Diseases: ICD-9/10 code
○ Diagnosis 4. LFT (liver function test): ALT (alanine aminotransferase), AST (aspartate aminotransferase), etc.
○ Liver Fibrosis
○ Hepatitis: Acute liver failure etc.
○ Acute liver failure (ALF): ALF caused by acetaminophen (APAP) accounts for about 40% of all ALF cases.
○ In South Korea, hepatitis B is more common than hepatitis C.
○ In Western countries, hepatitis C is more common than hepatitis B.
○ Cirrhosis
○ Hepatocellular Carcinoma (HCC): In many cases, it develops from hepatitis through cirrhosis to liver cancer.
○ Cause 1: Hepatitis B virus: Over 70%
○ Cause 2: Hepatitis C virus: 10%
○ Cause 3: Alcoholic hepatitis: 5 ~ 10%
○ These figures are based on Korean standards
○ Cholangitis
⑷ Gallbladder
① Normally, bile is stored and then released when fat enters the duodenum
② Emulsification action
○ Bile has both hydrophilic and lipophilic properties (amphiphilic)
○ As a result, bile forms small micelle particles, increasing the exposed surface area to lipase
Figure 8. Action of the Gallbladder
③ Bile salt recirculation (95%): absorbed in the ileum (the distal part of the small intestine) and recirculated, while about 5% is lost in the feces.
④ Gallstones
○ Components of bile, including bile acid salts, cholesterol, and phospholipids, exist in an insoluble, micellar state
○ Excess cholesterol or precipitation of bile pigments → bile crystallizes, leading to gallstone formation.
○ Gallstones are more common in females than males (influenced by sex hormones)
⑤ Jaundice
○ Bile duct obstruction: accumulation of bilirubin in the blood causes yellowing of the skin and eyes (jaundice)
○ Hemolytic jaundice: occurs when liver disease leads to red blood cell destruction or when bilirubin secretion into the gallbladder is abnormal
⑸ Regulation by Enterogastrones: Occurs in the duodenum
Figure 9. Regulation by Enterogastrones
① Secretin: Responds to acidity, stimulates pancreatic bicarbonate secretion, neutralizes gastric acid, and inhibits gastrin secretion
○ 1st. Acidic chyme from the stomach stimulates cells in the duodenal wall: Secretin secretion from the duodenal wall cells into the blood
○ 2nd - 1st. Secretin circulates in bloodstream and stimulates the gastric wall → decreases gastric acid secretion
○ 2nd - 2nd. Secretin stimulates the pancreas → promotes secretion of pancreatic juice rich in bicarbonate (NaHCO3) → neutralizes acidic chyme.
○ H+ and Na+ are expelled from the liver interstitial fluid into the blood → lowers blood pH
○ Stomach raises blood pH, offsetting the decreased pH in the duodenum
○ 3rd. Closure of the pyloric sphincter → prevents further entry of acidic chyme from the stomach into the duodenum
② Cholecystokinin (CCK): Responds to fats. Stimulates pancreatic and bile secretion for fat digestion
○ 1st. Fatty acids from the stomach stimulate cells in the duodenal wall
○ 2nd. CCK is secreted into the blood from cells in the duodenal wall
○ 3rd - 1st. CCK stimulates the gastric wall → decreases gastric acid secretion
○ 3rd - 2nd. CCK stimulates the pancreas → increases secretion of pancreatic juice rich in digestive enzymes
○ 3rd - 3rd. CCK stimulates the gallbladder → bile secretion → emulsification of fats
③ GIP, VIP, GLP-1: Respond to high osmolarity in the small intestine due to sugars and fats, and inhibit gastric acid secretion
○ 1st. Chyme from the stomach stimulates cells in the duodenal wall
○ 2nd. GIP is secreted into the blood from cells in the duodenal wall
○ 3rd - 1st. GIP stimulates the gastric wall → decreases gastric juice secretion
○ 3rd - 2nd. GIP stimulates the intestinal glands → promotes intestinal juice secretion.
○ 3rd - 3rd. GIP stimulates the pancreas → stimulates insulin secretion
⑹ Regulation of digestive functions by the enteric nervous plexus
Figure 10. Regulation by the Enteric Nervous System
① Basically, the regulation of digestion is controlled by the medulla oblongata, the center of the autonomic nervous system.
② Conditioned reflex: Influenced by the cerebrum, occurs when there is prior experience.
③ Unconditioned reflex: When food exerts pressure on the stomach wall, causing stomach distension.
○ Sensory nerve → medulla oblongata → vagus nerve → stimulation of chief cells and parietal cells → secretion of gastric juice
○ Sensory nerve → medulla oblongata → vagus nerve → stimulation of G cells → secretion of gastrin
5. Stage 4: Large Intestine
⑴ Composition: Colon, Cecum, Rectum
① Colon
○ About 1.5 m
○ Site of water and ion absorption, formation of semisolid stools from undigested material
○ About 7 L of fluid secreted into the digestive tract daily, with about 90% reabsorbed
○ No active transport mechanism for water → water reabsorption induced by osmotic pressure created by pumping ions (like salt) out of the lumen
○ Connected to the small intestine at the T-shaped junction leading to the cecum.
② Cecum
○ Not involved in digestion
○ Appendix is involved related to the immune system
○ Appendix: Protrudes from the cecum in a finger-like shape, partially involved in immune responses
○ Herbivores possess special chambers, including the cecum, within their digestive tract where numerous symbiotic microorganisms reside to digest cellulose.
③ Rectum
○ Site where feces are stored until elimination
○ End part of the colon
○ Two sphincters between the colon and anus: one is involuntary, and the other is voluntary
⑵ Intestinal Microbiota
① There are approximately 100 trillion intestinal bacteria, about the same number as the body’s cells
② Because of the significant number of intestinal bacteria, about 70% of the body’s immune cells are present in the intestines
③ Types
○ Harmful bacteria
○ Beneficial bacteria
○ Intermediate bacteria: Act in ways similar to both beneficial and harmful bacteria
④ Example 1: E. coli (Escherichia coli)
○ Produces Vitamin K, Vitamin B7 (biotin), and Vitamin B9 (folic acid), leading to vitamin deficiencies if antibiotics are overused
○ Engages in anaerobic metabolic reactions, producing methane and hydrogen sulfide gases as byproducts
○ Accounts for one-third of the weight of feces, excluding water
○ E. coli found in lakes and rivers serves as an indicator of pollution from untreated sewage
○ E. coli O157 is a harmful strain of E. coli
⑤ Example 2: Lactic acid bacteria
⑶ Sphincter: Two exist between the colon and anus
① Internal Anal Sphincter: Involuntary muscle
② External Anal Sphincter: Voluntary muscle
⑷ Defecation
① Defecation: Spinal reflex related to rectal wall distension, reflex begins when feces enter the empty rectum
② Diarrhea: Caused by lower digestive tract infection and nerve stimulation, leads to dehydration, disturbance in cardiac contraction due to blood electrolyte imbalance
③ Constipation: Dietary fiber (e.g., cellulose) intake helps prevent constipation
④ Flatulence: Ammonia, methane gas, nitrogen, hydrogen sulfide, benzophenone, skatole (odor-causing substances)
⑤ A significant portion of feces consists of bacteria, fecal color is due to bilirubin, a breakdown product of red blood cells
⑸ Lactose intolerance
**6. Absorption of Nutrients **
Figure 11. Absorption of Nutrients
⑴ Most nutrients are absorbed through the mucosa of the small intestine and transported via blood vessels and lymphatic vessels in the submucosal tissue
① Intestinal mucosal cells are connected by desmosomes and tight junctions: prevent digestive products from moving through intercellular gaps and block membrane protein migration
② Absorption in the stomach: alcohol and small amounts of drugs
③ Absorption in the large intestine: water and inorganic salts
⑵ Amino acids, monosaccharides: Secondary active transport with Na+
Figure 12. Absorption of Amino Acids, Monosaccharides
① Unlike glucose, fructose is absorbed via facilitated diffusion from the first time.
② Small intestine villus capillaries → Hepatic portal vein → Liver → Hepatic vein → Inferior vena cava → Right atrium → Whole body
○ The hepatic portal vein and the hepatic artery, which carries blood directly from the heart to the liver, are completely different.
○ Various metabolic functions in the liver: regulation of blood glucose levels
○ Removal of harmful substances before blood circulates through the body.
○ Water-soluble nutrients such as monosaccharides, amino acids, minerals, and water-soluble vitamins are mainly absorbed into the capillaries of the small intestine.
③ Amino acids vs. dipeptides, tripeptides
○ Amino acids: Absorbed into small intestine epithelial cells through co-transport with sodium
○ Dipeptides, tripeptides: Absorbed into small intestine epithelial cells through co-transport with hydrogen ions
⑶ Lipids: Absorbed via diffusion
Figure 13. Absorption of Lipids
① Absorption of lipids
○ 1st. Emulsification: Bile salts emulsify fat globules into fat particles
○ Bile salts are synthesized in the liver
○ Bile salts act as surfactants to promote emulsification
○ Bile salts are recycled
○ 2nd. Lipase activity: lipase secreted from the pancreas breaks down micelles into monoglycerides and free fatty acids.
○ 3rd. Lipid absorption: Monoglycerides, free fatty acids are absorbed via diffusion
○ 4th. Lipid synthesis: Monoglycerides are resynthesized into triglycerides by enzymes in the smooth endoplasmic reticulum
○ 5th. Chylomicrons: Expelled through vesicles in a form bound with cholesterol and proteins, allowing them to move into the plasma.
○ Chylomicrons are water-soluble
○ 6th. Chylomicrons are excreted by vesicles.
○ 7th. Chylomicrons are transported to lacteals in the center of villi since they are too large to pass through capillaries
○ Lacteals are part of the lymphatic system
○ Free fatty acids are utilized in cellular respiration after beta-oxidation
○ Can also be absorbed as diglycerides in the small intestine villi
② Lacteals → Lymphatic vessels → Thoracic duct → Left subclavian vein → Superior vena cava → Right atrium → Whole body
○ Lipids, glycerol, fat-soluble vitamins, cholesterol, etc., are primarily absorbed through lacteals
○ Serum albumin assists in the movement of fatty acids in the blood
③ Lipoprotein: A particle in which lipids are enclosed at the core and surrounded by proteins, making it soluble in water.
○ Lipids themselves are not soluble in water, so they must form complexes with proteins to move in the bloodstream
○ Components: Apolipoproteins (Apo), phospholipids, cholesterol, triacylglycerol, proteins
○ Types of lipoproteins
Figure 14. Types of Lipoproteins
○ Type 1: Chylomicron
○ Largest
○ Function 1: Produced from fats and delivers fat to the liver, skeletal muscle, and lacteals.
○ Function 2: Regulate lipolysis enzyme activity.
○ Contains the highest amount of triglycerides.
○ Major Apo lipoproteins: B-48, ApoC, ApoE
○ Core lipid: dietary triglycerides
○ Synthesized in: Intestines
○ Type 2: Low-Density Lipoprotein (LDL)
○ Function 1: Delivers fats from the liver to various tissues via blood circulation, and transports 60% of blood cholesterol
○ Function 2: Receptor-mediated endocytosis
○ Contains the highest amount of cholesterol
○ Directly associated with atherosclerosis, thus often referred to as “bad cholesterol”
○ A distinguishing feature is the presence of ApoB-100 and ApoE proteins on the surface: LDL receptors recognize ApoB-100.
○ Core lipid: endogenous cholesterol ester
○ LDL primarily targets the endothelial cells of vascular tissue.
○ Synthesized in Liver.
○ Type 3: High-Density Lipoprotein (HDL)
○ Collects fats from tissues and cells, delivers them to the liver for excretion via bile
○ Involved in cholesterol removal, thus often referred to as “good cholesterol”
○ Highest density
○ Core lipid: endogenous cholesterol ester
○ Synthesized in liver and intestines.
○ Type 4: Very-Low-Density Lipoprotein (VLDL), Intermediate Density Lipoprotein (IDL)
○ Intermediate forms between chylomicrons and LDL
○ Major Apo lipoproteins: ApoB-100, ApoC, ApoE
○ Function: Regulate lipolysis enzyme activity
○ Core lipid: endogenous triglycerides
○ Synthesized in liver.
○ LDL/HDL ratio: High ratio linked to atherosclerosis. 3.5 is the normal value.
○ Familial Hypercholesterolemia (FH)
○ A genetic disorder caused by an autosomal mutation.
○ 1st: Because LDL receptors are absent, abnormalities occur in IDL intake
○ 2nd: LDL accumulates in the blood and becomes oxidized into oxLDL
○ 3rd: Macrophages uptake oxLDL and become foam cells
○ 4th: Foam cells form plaques in blood vessels, leading to arteriosclerosis
○ From birth, blood cholesterol levels are elevated, and cardiovascular diseases develop at an early age
○ Increased blood LDL leads to increased LDL uptake by hepatocytes, resulting in increased ApoB production
○ Plasma LDL cholesterol levels are about four times higher than in normal individuals
○ Causes atherosclerosis
○ Most patients die before the age of 20
⑷ Minerals: Active transport
① Examples: Sodium transporters, calcium transporters, iron transporters
② Absorption of cobalamin (Vitamin B12)
○ 1st. Separation of “Vitamin B12 + haptocorrin complex” by stomach’s pepsin
○ 2nd. Separation of Vitamin B12 and haptocorrin by pancreatic enzymes
○ 3rd. Secretion of intrinsic factor by stomach wall cells
○ 4th. Binding of Vitamin B12 and intrinsic factor in the duodenum
○ 5th. Uptake of “Vitamin B12 + intrinsic factor” by receptor-mediated endocytosis in the ileum terminal: ATP required
⑸ Vitamin: Active transport
⑹ Water: About 90% is absorbed in the small intestine and colon (part of the large intestine)
⑺ Junctions of the small intestine epithelial cells
① Tight junctions: Prevent substances from moving between the lumen and blood vessels just beneath the villi
② Adherens junctions (anchoring junctions): Desmosomes and hemidesmosomes maintain strong cell-to-cell and cell-to-matrix adhesion through intermediate filaments
③ Gap junctions: Allow the movement of ions and small molecules between adjacent cells
7. Regulation of Appetite Hormones
⑴ Overview
① Appetite-regulating hormones all act on neuropeptide Y
② Neuropeptide Y induces overeating
⑵ Leptin
① Produced by fat cells
② Increased fat tissue leads to higher concentration of fats in the blood, suppressing appetite in the brain
③ Leptin receptor is located in the hypothalamus.
④ Leptin experiments
○ Ob protein: Leptin protein
○ Db protein: Cell membrane receptors in the appetite-regulating center for leptin.
○ ob/ob mice: Become obese due to lack of leptin secretion from fat cells
○ db/db mice: Produce more leptin than normal mice but their brain’s leptin receptors don’t function, leading to obesity
○ Application: Can connect blood vessels of two mice and observe phenotypic changes after a certain time
○ Example: ob / ob , Db / Db × Ob / Ob , db / db: ob / ob , Db / Db mice return to normal while Ob / Ob , db / db mice remain obese
⑶ PYY
① Produced in the small intestine after a meal
② Acts as an appetite suppressant, opposite to ghrelin
⑷ Insulin
① Secreted by the pancreas
② Acts on the brain to suppress appetite
⑸ Cholecystokinin (CCK)
① Hormone secreted when food enters the duodenum
② Function 1: Appetite suppression: Acts on the axon terminals of the vagus nerve to send satiety (fullness) signals to the brain.
③ Function 2: Inhibition of gastric juice secretion
④ Function 3: Stimulation of digestive enzyme secretion
⑤ Function 4: Stimulation of bile secretion
⑹ Ghrelin
① Secreted from the stomach wall, stimulates appetite
② One of the signals that induces the sensation of hunger as mealtime approaches.
③ Concentration increases in people undergoing weight loss and dieting, leading to hunger
**8. Digestive System Disorders **
⑴ Obesity
① Assessment of healthy body fat
○ Overweight: Definition varies by era and culture
○ Women require more body fat than men for reproductive purposes
○ Women: 22% (12 ~ 32 %)
○ Men: 14 % (3 ~ 29%)
○ The larger the body frame and the older the age, the greater the increase in body fat.
○ Body Mass Index (BMI): Weight (kg) ÷ (Height(m))^2. Imperfect criterion
○ Underweight: Below 18.5
○ Normal: 18.5 ~ 24.9
○ Overweight: 25.0 ~ 29.9
○ Obesity: 30 or above
② Causes of obesity: Lifestyle and genetics both influence obesity
③ Classification of obesity according to adipocyte type
○ Adipocyte hyperplasia type: the size of adipocytes is normal, but the number of adipocytes increases
○ Even with weight reduction, the increased number of adipocytes does not decrease
○ As a result, it is prone to relapse and often leads to moderate to severe obesity
○ Adipocyte hypertrophy type: the number of adipocytes is normal, but the size of adipocytes increases
○ When weight is reduced, the size of adipocytes decreases
○ Obesity in adults: usually adipocyte hypertrophy type
○ Mixed type: both the number and the size of adipocytes increase
○ Even with weight reduction, the increased number of adipocytes does not decrease
○ As a result, it is prone to relapse and often leads to moderate to severe obesity
○ Obesity in children and adolescents: usually the mixed type
④ Subcutaneous fat
○ Location where fat is stored for the first time.
○ When the subcutaneous space becomes insufficient, fat accumulates in the abdomen and internal organs → abdominal fat, visceral fat, and fatty liver.
○ Increased risk of metabolic complications
⑤ Related complications
○ Diabetes
○ Insulin: Hormone produced by pancreatic beta cells, promotes absorption of glucose by other cells
○ Type 1 Diabetes (Insulin-dependent): Unrelated to obesity, genetically absence of pancreatic beta cells, no insulin production
○ Type 2 Diabetes (Insulin-independent): Insulin resistance, related to obesity, common in adults, managed through diet and exercise
○ High Blood Pressure: Elevated blood pressure
○ Systolic: Blood pressure when the heart contracts
○ Diastolic: Blood pressure when the heart relaxes
○ Normal Blood Pressure: 120/80
○ High Blood Pressure: Consistently 140/90 or higher
○ Heart attack: Sudden blockage of blood flow to the heart muscle due to obstruction of a coronary artery.
○ Stroke (Seizure): Sudden blockage of brain blood flow due to cerebral artery obstruction or rupture
⑵ Atherosclerosis
① Cholesterol: Constituent of cell membranes, precursor to steroid hormones
○ LDL (Low-Density Lipoprotein): Synthesized in the liver, transports cholesterol from food to tissues
○ HDL (High-Density Lipoprotein): Transports cholesterol from tissues to the liver (for excretion in bile)
② Cholesterol is carried in the blood by lipoproteins, and accumulation in arteries leads to atherosclerosis
⑶ Appetite Regulation Disorders
① Anorexia nervosa
○ Self-induced starvation, an eating disorder driven by weight consciousness
○ Can lead to starvation of the heart muscle and cause irregular heartbeat
○ Suppresses estrogen, resulting in cessation of menstruation and risk of infertility
○ Increases risk of osteoporosis
② Bulimia nervosa
○ Binge eating followed by laxative use
○ Risk of gastric rupture, dental and gum damage from stomach acid, and dehydration
⑷ Nutritional Imbalance
① Undernutrition
○ A chronic lack of caloric intake, resulting in a continuous shortage of necessary chemical energy supply
○ Breakdown of stored glycogen, fat, and protein → muscle mass decreases and protein deficiency occurs in the brain
○ Example: In sub-Saharan Africa, where drought, war, and AIDS epidemics persist, about 200 million people cannot obtain sufficient nutrition
○ Example: Anorexia nervosa (obsessive fasting)
② Overnutrition
○ When food intake exceeds an animal’s energy requirements, the three main nutrients (i.e., carbohydrates, proteins, fats) are stored as glycogen or body fat
○ Important for hibernating animals
③ Malnutrition
○ A state in which one or more essential nutrients are lacking
○ Example 1: Vitamin A deficiency → solved by supplying beta-carotene (e.g., Golden Rice)
○ Example 2: When herbivores eat plants grown in phosphorus-deficient soil → bones break easily
○ Example 3: Diet lacking sufficient essential amino acids → protein deficiency disorders
④ Kwashiorkor: severe protein deficiency caused by extreme starvation
○ Albumin is used as an energy source → increase in water content of tissue cells → edema develops
○ Carbohydrates → acetyl-CoA → fatty acid synthesis
○ Symptoms:
○ Muscle wasting
○ Growth retardation
○ Neurological disorders
○ Digestive problems
○ Twisting of hands and feet
○ Skin lesions
○ Hair becomes brown and stops growing
○ Edema
⑤ Marasmus: deficiency of both protein and calories
○ Common in infants and young children during the weaning period
○ Skin, hair, and liver function are relatively normal, but with excessive wrinkling
○ Body reduced to “skin and bones,” with severe dehydration
⑸ Inflammatory Bowel Disease (IBD)
① Crohn’s Disease
○ A chronic inflammatory bowel disease that can occur anywhere in the digestive tract, from the mouth to the anus
○ One of the autoimmune diseases
② Colitis
○ Ulcerative Colitis
③ Gastric Ulcer
○ Helicobacter pylori’s secreted ammonia allows survival of themselves in stomach acid
○ Ammonia further disrupts mucin action, causing gastric ulcers
⑹ Indigestion
① Cause: Excess stomach acid
⑺ Liver Disease
① Fatty Liver: Non-alcoholic steatohepatitis (NASH, MASH), etc.
② Liver Fibrosis
③ Hepatitis: Acute liver failure, etc.
④ Cirrhosis
⑤ Hepatocellular Carcinoma (HCC)
⑥ Cholangitis
⑻ Gastric Cancer
① Annually, 1,080,000 people are diagnosed with gastric cancer, and over 760,000 die from it
⑼ Hernia: Abnormal protrusion of an organ
① Type 1: Groin Area Hernia
② Type 2: Ventral Hernia
○ 2-1: Incisional Hernia
○ 2-2: Umbilical Hernia
○ 2-3: Epigastric Hernia
○ 2-4: Hypogastric Hernia
③ Type 3: Femoral Hernia
Input: 2015.07.16 11:24
Modified: 2022.05.19 14:17