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
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 2. 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 3. 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. 4. 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 5. Movement of Bicarbonate Ions
⑶ Liver
① Structure
Figure 6. Structure of the liver
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: Non-alcoholic steatohepatitis (NASH, MASH) 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 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