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
4. Step 3: Small Intestine, Jejunum, Liver, Appendix
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., baleen whales)
○ Substrate feeders : Animals that eat their environment (substrate) (e.g., dung beetles, maggots)
○ Fluid feeders : Animals that feed on bodily fluids (e.g., mosquitoes)
○ Bulk feeders : Animals that consume relatively large food pieces (e.g., silk worms)
② Digestion : The process of breaking down food into small molecules that can be absorbed by the body
○ Peristalsis (mechanical digestion) : Wave-like contractions of smooth muscles in the digestive tract that propel food
○ Segmentation (mechanical digestion) : Contraction of smooth muscles that breaks down larger food particles
○ Chemical digestion : Enzymatic breakdown of polymers into monomers
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 : Specialized compartments for digestion to reduce self-digestion risk
② Intracellular digestion
○ Phagosome : Simplest digestive compartment, where digestion occurs after fusion with lysosomes
○ Endocytosis, Exocytosis
○ Marine 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 cnidarians
○ Hydra, secretes digestive enzymes in the gastrovascular wall
○ 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 Digestive tract usually has a contractile longitudinal muscle layer and a relaxant circular muscle layer, increasing surface area
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, flat teeth with a wide grinding surface for chewing tough plants
③ Omnivores : Less specialized dentition
④ Human Teeth : Enamel layer, dentin layer, pulp cavity
⑵ Saliva
① Secreted by salivary glands even before eating through reflexes or conditioning
② Salivary Glands
○ Salivary amylase secreted from acinar cells
○ Three pairs: sublingual, submandibular, parotid glands
○ Pathway of salivary secretion : Food → Taste receptors on the tongue → Sensory nerves → Salivary nuclei → 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 linkages in starch into maltose and dextrins
○ Fatty acids : Make saliva slightly acidic, inhibits bacterial growth
○ Lingual lipase
○ In infants, helps digest fats as their digestive enzymes are immature
○ IgA : Antibodies that remove parasites, etc.
④ Regulation of Saliva Secretion
○ Controlled by the autonomic nervous system in the medulla
○ Sympathetic nervous system inhibits saliva secretion, parasympathetic nervous system stimulates it
○ Sympathetic nervous system’s inhibition increases saliva viscosity
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)
○ Epiglottis : Upper part of the respiratory tract
○ Epiglottis moves upward, tipping epiglottis over the glottis (closing the airway) → Epiglottic lid closes the glottis (closure of airway)
○ Relaxation of esophageal sphincter → Opening of esophagus
○ Controlled by the medulla
③ 3rd. Food enters the esophagus, the larynx moves downward, and the glottis opens
○ Uvula rises to prevent food from going backward
④ 4th. Peristaltic wave (contraction) of muscles moves the bolus downward to reach the stomach
⑷ Esophagus: Peristalsis, Segmentation, Mucus (lubrication)
① Upper Esophageal Sphincter (= Upper Esophageal Muscle) : Striated muscle, initiated by deglutition reflex
② Lower Esophageal Sphincter (= Lower Esophageal Muscle, Cardiac Sphincter) : Smooth muscle, peristalsis
○ Below diaphragm, regulated under similar abdominal pressure as stomach
○ Prevents food from regurgitating into the esophagus
○ Normally closed; when stimulated by food, the lower esophagus reflexively opens
○ In late pregnancy, increased abdominal pressure due to displacement of the abdominal cavity (toward thoracic cavity) often causes lower esophagus to open
○ Heartburn : Dysregulation of lower esophageal muscle control → Acid reflux damages the esophagus → Ulceration
○ Vomiting
○ Vomiting center in the medulla
○ Innate protective function against toxic substances
○ Triggered by stomach expansion, intestinal wall, chemical receptors in the brain, rotation of the head
○ As exit is blocked and entrance is relaxed, food is pushed backward
○ Vomiting not only damages the esophagus but also increases stomach pH, inhibiting proper digestion
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 mucous barrier by mucus
○ Epithelial cells of stomach wall form tight junctions to protect the stomach wall
○ Rapid cell division of basal cells helps repair damaged cells, regeneration every 3 days
○ Secretion of inactive enzymes (e.g., pepsinogen, trypsinogen, chymotrypsinogen) due to cell damage
④ Structure of Gastric Musculature : Mucosa - Submucosa - Muscularis Externa - Longitudinal Muscle - Serosa
○ Submucosa : Distribution of blood vessels like veins and arteries. Helicobacter pylori commonly resides here
○ Muscularis Externa : Connected to serosa through mesentery
○ Myenteric Plexus : Controls the alimentary canal
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 : Generated by pacemaker cells (Cajal cells) rhythmically contracting, promoting smooth muscle contractions
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.
○ H+-K+-ATPase in parietal cells (wall cells) actively transports H+ and Cl- passively for HCl secretion.
○ 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, prorenin to renin.
○ Acidic tide : Phenomenon in reptiles like crocodiles or snakes after overeating, blood pH rises to 0.5 ~ 1.0, causing torpor.
○ Increased secretion of hydrochloric acid due to buffering effect of protein-rich food.
④ Mucus
○ Mucin (glycoprotein), cells, salt, water mixture secreted by goblet cells.
○ Function : Protects stomach lining, provides lubrication.
○ Stomach epithelial cells regenerate every 3 days to protect the stomach wall.
○ Gastric ulcer
○ Ammonia secreted by Helicobacter pylori helps survival in stomach acid.
○ Ammonia additionally disrupts mucin function, leading to gastric ulcers.
⑤ Gastrin
○ G cells in the pyloric antrum secrete gastrin upon physical, chemical (protein, peptide) stimuli.
○ Gastrin, histamine, vagus nerve : Promote 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 Prorenin : Renin facilitates clotting of milk protein casein for easy digestion, found in milk-fed infants’ gastric juice.
⑧ Mucin : Secreted by mucous cells, 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, 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, controls pyloric reflex.
○ Closed case: Expansion of duodenum, ↑ acidity, ↑ fat, ↑ cholecystokinin, long reflex, short reflex → Inhibition of pyloric sphincter muscle relaxation → Maximizes digestion efficiency.
○ Open case: When contents of duodenum become alkaline (after digestion in duodenum), pylorus reflexively opens.
○ Regulated by enterogastrones (general term for intestinal hormones).
○ Long reflex : Sympathetic nervous system ↑, parasympathetic nervous system ↓.
○ Short reflex : Involves enteric nervous system.
⑹ Regulation of Gastric Acid Secretion, 3 Stages: Cephalic, Gastric, Intestinal
Figure. 3. Regulation of Gastric Acid Secretion, 3 Stages
① Stage 1: Cephalic Phase
○ 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 Stage 2: Gastric Phase
○ 1st Distension 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 Phase
○ 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 acidic conditions by secreting mucin that neutralizes acid.
② Unable to cover the wall where these bacteria are attached, leading to damage and the possibility of gastric ulcers.
⑻ Stomach of Ruminants
① Fore-stomach (rumen) : Synthesis of cellulase by symbiotic microorganisms.
② Honeycomb stomach : Synthesis of cellulase by symbiotic microorganisms.
③ Multi-fold stomach (reticulum, omasum) : Absorption of water.
④ Folded stomach (abomasum) : Digestion.
4. Stage 3: Small Intestine, Jejunum, Liver, Gallbladder
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 : Performed by parietal cells, as described above
③ Villi of the small intestine
Figure. 4. Villi of the Small Intestine
○ Mucosal Folds : The surface of the mucosa is wrinkled in a state where the circular muscle is somewhat contracted (×3)
○ Microvilli : When the surface of each fold is magnified, a structure called microvilli can be observed (×10)
○ Microvillus Brush Border : Epithelial cells of microvilli
○ Various digestive enzymes are fixed · secreted at the brush border, from the brush border
○ Above each microvillus, threads of glycocalyx protrude (×20), surface area 300 ㎡
○ Villi of the Small Intestine : Maximization of absorption surface area, ×600
④ Duodenum (25 cm)
○ Intestinal Juice : Secreted in the intestinal wall → basic unit for final breakdown of carbohydrates and proteins for absorption in the small intestine
○ Enterokinase : Present in the intestinal epithelial membrane
○ Trypsin : Secreted from the jejunum, converted from trypsinogen to trypsin by enterokinase
○ Chymotrypsin : Secreted from the jejunum, converted from chymotrypsinogen to chymotrypsin by trypsin
○ Carboxypeptidase : Secreted from the jejunum as pro-carboxypeptidase, activated by chymotrypsin to carboxypeptidase
○ Maltase : Maltose → Glucose + Glucose
○ Sucrase : Sucrose → Glucose + Fructose
○ Lactase : Lactose → Glucose + Galactose
○ (Di)aminopeptidase, Dipeptidase, Carboxypeptidase : Poly-, tri-, dipeptides → Amino acids
○ Nucleases : DNA, RNA → Nucleotides
○ Mucus : Maintains flexibility of the contents in the lumen (lubricant function)
○ Acidic chyme mixes with digestive enzymes secreted from the jejunal, hepatic, and pancreatic cells in the small intestine, leading to digestion
⑤ Jejunum, Ileum (260 cm) : Absorption of nutrients, water
○ Jejunum : Secretion of N-peptidases
○ N-peptidase : Exopeptidase recognizing and cleaving the amino terminus of proteins, active from the beginning
○ About 90% of water is absorbed in the small intestine
⑵ Pancreas
① Location : Large gland right below the stomach, 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
○ Endopeptidases : Intracellular protein digestion enzymes, trypsin, chymotrypsin, etc.
○ Exopeptidases : Extracellular protein digestion enzymes, carboxypeptidase, etc.
○ N-peptidases secreted by the pancreas are also exopeptidases
○ Trypsin : Enteropeptidase in the duodenum activates trypsinogen to trypsin
○ Chymotrypsin : Trypsin in the duodenum activates chymotrypsinogen to chymotrypsin
○ Carboxypeptidase : Trypsin in the duodenum activates pro-carboxypeptidase to carboxypeptidase
○ Lipid digestion enzyme : Lipase
○ Nucleic acid digestion enzymes : RNase, DNase
○ Elastase : Elastin digestion enzyme
④ Bicarbonate ions (HCO3-) : Neutralizes the pH of chyme along with pancreatic juice, secreted from acinar cells
○ 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 facilitated by chloride ions in cotransporters for acidification
**Figure. 5. Movement of Bicarbonate Ions]
⑶ Pancreas (Pancreas)
① Structure
Figure. 6. Structure of the Pancreas
Figure. 7. 3 Zones of the Pancreas
○ Zone 1. Periportal Zone : High oxygen saturation, gluconeogenesis, cholesterol synthesis, synthesis of elements, beta oxidation, albumin expression
○ Zone 2. Intermediate Zone (Midlobular Zone)
○ Zone 3. Pericentral (Centrilobular) Zone : Process, lipid synthesis, glutamine synthesis, synthesis of bile juice, 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%
○ Biliary Epithelial Cells : 5%
○ Hepatic Stellate Cells : 8%
○ Lymphocytes : 25%
○ Natural Killer (NK) Cells : 31%
○ B Cells : 6%
○ Plasmacytoid 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 Endocrine Function : Secretion of Bile, Bicarbonate
○ Bile : Stored in the gallbladder
○ Synthesized from cholesterol (0.5 L/day)
○ Greenish liquid
○ Passed through bile ducts to the gallbladder, then to the duodenum through the common bile duct
○ Gallbladder contraction muscle relaxes during absorption, contracts during post-absorption
○ Food stimulates the duodenal wall → cholecystokinin secretion → enters the bloodstream → stimulates gallbladder → bile secretion
○ Major components : Water, bile salts, bile acids, cholesterol, bile pigments
○ Characteristics of bile : No digestive enzymes, aids in fat digestion, prevents food spoilage, alkaline
○ 85% of bile salts are reabsorbed in the ileum, 10% 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
○ Synthesis of Elements
○ Synthesis of Plasma Proteins
○ Blood clotting-related proteins : Fibrinogen, Heparin (anticoagulant), Prothrombin
○ Albumin, Inorganic Salts, Fatty Acids, etc. : Transport roles and regulation of 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
○ Also responsible for red blood cell production during infancy
⑨ Excretion
○ 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
○ Urea Cycle : Converts ammonia to urea
○ Example : Cytochrome P450 (particularly abundant in smooth endoplasmic reticulum)
⑪ Blood Coagulation : Synthesis of antithrombin (anticoagulant), prothrombin, fibrinogen, and other blood clotting factors
⑫ Thermoregulation : Releases a large amount of heat through vigorous metabolic activity, indirectly regulates body temperature
⑬ Liver-Related Disorders
○ Fatty Liver : Non-alcoholic fatty liver (NASH, non-alcoholic steatohepatitis) etc.
○ Liver Fibrosis
○ Hepatitis : Acute liver failure etc.
○ Acute liver failure (ALF) : Acute liver failure caused by acetaminophen (APAP) accounts for 40% of total ALF cases
○ According to South Korean standards, Hepatitis B is more prevalent than Hepatitis C
○ According to Western standards, Hepatitis C is more prevalent than Hepatitis B
○ Cirrhosis
○ Hepatocellular Carcinoma (HCC) : Many cases of liver cancer develop from cirrhosis due to hepatitis
○ 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 (Gallstone) **
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 Acid Recirculation (95%) : Absorbed from the ileum (back of the small intestine) and recirculated, 5% is lost through feces
④ Gallstones
○ Components of bile, including bile acids, cholesterol, and phospholipids, exist in an insoluble, micellar state
○ Excess cholesterol or precipitation of bile pigments → formation of gallstones due to impaired bile secretion
○ Gallstones are more common in females than males (influenced by sex hormones)
⑤ Jaundice
○ Biliary obstruction : Accumulation of bilirubin in the blood leads to yellowing of the skin and sclera (jaundice)
○ Hemolytic Jaundice : Occurs when red blood cells are destroyed in the liver, and bilirubin secretion from the gallbladder is abnormal
⑸ Regulation by Enterogastrones : Occurs in the duodenum
Figure 9. Regulation by Enterohormones
① Secretin : Responds to acidity, stimulates pancreatic bicarbonate secretion, neutralizes gastric acid, inhibits gastrin secretion
○ 1st. Acidic chyme from the stomach stimulates cells in the duodenal wall : Secretin secretion into the blood
○ 2nd - 1st. Secretin stimulates the gastric wall → decreases gastric acid secretion
○ 2nd - 2nd. Secretin stimulates the duodenal mucosa → promotes secretion of pancreatic juice rich in bicarbonate (NaHCO3) → neutralizes acidic chyme
○ H+ and Na+ are expelled from the liver 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 duodenal mucosa → 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, 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 acid secretion
○ 3rd - 2nd. GIP stimulates the crypts of Lieberkühn → promotes secretion of intestinal juice
○ 3rd - 3rd. GIP stimulates the mucosa → stimulates insulin secretion
⑹ Regulation by the Enteric Nervous System
Figure 10. Regulation by the Enteric Nervous System
① Basic digestion control is regulated by the autonomic nervous system’s central controller, the medulla
② Conditioned Reflex : Influenced by the brain and previous experience
③ Unconditioned Reflex : When food causes pressure on the gastric wall, leading to expansion
○ Sensory nerves → medulla → vagus nerve → stimulates chief cells and parietal cells → gastric juice secretion
○ Sensory nerves → medulla → vagus nerve → stimulates G cells → gastrin secretion
5. Stage 4: Large Intestine (1.5 m)
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 cecum at the junction resembling a T-shape
② Cecum
○ Not involved in digestion
○ Appendix is involved in the immune system
○ Appendix: Protrudes from the cecum in a finger-like shape, partially involved in immune responses
○ Herbivores have a specialized section in their digestive tract, including the cecum, where numerous symbiotic microorganisms reside, aiding in cellulose digestion
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: Probiotics
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 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
**6. Absorption of Nutrients **
Figure 11. Absorption of Nutrients
⑴ Mostly absorbed through the mucosa of the small intestine and transported through blood vessels and lymphatics in mucosal tissue
① Mucosal cells of the small intestine are connected by desmosomes and tight junctions : Prevent movement of digestive products between cells, prevent movement of membrane proteins
② Absorption from the stomach : Alcohol and small amounts of drugs
③ Absorption from the large intestine : Water and electrolytes absorption
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
② Small intestine villus capillaries → Hepatic portal vein → Liver → Hepatic vein → Inferior vena cava → Right atrium → Whole body
○ (Note) Hepatic artery and hepatic portal vein, which carry blood directly from the heart to the liver, are distinct
○ Liver performs various metabolic functions: glucose regulation, removal of harmful substances before blood circulation, absorption of water-soluble nutrients
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 synthesized in the liver
○ Bile salts act as surfactants to promote emulsification
○ Bile salts are recycled
○ 2nd. Lipolysis : Released by pancreas, lipase breaks micelle particles 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 : Transported through lymphatics in the form of cholesterol-protein complexes (micelles) for excretion
○ Chylomicrons are water-soluble
○ 6th. Chylomicrons are excreted via lymphatics
○ 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 Intestinal lymphatics → 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 intestinal lymphatics
○ Serum albumin assists in the movement of fatty acids in the blood
③ Lipoproteins : Particles surrounded by proteins to enable solubility 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: Transport fats from intestines to liver, skeletal muscles, adipose tissues
○ Function 2: Regulate lipolysis enzymes
○ Contains the most triglycerides
○ Major Apo proteins: B-48, ApoC, ApoE
○ Core lipid: dietary triglycerides
○ Synthesized in: Intestines
○ Type 2: Low-Density Lipoprotein (LDL)
○ Function 1: Deliver fats from the liver to various tissues via blood circulation, transports 60% of blood cholesterol
○ Function 2: Receptor-mediated endocytosis
○ Contains the most cholesterol
○ Directly associated with atherosclerosis, often referred to as “bad cholesterol”
○ LDL receptors recognize ApoB-100 on LDL surface
○ Core lipid: endogenous cholesterol ester
○ Targets endothelial cells of blood vessels
○ 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, often referred to as “good cholesterol”
○ Highest density
○ Core lipid: endogenous cholesterol ester
○ Synthesized in: Liver, intestines
○ Type 4: Very-Low-Density Lipoprotein (VLDL), Intermediate Density Lipoprotein (IDL)
○ Intermediate forms between chylomicrons and LDL
○ Major Apo proteins: ApoB-100, ApoC, ApoE
○ Function: Regulate lipolysis enzymes
○ Core lipid: endogenous triglycerides
○ Synthesized in: Liver
○ LDL/HDL ratio: High ratio linked to atherosclerosis. 3.5 is the normal value.
○ Familial Hypercholesterolemia (FH)
○ Autosomal dominant mutation
○ 1st. Lack of LDL receptors leads to IDL accumulation
○ 2nd. Oxidation of LDL in the blood
○ 3rd. Foam cells form from macrophages that uptake oxidized LDL
○ 4th. Foam cells create plaque in blood vessels, leading to atherosclerosis
○ High cholesterol levels from birth, early cardiovascular diseases
○ Increased LDL uptake by liver cells, increased ApoB production
○ Plasma LDL cholesterol levels four times higher than normal
○ Results in atherosclerosis
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 : ATP required
⑸ Vitamin : Active transport
⑹ Water : About 90% is absorbed in the small intestine and colon (part of the large intestine)
⑺ Adjacent to the small intestine epithelial cells
① Tight junction : Prevents the movement of substances between the hair cells and blood vessels just below
② Adherent junction : Desmosomes and hemidesmosomes use intermediate fibers to maintain strong cell-cell and cell-matrix connections
③ Gap junction : Allows the movement of ions and small molecules between adjacent cells
**7. Appetite-regulating 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 concentrations in the blood, suppressing appetite in the brain
③ Leptin experiments
○ Ob protein : Leptin protein
○ Db protein : Receptor on hypothalamic cell membranes 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 nerve endings of the vagus nerve, sending signals of fullness 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 Signals 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%)
○ Increased skeletal size and age correlate with increased 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 based on fat cell type
○ Hyperplastic obesity : Normal-sized fat cells, but the number increases
○ Reduced fat cell number doesn’t occur even with weight loss
○ Often leads to recurrence and moderate to severe obesity
○ Hypertrophic obesity : Normal number of fat cells, but they become larger
○ Reduced fat cell size with weight loss
○ Adult obesity : Hypertrophic fat cells
○ Mixed-type obesity : Both increased fat cell number and larger fat cell size
○ Reduced fat cell number doesn’t occur even with weight loss
○ Often leads to recurrence and moderate to severe obesity
○ Pediatric and adolescent obesity : Mixed-type fat cells
④ Subcutaneous fat
○ Location where fat is primarily stored
○ Limited subcutaneous space leads to fat accumulation in the abdomen and viscera → visceral fat, intrahepatic fat
○ 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
○ Cardiac Arrest : Sudden blockage of cardiac muscle blood flow due to coronary artery obstruction
○ 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 Decreased Appetite
○ Self-imposed starvation, eating disorders driven by body image
○ Can affect cardiac muscles, irregular heartbeat
○ Blocks estrogen, halts menstruation, risk of infertility
○ Increases risk of osteoporosis
② Increased Appetite
○ Laxative use after overeating
○ Risk of stomach rupture, dental and gum issues from stomach acid, dehydration
⑷ Nutritional Imbalance
① Malnutrition
○ Chronic insufficient caloric intake, sustained inadequate supply of necessary chemical energy
○ Breakdown of stored glycogen, fat, protein → Reduced muscle size, protein deficiency in the brain
○ Example: About 200 million people in sub-Saharan Africa, south of the Sahara Desert, cannot access sufficient nutrition due to drought, war, AIDS pandemic
○ Example: Anorexia nervosa (compulsive fasting)
② Overnutrition
○ Consuming more food than energy demands, storing the excess as glycogen or body fat ○ Relevant to hibernating animals
③ Malnutrition
○ Deficiency of one or more essential nutrients
○ Example 1: Vitamin A deficiency → Resolved through beta-carotene supplementation (golden rice)
○ Example 2: Animals eating plants grown in soil deficient in phosphorus → Herbivores’ bones become brittle
○ Example 3: Diet lacking essential amino acids → Protein deficiency
④ Kwashiorkor: Protein deficiency due to severe fasting
○ Albumin used as an energy source → Increased tissue cell water → Edema
○ Carbohydrates → Acetyl coA → Fatty acid synthesis
○ Symptoms
○ Muscle atrophy
○ Growth retardation
○ Neurological disorders
○ Digestive disorders
○ Swelling of hands and feet
○ Skin lesions
○ Hair turns brown and stops growing
○ Edema
⑤ Marasmus: Protein and calorie deficiency
○ Common in infants during weaning and early childhood
○ Skin and hair appear fine, liver functions relatively normal, but excessive wrinkling
○ Skin and bones remain, severe emaciation of the neck
⑸ Inflammatory Bowel Disease (IBD)
① Crohn’s Disease: Chronic inflammatory bowel disease affecting the entire digestive tract from mouth to anus
② Colitis
○ Ulcerative Colitis
③ Gastric Ulcer
○ Helicobacter pylori’s secreted ammonia allows survival 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), 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: Inguinal 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