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Questions and Answers
Which sphincter prevents gastric reflux by maintaining a high resting tone?
Which sphincter prevents gastric reflux by maintaining a high resting tone?
What function does the liver not perform?
What function does the liver not perform?
Which part of the small intestine is the longest?
Which part of the small intestine is the longest?
Which layer of the gastrointestinal tract is responsible for peristalsis?
Which layer of the gastrointestinal tract is responsible for peristalsis?
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Which structure connects the small intestine to the abdominal wall and carries blood vessels?
Which structure connects the small intestine to the abdominal wall and carries blood vessels?
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What type of hormone controls gastric secretion and motility in the GI tract?
What type of hormone controls gastric secretion and motility in the GI tract?
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The ampulla of Vater is an important junction for which ducts?
The ampulla of Vater is an important junction for which ducts?
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Which nerve system is responsible for increasing motility and secretion in the GI tract?
Which nerve system is responsible for increasing motility and secretion in the GI tract?
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What is the primary function of the Kupffer cells in the liver?
What is the primary function of the Kupffer cells in the liver?
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What condition is characterized by increased pressure in the portal system due to hepatic resistance?
What condition is characterized by increased pressure in the portal system due to hepatic resistance?
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What is the primary role of gastric acid in the stomach?
What is the primary role of gastric acid in the stomach?
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Which cell type in the stomach is responsible for producing the hunger hormone Ghrelin?
Which cell type in the stomach is responsible for producing the hunger hormone Ghrelin?
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What triggers the release of cholecystokinin (CCK) from duodenal wall cells?
What triggers the release of cholecystokinin (CCK) from duodenal wall cells?
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Which components are important for the formation of micelles during fat digestion?
Which components are important for the formation of micelles during fat digestion?
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What is one of the main functions of the large intestine?
What is one of the main functions of the large intestine?
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What is the role of intrinsic factor in the digestive system?
What is the role of intrinsic factor in the digestive system?
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Which dietary components are broken down by pancreatic amylase?
Which dietary components are broken down by pancreatic amylase?
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What neural response initiates the defecation reflex?
What neural response initiates the defecation reflex?
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How are amino acids absorbed in the small intestine?
How are amino acids absorbed in the small intestine?
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What substance helps reduce the surface tension of fat globules in digestion?
What substance helps reduce the surface tension of fat globules in digestion?
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Study Notes
Mouth
- Teeth: 32 in total, divided into incisors, cuspids, premolars, and molars
- Oral vestibule: The space between the teeth and cheeks
- Gingiva: The gums
- Superior/Inferior labial frenula: Tissues connecting upper and lower lips to gums
- Lingual frenulum: Tissue connecting the tongue to the floor of the mouth
- Uvula: Soft tissue hanging from the soft palate
- Palatine tonsil: Lymphoid tissue at the back of the throat
- Hard and soft palates: Roof of the mouth, hard palate is bony, soft palate is muscular
Pharynx
- Nasopharynx: Top part of the pharynx, behind the nose
- Oropharynx: Middle part of the pharynx, behind the mouth
- Laryngopharynx: Lower part of the pharynx, connecting to the larynx
- Tongue: Muscular organ in the mouth, important for speech and swallowing
- Vallecula: Space between the tongue and epiglottis
Oesophagus
- Upper oesophageal sphincter (UOS): Muscle separating the pharynx from the oesophagus
- Lower oesophageal sphincter (LOS): Muscle separating the oesophagus from the stomach
- Gastro-oesophageal junction (GOJ): Point where the oesophagus and stomach meet, the LOS prevents acid reflux
- Diaphragm: Muscle surrounding the oesophagus
- Upper endoscope generally measures 40cm from incisors to GOJ
Stomach
- Fundus: Dome-shaped upper part of the stomach
- Body: Main central part of the stomach
- Antrum: Lower part of the stomach, leading to the pylorus
- Pyloric sphincter: Muscle controlling the flow of food from stomach to duodenum
Small Bowel
- Duodenum: First and shortest part of the small intestine, approx 30cm long
- Jejunum: Second part of the small intestine
- Ileum: Third and longest part of the small intestine, connected to the colon by the ileocaecal valve
Liver
- Location: Right upper quadrant of the abdomen, protected by ribs
- Functions: Metabolic (e.g., glucose metabolism), storage (e.g., vitamins), immune (e.g., filters blood), digestion (e.g., makes bile), detoxification (e.g., removes toxins)
Pancreas
- Exocrine function: Produces digestive enzymes via acinar cells, which enter the duodenum through the ampulla of Vater
- Endocrine function: Produces hormones (e.g., insulin, glucagon) via islets of Langerhans
Liver and Pancreas
- Biliary tree: System of ducts carrying bile from the liver to the gallbladder and duodenum
- Common bile duct (CBD): Connects the liver and gallbladder to duodenum
- Gallbladder: Stores bile
- Cystic duct: Connects gallbladder to CBD
- Pancreatic duct: Carries pancreatic juice to the duodenum
- Ampulla of Vater: Point where CBD and pancreatic duct join, opens into the duodenum
- Sphincter of Oddi: Muscle controlling the flow of bile and pancreatic juice into the duodenum
Colon
- Caecum: First part of the colon
- Ascending colon: Part of the colon ascending from the caecum, turns at the hepatic flexure
- Transverse colon: Part of the colon running across the abdomen, turns at the splenic flexure
- Descending colon: Part of the colon descending from the splenic flexure
- Sigmoid: S-shaped part of the colon leading to the rectum
- Rectum: Final part of the colon, stores faeces before defecation
- Anus: Opening at the end of the rectum through which faeces are expelled
Lengths
- Tongue: ~50cm
- Oesophagus: ~ 150cm
- Stomach: Variable
- Small bowel: ~550 to 600cm
- Colon: Variable
Layers of the GI Tract
- Mucosa: Innermost layer, contains epithelium, lamina propria, and muscularis mucosae
- Epithelium: Protective layer, varies in different parts of the GI tract
- Lamina propria: Connective tissue containing blood vessels, nerves, and lymph vessels
- Muscularis mucosae: Thin layer of muscle, helps with movement of mucosa
- Submucosa: Connective tissue containing blood vessels, nerves, and lymph vessels
- Muscularis: Muscle layer responsible for peristalsis, divided into circular and longitudinal muscles
- Circular muscle: Contracts behind the food bolus, prevents backflow
- Longitudinal muscle: Contracts in front of the food bolus, shortens the GI tract
- Serosa: Outermost layer, protective layer composed of epithelial cells and connective tissue
Mesentery
- Attaches bowel to the posterior abdominal wall
- Connection for blood vessels, lymphatic vessels, and nerves
Arteries
- Coeliac artery: Supplies blood to the stomach, liver, and spleen
- Superior mesenteric artery (SMA): Supplies blood the small intestines and part of the colon
- Inferior mesenteric artery (IMA): Supplies blood to part of the colon
- Hepatic artery: Supplies blood to the liver
Veins
- Veins leading to the IVC drain venous blood from the kidneys, ovaries/testes, left adrenal gland, common iliac veins, the liver, diaphragm
- Veins leading to the hepatic portal vein: Collect venous blood from the spleen, inferior mesenteric vein, superior mesenteric vein
- Hepatic Portal vein: Supplies 70% of the blood supply to the liver (50% oxygen)
- Hepatic Artery: Supplies 30% of the blood supply to the liver (50% oxygen)
- Hepatic vein: Drains the liver and joins the IVC
Hepatic circulation
- Blood flows slowly through hepatic sinusoids
- Into hepatic venules, eventually into the hepatic vein
- Re-join systemic circulation at the inferior vena cava
Liver
- Removes toxins ingested with food
- Metabolises medicines
- Kupffer cells clear microorganisms
Portal hypertension
- Increased pressure in the portal system, caused by resistance in the hepatic system
- Can be caused by cirrhosis or thrombosis
- Oesophageal and rectal varices can form as a result of re-routed portal blood
- Ascites can form due to high pressure and low albumin
Lymph
- Lymph: Fluid from cells and tissues that doesn't enter the capillary system
- Lymphatic vessels: Transport lymph, connect lymph nodes
Lymphatic drainage
- Right lymphatic duct: Drains the right upper body
- Thoracic duct: Drains the left upper and bilateral lower body
Nerves
- Enteric nervous system: Intrinsic nervous system of the GI tract, controls sensory and motor functions, receives signals from extrinsic nerves (voluntary and autonomic)
- Voluntary nerves: Control lips, tongue, mastication, pelvic floor muscles, and external anal sphincter
- Autonomic nerves: Control involuntary functions of the GI tract
- Sympathetic: 'Fight or flight' response, decreases motility and secretion, increases sphincter tone
- Parasympathetic: 'Rest and digest' response, increases motility and secretion
Types of molecules in the GI tract
- Enteric hormones: Control gastric secretion and motility, mediate communication between different parts of the GI system or the CNS
- Examples: Gastrin, Histamine, Ghrelin
- Enzymes: Break down food molecules
- Amylases: Break down carbohydrates
- Oligosaccharidases: Break down small carbohydrates
- Peptidases: Break down proteins
- Lipases: Break down fats
- Special molecules: Various molecules with specific functions (e.g., intrinsic factor, bile salts)
Mouth
- Mastication: Mechanical breakdown of food by teeth
- Saliva: Secretion of saliva by salivary glands, contains salivary amylase, mucus and serous fluid
- Salivary amylase: Digests polysaccharides into monosaccharides and disaccharides
- Mucus and serous fluid: Lubricates food, binds food particles
Swallowing/deglutition
- Oropharyngeal phase: Food moved back through the mouth by the tongue, epiglottis closes off the trachea, food is forced down the oesophagus
- Oesophageal phase: Upper oesophageal sphincter opens, food moved down by peristalsis and gravity, lower oesophageal sphincter opens to allow food into the stomach
Stomach
- Gastric juice: Secretion of gastric juice by gastric glands
- Digestive enzymes: Pepsinogen (converted to pepsin by HCl), gastric lipase
- Acidic and alkaline secretions: HCl and bicarbonate ions, pH 1.5-3.5 (mainly acidic)
- HCl: Denatures proteins, activates pepsinogen, kills bacteria
- Bicarbonate ions: Neutralizes HCl
- Regulatory molecules: Gastrin, histamine, ghrelin
- Mechanical: Mixing of food with gastric juice to form chyme, propulsion of chyme into the duodenum through the pylorus
- Absorption: Water, some salts, lipid soluble drugs, alcohol (also in small intestine)
Gastric pits and cell types
- Mucous cells: Secrete mucus to protect stomach lining
- Parietal cells: Secrete HCl and intrinsic factor
- Chief cells: Secrete pepsinogen
- Enteroendocrine cells: Secrete hormones (e.g., gastrin, histamine, ghrelin)
Stomach Enteroendocrine Cells
- Gastrin: Stimulates gastric juice production
- Histamine: Binds to receptors on parietal cells, stimulating HCl release
- Ghrelin: 'Hunger hormone', regulates appetite, increases prior to a meal, targets the hypothalamus
Stomach- Chief Cells
- Pepsinogen: Secreted into the gastric pit, becomes pepsin in the presence of HCl, digests protein into polypeptides
Stomach- Parietal Cells
- Intrinsic Factor: Required for B-12 absorption in the ileum
- Hydrochloric acid (HCl): Creates optimal pH for enzyme activity, activates pepsinogen, denatures proteins, kills ingested bacteria
Vomiting
- Complex reflex managed by parts of the medulla oblongata
- Chemoreceptor trigger zone (CTZ): Receives stimuli from signals in the blood, communicates with the vomiting centre, has multiple receptors: 5HT3 (serotonin), D2 (opiates), Histamine (target for motion sickness medications)
- Vomiting centre: Directs motor responses, closes nasal cavity and glottis, relaxes LOS, contracts diaphragm and abdominal wall
Vomiting Stimuli
- In the GI tract: Drugs, toxins, distention, vagus nerve
- Outside the GI tract: Higher brain centres (sights, sounds, odours, tastes, emotions), inner ear (labyrinths of inner ear with changes in motion)
- Mechanical stimulus: Back of the pharynx
Leaving the stomach
- Carbohydrates: Fastest digestion and absorption
- Proteins: Moderate digestion and absorption
- Fatty foods: Slowest digestion and absorption, takes 3-6 hours
Intestinal phase
- Begins when chyme leaves the stomach
- Involves coordination of the pancreas, liver and gallbladder, and small intestine (duodenum)
Liver - Bile
- Hepatocytes make bile
- Bile travels down the CBD, fills gallbladder when Sphincter of Oddi (SOD) is closed
- Bile contains bile salts, bilirubin (yellowish green liquid), cholesterol, water
- Bile salts: Reduce surface tension, emulsify fat globules
- Bilirubin: Breakdown product of heme, excreted in faeces, in urine
Cholecystokinin CCK (duo)
- Hormone secreted by duodenal wall cells
- Triggered by fatty food in the duodenum
- Inhibits gastrin
- Stimulates gallbladder contraction
- Stimulates pancreatic secretion
Pancreatic Enzymes
- Enzymes secreted in zymogen granules, activated when chyme is neutralized
- Pancreatic amylase: Breaks down starch
- Pancreatic lipase: Breaks down fats
- Peptidases: Break down proteins
- Trypsin: One of the main peptidases
Small Intestine Absorption
- Plicae circulares: Folds in the intestinal wall, increase surface area
- Villi: Finger-like projections on the inner wall of the small intestine, increase surface area
- Microvilli: Microscopic projections on the surface of cells lining the villi, increase surface area
- This increased surface area aids in absorption
Digestion and Absorption of Carbohydrates
- Starch and glycogen are broken down by amylase into disaccharides (maltotriose, sucrose, lactose)
- Disaccharidases (maltase, sucrase, lactase) break down disaccharides into monosaccharides (glucose, fructose, galactose)
- Monosaccharides enter capillaries via facilitated diffusion
- Glucose is either stored as glycogen in the liver or utilized in cellular respiration (citric acid cycle, oxidative phosphorylation)
Digestion and Absorption of Proteins
- Proteins are digested into oligopeptides by pepsin and trypsin.
- Oligopeptides are digested into amino acids.
- Amino acids enter the cell via a Na+ co-transporter.
- Amino acids are absorbed into the blood for transport to other cells.
Digestion and Absorption of Fats
- Triglycerides and phospholipids are broken down by lipase into fatty acids and glycerol.
- Bile salts help form micelles.
- Micelles are absorbed across the cell membrane and then form chylomicrons.
- Chylomicrons are secreted into the basolateral space and carried in the lacteals.
Vitamins and Minerals
- Iron:
- Haem-iron (from animal sources) is easily absorbed.
- Non-haem iron (from plant sources) is converted from ferric Fe3+ to ferrous Fe2+ for better absorption.
- HCl and Vitamin C promote this conversion.
- Absorbed in the duodenum.
- Vitamin B12:
- Binds to intrinsic factor (from parietal cells).
- Absorbed in the terminal ileum.
- Sources: Meat, eggs, milk.
- Folic acid:
- Absorbed in the jejunum.
- Sources: Green leafy vegetables.
- Vitamins A, D, E, K:
- Fat-soluble vitamins.
- Require adequate bile acid secretion.
Large Intestine/Colon
- Major function: Reabsorption of water and electrolytes
- Undigested, unabsorbed material turns into faeces
- Contains up to 500 species of bacteria
Large Intestine/Colon
- Elimination of faeces:
- Movement of faecal material into the rectum signals parasympathetic neurons in the spinal cord.
- Defecation reflex:
- Rectum contracts.
- Internal sphincter relaxes.
- Brainstem and thalamus send signals to relax the external sphincter at a convenient time.
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