The Digestive System Biology 12
Intro <ul><li>The human body’s “gastrointestinal tract” or “alimentary canal” is up to 10m long </li></ul><ul><li>Along th...
Digestion Defined <ul><li>Digestion is the breakdown of substances (other organisms!) into small molecules that can be abs...
Physical Digestion <ul><li>The breakdown of food into small pieces without chemically changing them into different substan...
Chemical Digestion <ul><li>The breakdown of food through chemical reactions into small soluble molecules that cells can ab...
Chemical Digestion <ul><ul><li>Carbohydrates are also hydrolyzed with the help of enzymes into dissacharides…and then into...
Essential nutrients <ul><li>Amazingly, our body can construct most of the molecules necessary for proper function </li></u...
Part 1: The Pathway to the Stomach Stomach Esophagus Mouth
The Mouth <ul><li>The mouth has a number of digestive features: </li></ul><ul><ul><li>Teeth </li></ul></ul><ul><ul><ul><li...
The Mouth <ul><ul><ul><li>Dissolves substances into aqueous solutions (water) for higher reaction rates </li></ul></ul></u...
Swallowing <ul><li>Food entering the mouth is transported to the stomach by swallowing </li></ul><ul><li>Swallowing is a v...
Swallowing (“Deglutition”) <ul><li>Pressure of bolus on the pharynx stimulates muscle contractions in the pharynx </li></u...
 
 
Part 2: The Stomach <ul><li>The stomach is a stretchable, muscular sac </li></ul><ul><ul><li>J-shaped organ (0.5 – 2L capa...
Stomach Wall Secretions <ul><li>Parietal Cells </li></ul><ul><ul><li>Secretes roughly 2L of concentrated (pH ~1-3) HCl eac...
Stomach Wall Secretions <ul><li>Peptic or Chief Cells </li></ul><ul><ul><li>Secrete the enzyme pepsinogen </li></ul></ul><...
Stomach Wall Secretions <ul><li>Epithelial “Mucus” Cells </li></ul><ul><ul><li>Secrete mucus from gastric pits in stomach ...
Stimuli in the Stomach <ul><li>The nervous system controls the digestive activity of the stomach (secretions, muscle contr...
 
 
So…where are we? <ul><li>It takes the bolus about 9 seconds to get from your mouth to your stomach </li></ul><ul><li>In th...
 
Part 3: The Small Intestine <ul><li>“ Small” refers to diameter, not length </li></ul><ul><ul><li>About 6m long in an adul...
Big Events in the Small Intestine <ul><li>The  acid chyme is neutralized  in the small intestine by the bicarbonate ions i...
Specialized Structure <ul><li>The small intestine has a number of special features that allows it to absorb nutrients: </l...
 
Starch Digestion and Absorption <ul><li>Pancreatic juice is released into the duodenum through the pancreatic duct </li></...
 
Protein Digestion and Absorption <ul><li>Pancreatic juice contains (trypsin – active form of trypsinogen) which, like peps...
 
Fat Digestion and Absorption <ul><li>The liver makes ~1L of bile each day </li></ul><ul><ul><li>Contains water, electrolyt...
 
Fat Digestion and Absorption <ul><li>Bile emulsifies fats (mechanically) into droplets that are broken down further into f...
 
Nucleic Acid Digestion and Absorption <ul><li>Pancreatic juice released into the small intestine contains nuclease enzymes...
Part 4: Accessory Organs <ul><li>The Pancreas </li></ul><ul><ul><li>An elongated, flattened organ that lays partially behi...
 
Part 4: Accessory Organs <ul><ul><li>The exocrine (digestive) aspect of the pancreas is stimulated when chyme enters the d...
 
Part 4: Accessory Organs <ul><li>The Liver </li></ul><ul><ul><li>The largest internal organ! (about 1.5 kg) </li></ul></ul...
 
Part 4: Accessory Organs <ul><ul><li>The liver also… </li></ul></ul><ul><ul><ul><li>Converts toxic ammonia (from amino aci...
Part 5: The Large Intestine <ul><li>The last section of the gastrointestinal tract is large in diameter (6.5cm) and extend...
 
Large Intestine Functions <ul><li>Movement and compaction of undigested food (soon to be called feces) toward the anus </l...
 
Digestive System Issues <ul><li>Your turn! Put together a 5 minute presentation of a digestive issue of interest to you. Y...
Ideas… <ul><li>Ulcers </li></ul><ul><li>Indigestion </li></ul><ul><li>Heartburn </li></ul><ul><li>Cirrhosis </li></ul><ul>...
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The Digestive System

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The Digestive System

  1. 1. The Digestive System Biology 12
  2. 2. Intro <ul><li>The human body’s “gastrointestinal tract” or “alimentary canal” is up to 10m long </li></ul><ul><li>Along this pathway: </li></ul><ul><li>Ingestion  Digestion  Absorption  Egestion </li></ul><ul><li>About 95% of ingested food is absorbed and made available to our body, the rest is egested </li></ul>
  3. 3. Digestion Defined <ul><li>Digestion is the breakdown of substances (other organisms!) into small molecules that can be absorbed by cells </li></ul><ul><li>There are two classes of digestion: </li></ul><ul><ul><li>Physical </li></ul></ul><ul><ul><li>Chemical </li></ul></ul>
  4. 4. Physical Digestion <ul><li>The breakdown of food into small pieces without chemically changing them into different substances </li></ul><ul><li>Helps facilitate chemical digestion by increasing surface area </li></ul><ul><ul><li>Increased surface area exposes food pieces to enzymes and other substrate molecules and increases the rate of metabolic reactions </li></ul></ul><ul><ul><li>Examples: teeth, stomach (HCl), intestines (bile) </li></ul></ul>
  5. 5. Chemical Digestion <ul><li>The breakdown of food through chemical reactions into small soluble molecules that cells can absorb </li></ul><ul><ul><li>Lipids are broken down through hydrolysis reactions – assisted by enzyme activity: </li></ul></ul><ul><ul><li>lipid + water  glycerol + fatty acids </li></ul></ul><ul><ul><li>(lipase enzyme) </li></ul></ul>
  6. 6. Chemical Digestion <ul><ul><li>Carbohydrates are also hydrolyzed with the help of enzymes into dissacharides…and then into glucose </li></ul></ul><ul><ul><li>Starch + water  maltose </li></ul></ul><ul><ul><li>(amylase enzyme) </li></ul></ul><ul><ul><li>Proteins are broken into shorter polypeptides by the enzyme pepsin, and then cleaved into amino acids by other protease enzymes </li></ul></ul><ul><ul><li>More on these processes later… </li></ul></ul>
  7. 7. Essential nutrients <ul><li>Amazingly, our body can construct most of the molecules necessary for proper function </li></ul><ul><li>However, there are a number of substances that the body cannot synthesize and must ingest </li></ul><ul><ul><li>Essential Amino Acids (8) </li></ul></ul><ul><ul><li>Essential Fatty Acids (2) </li></ul></ul><ul><ul><li>Essential Vitamins (4 water and 5 fat soluble) </li></ul></ul><ul><ul><li>Trace Elements (many…) </li></ul></ul>
  8. 8. Part 1: The Pathway to the Stomach Stomach Esophagus Mouth
  9. 9. The Mouth <ul><li>The mouth has a number of digestive features: </li></ul><ul><ul><li>Teeth </li></ul></ul><ul><ul><ul><li>Omnivores, like ourselves, have incisors (rip/shred), canines (hold/tear), premolars and molars (grind) </li></ul></ul></ul><ul><ul><ul><li>Teeth mechanically break down food into pieces </li></ul></ul></ul><ul><ul><li>Saliva </li></ul></ul><ul><ul><ul><li>Secreted into mouth by salivary glands (roof of mouth and under tongue) </li></ul></ul></ul><ul><ul><ul><li>Moistens food to allow “taste” sensations and lubricate it for transport through the digestive system </li></ul></ul></ul>
  10. 10. The Mouth <ul><ul><ul><li>Dissolves substances into aqueous solutions (water) for higher reaction rates </li></ul></ul></ul><ul><ul><ul><li>Contains the enzyme amylase which converts starch into dissacharides </li></ul></ul></ul><ul><ul><li>Mucus </li></ul></ul><ul><ul><ul><li>Secreted into mouth (and other organs) to coat epithelial tissues/surfaces </li></ul></ul></ul><ul><ul><ul><li>Made of water, mucins (glycoproteins), and salts </li></ul></ul></ul><ul><ul><ul><li>Lubricates the movement of food </li></ul></ul></ul><ul><ul><ul><li>Serves as a barrier against noxious substances </li></ul></ul></ul><ul><ul><ul><li>Holds food together in a “bolus” (ball of food) </li></ul></ul></ul>
  11. 11. Swallowing <ul><li>Food entering the mouth is transported to the stomach by swallowing </li></ul><ul><li>Swallowing is a very complex process controlled by nerve and muscle tissue coordination </li></ul>
  12. 12. Swallowing (“Deglutition”) <ul><li>Pressure of bolus on the pharynx stimulates muscle contractions in the pharynx </li></ul><ul><li>The soft palate seals off the nasopharynx to prevent food from entering the nose </li></ul><ul><li>The larynx (voice box) move up to stop the passage of food through the trachea (throat) </li></ul><ul><li>This stops breathing and widens the esophagus for passage of the bolus </li></ul><ul><li>Food passes the trachea to the stomach through muscle contractions called peristalsis </li></ul><ul><li>The trachea is ultimately blocked by a small flap called the epiglottis </li></ul>
  13. 15. Part 2: The Stomach <ul><li>The stomach is a stretchable, muscular sac </li></ul><ul><ul><li>J-shaped organ (0.5 – 2L capacity) </li></ul></ul><ul><ul><li>3 layers of muscle tissue </li></ul></ul><ul><ul><li>Upper section dotted with deep depressions called gastric pits – which secrete “gastric juice” and mucus (~500mL after a large meal!) </li></ul></ul><ul><ul><li>Has two sphincter portals: (like draw strings) </li></ul></ul><ul><ul><ul><li>Entrance: cardiac sphincter </li></ul></ul></ul><ul><ul><ul><li>Exit: pyloric sphincter </li></ul></ul></ul>
  14. 16. Stomach Wall Secretions <ul><li>Parietal Cells </li></ul><ul><ul><li>Secretes roughly 2L of concentrated (pH ~1-3) HCl each day </li></ul></ul><ul><ul><li>HCl mechanically breaks down tissues and proteins into smaller pieces </li></ul></ul><ul><ul><li>Acidic secretions make the stomach inhospitable to most bacteria (exception: heliobacter pyloria) </li></ul></ul><ul><ul><li>Activates pepsinogen… </li></ul></ul>
  15. 17. Stomach Wall Secretions <ul><li>Peptic or Chief Cells </li></ul><ul><ul><li>Secrete the enzyme pepsinogen </li></ul></ul><ul><ul><li>Pepsinogen’s shape is changed to an “active” state by HCl (pH denatures the protein-based enzyme into the desired form!) </li></ul></ul><ul><ul><li>The active form of the enzyme, pepsin, catalyzes the chemical breakdown of protein into short amino acid chains </li></ul></ul><ul><ul><li>No effect on carbs and fats! </li></ul></ul>
  16. 18. Stomach Wall Secretions <ul><li>Epithelial “Mucus” Cells </li></ul><ul><ul><li>Secrete mucus from gastric pits in stomach wall </li></ul></ul><ul><ul><li>Lubricates and facilitates food transport </li></ul></ul><ul><ul><li>Protects walls from abrasive food particles </li></ul></ul><ul><ul><li>Protects the protein-based walls of the stomach from autodigestion by gastric juices! </li></ul></ul><ul><ul><li>Failure of mucus results in ulcers (bacteria, stress, and diet causes) </li></ul></ul>
  17. 19. Stimuli in the Stomach <ul><li>The nervous system controls the digestive activity of the stomach (secretions, muscle contractions) by releasing the hormone gastrin when protein-rich food is sensed or an inhibitor when the stomach is empty </li></ul><ul><li>Other hormones stimulate the release of bile and pancreatic juices… </li></ul>
  18. 22. So…where are we? <ul><li>It takes the bolus about 9 seconds to get from your mouth to your stomach </li></ul><ul><li>In the stomach, the partially digested food mass is called “chyme” </li></ul><ul><li>This “acid chyme” remains in the stomach for 2-6 hours (depending on the type of food) before release by the pyloric sphincter into the small intestine… </li></ul>
  19. 24. Part 3: The Small Intestine <ul><li>“ Small” refers to diameter, not length </li></ul><ul><ul><li>About 6m long in an adult human </li></ul></ul><ul><ul><li>Tapers from ~3cm diameter at the pyloric sphincter to ~2cm diameter at the entrance to the large intestine (ileocecal valve) </li></ul></ul><ul><li>Three major sections of the small intestine: </li></ul><ul><ul><li>Duodenum (fixed C-shaped section ~25-30cm) </li></ul></ul><ul><ul><li>Jejunum (4-5m long) </li></ul></ul><ul><ul><li>Ileum (1-1.5m) </li></ul></ul>
  20. 25. Big Events in the Small Intestine <ul><li>The acid chyme is neutralized in the small intestine by the bicarbonate ions in pancreatic juices </li></ul><ul><li>A host of enzymes from the pancreas are released in the first two sections of the small intestine to break down all four basic nutrients </li></ul><ul><li>Bile salts are released from the liver  gall bladder  intestine to emulsify fats (mechanical digestion) </li></ul><ul><li>Most digestion occurs in the duodenum </li></ul><ul><li>Most absorption occurs in latter intestinal sections </li></ul>
  21. 26. Specialized Structure <ul><li>The small intestine has a number of special features that allows it to absorb nutrients: </li></ul><ul><ul><li>Length (increases surface area) </li></ul></ul><ul><ul><li>Folding (slows the passage of food and increases surface area) </li></ul></ul><ul><ul><li>Villi (finger-like projections, ~30 per mm 2 , ~1mm long, increases surface area) </li></ul></ul><ul><ul><li>Microvilli (villi epithelial cell projections increase surface area) </li></ul></ul><ul><li>The surface area of the small intestine is about the size of a tennis court! </li></ul>
  22. 28. Starch Digestion and Absorption <ul><li>Pancreatic juice is released into the duodenum through the pancreatic duct </li></ul><ul><li>This juice contains pancreatic amylase that helps hydrolyze starch into disaccharides like maltose, lactose, and sucrose </li></ul><ul><li>Additional enzymes such as maltase, lactase, and sucrase further reduce disaccharides into simple sugars – namely, glucose </li></ul><ul><li>Glucose is actively transported (against its concentration gradient) into villi capillaries and taken directly to the liver </li></ul><ul><li>The liver converts some glucose into fat, and some into glycogen (and vice versa) in the hours between meals – maintaining blood sugar levels </li></ul>
  23. 30. Protein Digestion and Absorption <ul><li>Pancreatic juice contains (trypsin – active form of trypsinogen) which, like pepsin in the stomach, aids the break down of proteins into shorter polypeptides </li></ul><ul><li>The juice also contains peptidases which help hydrolyze short polypeptides into amino acids </li></ul><ul><li>Amino acids are actively transported by proteins in the villi wall and taken to the liver by capilliaries for processing </li></ul><ul><li>The liver breaks down some amino acids to produce carbohydrates, fats, blood proteins, bile, etc. </li></ul><ul><li>The break down of amino acids into glucose releases toxic ammonia…which is converted to urea…which is transported to the kidneys for excretion! </li></ul>
  24. 32. Fat Digestion and Absorption <ul><li>The liver makes ~1L of bile each day </li></ul><ul><ul><li>Contains water, electrolytes, bile acids, cholesterol, phospholipids, and bilirubin </li></ul></ul><ul><ul><li>Green color due to bilirubin </li></ul></ul><ul><ul><li>Released into gall bladder for storage and concentration (up to five-fold concentration!) </li></ul></ul><ul><ul><li>Bile salts are able to form micelles to “dissolve” fat-soluble vitamins </li></ul></ul><ul><ul><li>Many waste products (500mg of cholesterol per day) are eliminated from the body by secretion into bile…if not  gall stones! </li></ul></ul>
  25. 34. Fat Digestion and Absorption <ul><li>Bile emulsifies fats (mechanically) into droplets that are broken down further into fatty acids and glycerol by pancreatic lipase enzymes </li></ul><ul><li>Fatty acids are lipid soluble and pass through the villi membranes into lacteals (central lymph vessel of the villi) </li></ul><ul><li>Solution called “lymph” is then carried to the heart and then distributed to the body’s cells through blood circulation </li></ul>
  26. 36. Nucleic Acid Digestion and Absorption <ul><li>Pancreatic juice released into the small intestine contains nuclease enzymes that help break down RNA and DNA into nucleotides </li></ul><ul><li>Nucleotidase enzymes secreted by the small intestine itself further aid in the break down nucleotides into phosphate, sugars, and nitrogen bases </li></ul><ul><li>These nucleotide components are then absorbed into capilliaries and taken to the liver </li></ul>
  27. 37. Part 4: Accessory Organs <ul><li>The Pancreas </li></ul><ul><ul><li>An elongated, flattened organ that lays partially behind the stomach – deep in the chest cavity </li></ul></ul><ul><ul><li>Acts as both an endocrine (hormone releasing) gland and exocrine (digestive processes) gland </li></ul></ul><ul><ul><li>Two key hormones released into blood from the “islets of Langerhans” (2000 of them!): </li></ul></ul><ul><ul><ul><li>Insulin – released when blood glucose high </li></ul></ul></ul><ul><ul><ul><li>Glucagon – released when blood glucose low </li></ul></ul></ul>
  28. 39. Part 4: Accessory Organs <ul><ul><li>The exocrine (digestive) aspect of the pancreas is stimulated when chyme enters the duodenum </li></ul></ul><ul><ul><li>The hormones secretin (in response to acid) and cholecystokinin or CCK ( in response to protein and fat) are sent by duodenum cells to pancreas </li></ul></ul><ul><ul><li>Pancreatic juice is produced and released into the duodenum in response to these hormones </li></ul></ul><ul><ul><li>A host of enzymes are released </li></ul></ul><ul><ul><li>HCO 3 - (bicarbonate ions) in the juice change the chyme pH from ~2-3 to ~7-8 (slightly basic!) </li></ul></ul>
  29. 41. Part 4: Accessory Organs <ul><li>The Liver </li></ul><ul><ul><li>The largest internal organ! (about 1.5 kg) </li></ul></ul><ul><ul><li>Lobed (large right lobe, small left lobe), triangular shape </li></ul></ul><ul><ul><li>All blood from the intestines is brought back to the liver through the hepatic portal vein </li></ul></ul><ul><ul><li>As we have seen… </li></ul></ul><ul><ul><ul><li>Liver produces bile (emulsifies fats) </li></ul></ul></ul><ul><ul><ul><li>Liver converts glucose  glycogen in response to pancreatic hormones </li></ul></ul></ul>
  30. 43. Part 4: Accessory Organs <ul><ul><li>The liver also… </li></ul></ul><ul><ul><ul><li>Converts toxic ammonia (from amino acid and nucleotide breakdown) into urea – excreted in urine by the kidneys </li></ul></ul></ul><ul><ul><ul><li>Detoxifies blood by breaking down: alcohol, hormones, antibiotics, drugs, excess vitamins, etc. </li></ul></ul></ul><ul><ul><ul><li>Stores some vitamins </li></ul></ul></ul><ul><ul><ul><li>Synthesizes blood proteins like albumin (regulates osmotic pressure of blood) and fibrinogen (aids in blood clotting) </li></ul></ul></ul><ul><ul><ul><li>Destroys old red blood cells (recycles iron) and converts hemoglobin into bilirubin/biliverdin (bile pigments) </li></ul></ul></ul>
  31. 44. Part 5: The Large Intestine <ul><li>The last section of the gastrointestinal tract is large in diameter (6.5cm) and extend ~1.5m from the end of the ileum to the anus </li></ul><ul><li>The entrance to the large intestine is called the ileo-caecal valve (another sphincter) </li></ul><ul><li>There are four sections of the large intestine: </li></ul><ul><ul><li>Caecum and Ascending Colon </li></ul></ul><ul><ul><li>Transverse Colon </li></ul></ul><ul><ul><li>Descending Colon </li></ul></ul><ul><ul><li>Rectum </li></ul></ul>
  32. 46. Large Intestine Functions <ul><li>Movement and compaction of undigested food (soon to be called feces) toward the anus </li></ul><ul><li>Absorption of water (about 10L of water per day!) and some salts (sodium, etc.) </li></ul><ul><li>E. coli bacteria present in large intestine… </li></ul><ul><ul><li>Produces feces (60% bacteria!) </li></ul></ul><ul><ul><li>Synthesizes vitamin K for liver </li></ul></ul><ul><li>Defecation – contraction of muscles in the rectum that forces open the anal sphincter (anus) </li></ul>10x more bacteria in your gut than cells in your body!
  33. 48. Digestive System Issues <ul><li>Your turn! Put together a 5 minute presentation of a digestive issue of interest to you. Your presentation should meet the following criteria: </li></ul><ul><ul><ul><li>Have clear linkage to digestion (approved) </li></ul></ul></ul><ul><ul><ul><li>Provide a single page handout (1 side) </li></ul></ul></ul><ul><ul><ul><li>Make use of visuals </li></ul></ul></ul><ul><ul><ul><li>Be referenced </li></ul></ul></ul>
  34. 49. Ideas… <ul><li>Ulcers </li></ul><ul><li>Indigestion </li></ul><ul><li>Heartburn </li></ul><ul><li>Cirrhosis </li></ul><ul><li>Jaundice </li></ul><ul><li>Diarrhea </li></ul><ul><li>Constipation </li></ul><ul><li>Flatulence </li></ul><ul><li>Incontinence </li></ul><ul><li>Gall stones </li></ul><ul><li>Diabetes </li></ul><ul><li>Polyps </li></ul><ul><li>Hepatitis </li></ul><ul><li>Dysphasia </li></ul><ul><li>Hernia </li></ul><ul><li>Hemorrhoids </li></ul><ul><li>Irritable Bowel Syndrome </li></ul>

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