The pathway of digestion Parts of digestive system Role of enzymes

1. Digestion
MS ANBREEN FATIMA

2. Introduction
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3. Digestive System
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4. Functions of the Digestive System
1. Ingestion: taking food into mouth.
2. Secretion: release of water, acid, buffers, and enzymes into lumen of GI
tract.
3. Motility (mixing and propulsion): churning and movement of food
through GI tract.
4. Digestion: mechanical and chemical breakdown of food.
5. Absorption: passage of digested products from GI tract into blood and
lymph.
6. Defecation: elimination of feces from GI tract.
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5. Alimentary Canal/Gastrointestinal Tract
•Digestion takes place in the alimentary canal with the aid of its accessory
organs.
•Length of the GIT : 5-7 m (16.5-23 ft.).
•Long, muscular tube from mouth to anus.
•It includes:
 Oral cavity
 Pharynx
 Esophagus
 Stomach
 Small intestine
 Large intestine
Accessory Organs
◦ Three organs located outside of the alimentary canal are considered
part of the digestive system—the liver, gallbladder, and pancreas.
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6. Alimentary Canal/Gastrointestinal Tract
Sphincters:
◦ Muscle rings, called sphincters, separate
segments of the alimentary canal.
◦ Act as valves to control the passage of food.
◦ When the muscles contract, the passageway
closes; when the muscles relax, the passageway
opens.
Mucosa:
◦ Lines the GIT and secretes mucus, which
lubricates the canal and helps facilitate the
smooth passage of food.
◦ Secretes the digestive enzymes of the stomach
and small intestine.
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7. Types of Digestion
Digestion is a two-part process — half mechanical, half chemical:
Mechanical digestion
◦ Takes place in mouth and stomach.
◦ Teeth break food into small pieces to swallow without choking.
◦ Stomach: churning action continues to break food into smaller particles.
Chemical digestion
◦ Occurs at every point in the digestive tract where enzymes and other substances, such as hydrochloric
acid (from stomach glands) and bile (from the liver), dissolve food, releasing the nutrients inside.
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8. The Mouth
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9. Mouth/Buccal or Oral Cavity
The mouth, also referred to as the oral or buccal cavity, is formed by the cheeks, hard and soft palates, and tongue.
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10. Functions of Oral Cavity
Sensory analysis
◦ Of material before swallowing
Mechanical processing
◦ Through actions of teeth, tongue, and palatal
surfaces
Lubrication
◦ Mixing with mucus and salivary gland
secretions
Limited digestion
◦ Of carbohydrates and lipids
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11. Salivary Gland
Gland that releases saliva into the oral cavity.
Ordinarily, just enough saliva is secreted to keep the mucous membranes of the mouth and
pharynx moist and to cleanse the mouth and teeth.
3 pairs of major salivary glands: the parotid, submandibular, and sublingual glands.
Saliva lubricates and dissolves foods and begins the chemical breakdown of carbohydrates and
lipids.
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12. Saliva
•Chemically, saliva is 99.5% water and 0.5%
solutes.
•Among the solutes are ions, including sodium,
potassium, chloride, bicarbonate, and
phosphate.
•Also present are some dissolved gases and
various organic substances, including urea and
uric acid, mucus, immunoglobulin A, the
bacteriolytic enzyme lysozyme, and salivary
amylase, a digestive enzyme that acts on
starch.
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13. Functions of Saliva
•The water in saliva provides a medium for dissolving foods so that they can be tasted by gustatory
receptors and so that digestive reactions can begin.
•Chloride ions activate salivary amylase (enzyme that lyses starch in the mouth into maltose,
maltotriose, and α-dextrin).
•Bicarbonate and phosphate ions buffer acidic foods that enter the mouth, so saliva is only slightly
acidic (pH 6.35–6.85).
•Salivary glands (like the sweat glands of the skin) help remove waste molecules from the body, which
accounts for the presence of urea and uric acid in saliva.
•Mucus lubricates food so it can be moved around easily in the mouth, formed into a ball, and
swallowed.
•Immunoglobulin A (IgA) prevents attachment of microbes so they cannot penetrate the epithelium,
and the enzyme lysozyme kills bacteria; however, these substances are not present in large enough
quantities to eliminate all oral bacteria
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14. Mechanical and Chemical Digestion
in the Mouth
Mechanical
•Results from chewing, or mastication, in which
food is manipulated by the tongue, ground by
the teeth, and mixed with saliva.
•As a result, the food is reduced to a soft,
flexible, easily swallowed mass called a bolus.
Chemical
•Salivary amylase and lingual lipase.
•Salivary glands  Salivary amylase  starts
breakdown of starch.
•Lingual glands  Lingual lipase  starts
breakdown of triglycerides.
• Enzyme becomes activated in the acidic
environment of the stomach and thus starts to
work after food is swallowed.
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15. Summary
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16. The Oesophagus
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17. Esophagus
Aka gullet
Hollow muscular tube
Connects pharynx to stomach.
About 25 cm (10 in.) long and 2 cm (0.80 in.)
wide.
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18. Esophagus
•Conveys solid food and liquids to the stomach
via peristalsis.
•Cardiac sphincter: opens to permit passage of
food.
•Sphincter closes to prevent backup of stomach
contents.
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19. Deglutition
Swallowing occurs in three stages:
1. The voluntary stage, in which
the bolus is passed into the
oropharynx;
2. The pharyngeal stage, the
involuntary passage of the
bolus through the pharynx
into the esophagus; and
3. The esophageal stage, the
involuntary passage of the
bolus through the esophagus
into the stomach.
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20. Summary of Digestive Activities in the
Pharynx and Esophagus
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21. The Stomach
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22. Gastric Anatomy

23. Digestive Secretions
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24. Digestive Secretions
•Produces mucus, hydrochloric acid, pepsin, and the hormone gastrin.
•Mucus protects the stomach from self-digestion,
•Hydrochloric acid destroys many organisms and provides the required acid environment for
pepsin activity,
•Pepsin converts proteins to polypeptides, and
•Gastrin produced in the stomach stimulates gastric acid secretion.
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25. Functions of the Stomach
1. Mixes saliva, food, and gastric juice to form chyme.
2. Serves as reservoir for food before release into small intestine.
3. Secretes gastric juice, which contains HCl (kills bacteria and denatures proteins), pepsin
(begins the digestion of proteins), intrinsic factor (aids absorption of vitamin B12), and
gastric lipase (aids digestion of triglycerides).
4. Secretes gastrin into blood.
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26. Digestion in the Stomach
•Stomach performs preliminary digestion of proteins by pepsin
–Some digestion of carbohydrates (by salivary amylase)
–Lipids (by lingual lipase)
•Stomach contents
–Become more fluid
–pH approaches 2.0
–Pepsin activity increases
–Protein disassembly begins
•Although digestion occurs in the stomach, nutrients are not absorbed there

27. Summary of
Digestive
Activities in the
Stomach
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28. Accessory Organs
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29. Liver
•Produces greenish fluid called bile that contains cholesterol, a fat substance, bile acids, and
several bile pigments.
•Liver  bile  hepatic duct  cystic duct  gall bladder (storage)
•After meals, the gallbladder contracts, forcing bile into the common bile duct joining with the
pancreatic duct, just before the entrance of the duodenum.
•The duodenum receives a mixture of bile and pancreatic juices.
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30. Pancreas
•Named because of fleshy appearance; Greek “pan” means “all,” and krea means “flesh”
•Produces amylase and lipase; pass via pancreatic duct to duodenum
•Secretes insulin (β cells) and glucagon
•Pancreas is made up of small clusters of glandular epithelial cells, about 99% of the clusters,
called acini, constitute the exocrine portion of the organ.
•Cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice.
• The remaining 1% of the clusters, called pancreatic islets, form endocrine portion; cells secrete
the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide.
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31. Pancreatic Juice Composition
•Produces 1200–1500 mL of pancreatic juice,
•Clear, colorless liquid
•Mostly of water, some salts, sodium bicarbonate, and several enzymes.
•Sodium bicarbonate - slightly alkaline pH (7.1–8.2) that buffers acidic gastric juice in chyme,
stops the action of pepsin from the stomach, and creates the proper pH for the action of
digestive enzymes in the small intestine.
•Enzymes:
Starch-digesting enzyme called pancreatic amylase
Enzymes that digest proteins into peptides - trypsin, chymotrypsin, carboxypeptidase and elastase
Triglyceride–digesting enzyme - pancreatic lipase
Nucleic acid–digesting enzymes called ribonuclease and deoxyribonuclease
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32. Gall Bladder
•Saclike structure; 3–4 inches long
•Part of the biliary tract (hepatic, cystic, and common bile ducts).
•Main function: bile storage site
•Cholecystokinin (CCK) is released in the small intestine in response to the presence of proteins
and lipids
•This hormone signals the gallbladder to contract
•Bile then emulsifies the lipids; that is, it reduces the lipids into
•Smaller globules and disperses them so they are more accessible to digestive enzymes
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33. The Small Intestine
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34. Small Intestine
90% of absorption occurs in the small intestine

35. Functions of the Small Intestine
1. Segmentations mix chyme with digestive juices and bring food into contact with mucosa for
absorption; peristalsis propels chyme through small intestine.
2. Completes digestion of carbohydrates, proteins, and lipids; begins and completes digestion
of nucleic acids.
3. Absorbs about 90% of nutrients and water that pass through digestive system.
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36. Digestion in Small Intestine
•The entry of chyme into the duodenum stimulates the secretion secretin and CCK.
◦ Secretin stimulates the production of bile and the secretion of sodium bicarbonate juice by the
pancreas.
◦ CCK stimulates the contraction of the gallbladder and pancreatic enzymes
•Bile emulsify fats
•Sodium bicarbonate juice neutralizes the gastric juice that enters the duodenum.
◦ Necessary to prevent damage to the lining of duodenum.
◦ Mucus provides some measure of protection against such damage.
•Intestinal juice is also secreted in response to the presence of chyme in the duodenum.
•Peristaltic action mixes the bile, pancreatic juice, and intestinal juice with the chyme
•The collective action of these juices yields the final products of digestion.
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37. Villi
Enlarged villus showing lacteal,
capillaries, intestinal glands, and cell
types
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38. Small Intestine
The Duodenum
◦ The segment of small intestine closest to stomach
◦ 25 cm (10 in.) long
◦ “Mixing bowl” that receives chyme from stomach and digestive secretions from pancreas and liver
◦ Functions of the duodenum
◦ To receive chyme from stomach
◦ To neutralize acids before they can damage the absorptive surfaces of the small intestine

39. Small Intestine
The Jejunum
◦ Is the middle segment of small intestine
◦ 2.5 meters (8.2 ft) long
◦ Is the location of most
◦ Chemical digestion
◦ Nutrient absorption
◦ Has few plicae circulares
◦ Small villi

40. Small Intestine
The Ileum
◦ The final segment of small intestine
◦ 3.5 meters (11.48 ft) long
◦ Ends at the ileocecal valve, a sphincter that controls flow of material from the ileum into the large intestine

41. Small Intestine

42. Small Intestine
Intestinal Secretions
◦ Watery intestinal juice
◦ 1.8 liters per day enter intestinal lumen
◦ Moisten chyme
◦ Assist in buffering acids
◦ Keep digestive enzymes and products of
digestion in solution
Intestinal Movements
◦ Chyme arrives in duodenum
◦ Weak peristaltic contractions move it slowly
toward jejunum
◦ Myenteric reflexes
◦ Not under CNS control
◦ Parasympathetic stimulation accelerates local
peristalsis and segmentation

43. 43
Summary of
Digestive
Activities in
the
Pancreas,
Liver,
Gallbladder,
and Small
Intestine

44. The Large Intestine
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45. Large Intestine Functions
◦ Reabsorption of water
◦ Compaction of intestinal contents
into feces
◦ Absorption of important vitamins
produced by bacteria
◦ Storage of fecal material prior to
defecation

46. Parts of Large Intestine
The Cecum
◦ Is an expanded pouch
◦ Receives material arriving from
the ileum
◦ Stores materials and begins
compaction
Appendix
◦ Also called vermiform appendix
◦ Is a slender, hollow appendage about
9 cm (3.6 in.) long
◦ Is dominated by lymphoid nodules (a
lymphoid organ)

47. Parts of Large Intestine
The Colon
◦ Has a larger diameter and thinner wall than small intestine
◦ The wall of the colon
◦ Forms a series of pouches (haustra)
◦ Haustra permit expansion and elongation of colon

48. Parts of Colon
Ascending Colon
◦ Begins at superior border of cecum
◦ Ascends along right lateral and posterior wall of peritoneal cavity to inferior surface
of the liver and bends at right colic flexure (hepatic flexure)
Transverse Colon
◦ Crosses abdomen from right to left; turns at left colic flexure (splenic flexure)
◦ Is supported by transverse mesocolon
◦ Is separated from anterior abdominal wall by greater omentum

49. Parts of Colon
The Descending Colon
◦ Proceeds inferiorly along left side to the iliac fossa (inner surface of left ilium)
◦ Is retroperitoneal, firmly attached to abdominal wall
The Sigmoid Colon
◦ Is an S-shaped segment, about 15 cm (6 in.) long
◦ Starts at sigmoid flexure
◦ Lies posterior to urinary bladder
◦ Is suspended from sigmoid mesocolon
◦ Empties into rectum

50. Parts of Colon

51. Parts of Large Intestine
The Rectum
◦ Forms last 15 cm (6 in.) of digestive tract
◦ Is an expandable organ for temporary
storage of feces
◦ Movement of fecal material into rectum
triggers urge to defecate
The anal canal is the last portion of the
rectum
◦ Contains small longitudinal folds called anal
columns
Anus
◦ Also called anal orifice
◦ Is exit of the anal canal
◦ Has keratinized epidermis like skin

52. Summary
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53. Overview of Digestive System
•Consists of three divisions including the mouth, the pharynx, and the alimentary canal,
comprising the esophagus, the stomach, the small intestine, and the large intestine.
•Performs specific functions: essential food-processing activities. In the mouth, the food is
moistened by saliva; is masticated and mixed by the mandible, teeth, and tongue; and is
propelled by the pharynx and esophagus into the stomach, where it is mixed with the gastric
juice and converted into chyme.
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54. Overview of Digestive System
•Performs specific functions: in the small intestine, the food or chyme is digested by secretions
from glands in the intestinal wall and from the liver, gallbladder, and pancreas; digested end
products are absorbed into the blood and lymph capillaries in the intestinal wall.
•Performs specific functions: in the large intestine, water and electrolytes are absorbed and the
waste products are transported to the rectum and anal canal, where they are eliminated as
feces.
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55. Digestive System
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56. 56
Summary
of
Digestive
Enzymes