OFFICIAL
Chapter 23: The Digestive System
1. Alternate Names for the Digestive System
The digestive system may also be referred to as the gastrointestinal (GI) tract or alimentary canal.
2. Sub-groups of the Digestive System
The digestive system is divided into two sub-groups:
Alimentary Canal: Includes the organs through which food passes and is digested (e.g., mouth, esophagus, stomach, intestines).
Accessory Organs: Organs that aid in digestion but do not form the pathway for food (e.g., liver, pancreas, gallbladder).
3. Main Organs of the Digestive System
Mouth
Esophagus
Stomach
Small intestine
Large intestine
4. Accessory Organs of the Digestive System
Teeth
Tongue
Salivary glands (parotid, submandibular, sublingual)
Liver
Gallbladder
Pancreas
5. Essential Activities of the Digestive System
There are six essential activities performed by the digestive system:
Ingestion: Intake of food and liquids via the mouth.
Propulsion: Involves swallowing and peristalsis which moves food through the digestive tract.
Mechanical digestion: Breakdown of food into smaller pieces (includes chewing and the churning action of the stomach).
Chemical digestion: Enzymatic breakdown of food into simpler substances.
Absorption: Process where the products of digestion are absorbed into the bloodstream or lymphatic system.
Defecation: Elimination of indigestible substances via the anus in the form of feces.
6. Controls of the Digestive System
Intrinsic controls: Governed by the enteric nervous system which coordinates reflexes such as peristalsis and secretion.
Extrinsic controls: Involve the central nervous system (CNS) influences such as autonomic nervous system and hormonal responses that regulate digestive processes.
7. The Peritoneum
The peritoneum is a continuous serous membrane that lines the abdominal cavity and covers the organs within, providing lubrication and allowing for smooth movements.
8. The Four Tunics of the Digestive System
The digestive system is composed of four tunics:
Mucosa: The innermost layer; contains epithelium, lamina propria, and muscularis mucosae. Function: secretion, absorption, and protection.
Submucosa: A layer of connective tissue; contains blood vessels, lymphatics, and nerves. Function: support for the mucosa and elasticity.
Muscularis externa: Comprised of two layers of muscle (circular and longitudinal) that facilitate peristalsis and segmentation.
Serosa: The outermost layer; also known as the visceral peritoneum, it reduces friction between digestive organs and surrounding structures.
9. Boundaries of the Oral Cavity
The oral cavity is bounded by the lips anteriorly, cheeks laterally, the hard and soft palates superiorly, and the tongue inferiorly.
10. Compact Mass of Food
A compact mass of food is called a bolus.
11. Types of Papillae on the Tongue
There are several types of papillae on the tongue:
Filiform papillae: Most numerous, providing texture but no taste sensation.
Fungiform papillae: Scattered among filiform; have taste buds and are mushroom-shaped.
Circumvallate papillae: Large and arranged in a V-shape at the back of the tongue; contain taste buds.
Foliate papillae: Located on the sides of the tongue; contain taste buds and are more prominent in infants.
12. Functions of Saliva
Saliva serves multiple functions:
Moistening food: Facilitates chewing and swallowing.
Digestion: Contains enzymes like amylase that initiate the breakdown of carbohydrates.
Protection: Helps wash away food particles and bacteria, contains antimicrobial agents.
13. Different Salivary Glands
Parotid Glands: Located anterior to the ears; largest salivary glands.
Submandibular Glands: Located under the jaw; produce the majority of saliva.
Sublingual Glands: Located beneath the tongue; smallest salivary glands.
The parotid glands swell during mumps infection.
14. Saliva and Microorganisms
Saliva protects against microorganisms through the presence of enzymes (like lysozyme), antibodies (such as IgA), and various antimicrobial compounds that inhibit bacterial growth.
15. Classifications of Teeth
There are four classifications of teeth, each with distinct functions:
Incisors: Sharp teeth at the front for cutting food.
Canines (cuspids): Pointed teeth for tearing food.
Premolars (bicuspids): Flat-topped teeth for crushing and grinding.
Molars: Larger teeth at the back for grinding food.
16. Tooth Structures
Crown: The visible part of the tooth above the gum line.
Enamel: Hardest substance in the body covering the crown, primarily composed of hydroxyapatite.
Root: Portion of the tooth embedded in the jawbone.
Periodontal ligament: Connects the tooth root to the surrounding alveolar bone.
Dentin: Tissue beneath the enamel, gives tooth its shape, and is less hard than enamel.
Pulp cavity: The center of the tooth containing nerves and blood vessels.
Root canal: The space within the root, containing pulp.
Apical foramen: Opening at the tip of the root through which nerves and vessels enter the tooth.
17. Hardest Substance in the Body
The hardest substance in the body is enamel. Once formed, enamel cannot be replaced or regenerated.
18. Regions of the Pharynx in Swallowing
During swallowing, food passes through two regions of the pharynx:
Oropharynx (middle part, behind the mouth).
Laryngopharynx (lower part before the esophagus).
19. Tube from Pharynx to Stomach
The muscular tube that passes food from the pharynx to the stomach is the esophagus.
20. Esophagus Piercing the Diaphragm
The esophagus pierces the diaphragm at the esophageal hiatus.
21. Esophagus Meets the Stomach
The esophagus meets the stomach at the cardia region of the stomach.
22. Structure Keeping Esophagus-Stomach Opening Closed
The lower esophageal sphincter (or cardiac sphincter) maintains closure between the esophagus and stomach when food is not passing through.
23. Digestion Start Point
Digestion begins in the mouth as food is mechanically and chemically processed.
24. Enzyme for Starch and Glycogen Digestion
The enzyme that starts to digest starch and glycogen in the mouth is salivary amylase.
25. Digestive Processes in the Mouth
The digestive processes occurring in the mouth include mastication (chewing) and the initial chemical breakdown of carbohydrates.
26. Definition of Mastication
Mastication: The process of chewing food into smaller pieces to facilitate swelling and digestion.
27. Definition of Deglutition
Deglutition: The act of swallowing, involving two phases:
Buccal phase: Voluntary phase where food is formed into a bolus and pushed to the back of the mouth.
Pharyngeal-esophageal phase: Involuntary phase where the bolus is pushed down the pharynx into the esophagus.
Multiple muscle groups are involved in deglutition, including those for the tongue, soft palate, pharynx, and esophagus.
28. Two Phases of Swallowing
The two phases of swallowing:
Voluntary Phase (buccal): Mainly under voluntary control, where the tongue pushes the bolus back.
Involuntary Phase (pharyngeal-esophageal): Reflexive process controlled by the medulla oblongata, preventing food from entering the airway during swallowing.
29. Definition of Peristalsis
Peristalsis: The process of wave-like muscle contractions that moves food through the digestive tract, essential for propulsion.
30. Food Conversion in the Stomach
Food is converted to chyme in the stomach as it mixes with gastric juices.
31. Stomach Structures
Structures of the stomach include:
Rugae: Folds in the stomach lining that allow expansion.
Cardia: The region where the esophagus connects to the stomach.
Fundus: The upper dome-shaped region of the stomach.
Body: The main part of the stomach that contains most gastric glands.
Pyloric region: The lower section that connects to the duodenum of the small intestine.
Pyloric antrum: The initial section of the pyloric region before the pyloric canal.
Pyloric canal: The passage leading from the pyloric antrum to the pylorus.
Pylorus: The terminal part of the stomach that opens into the small intestine.
Pyloric sphincter: A muscle that regulates the passage of chyme into the small intestine.
Greater curvature: The longer curve of the stomach's outer edge.
Lesser curvature: The shorter curve along the inner edge of the stomach.
Greater omentum: A fold of peritoneum hanging from the greater curvature.
Lesser omentum: A fold of peritoneum connecting the lesser curvature to the liver.
32. Unique Muscular Layer of Stomach
The muscular layer in the stomach differs from other digestive organs due to the presence of three layers of muscle (oblique, circular, longitudinal) instead of just two. This allows for more vigorous churning and mixing of food with gastric juices, enhancing mechanical digestion.
33. Stomach Glandular Cell Types
Gastric glands consist of:
Mucous neck cells: Produce mucus to protect the stomach lining.
Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor (essential for vitamin B12 absorption).
Chief cells: Produce pepsinogen, an inactive enzyme that becomes the active enzyme pepsin in acidic conditions.
34. Mucosal Barrier Necessity
A mucosal barrier is necessary in the stomach to protect the epithelial cells from the harsh acidic environment and digestive enzymes which can lead to ulcer formation if the lining is compromised.
35. Gastric Ulcers Formation
Gastric ulcers form when there is damage to the stomach lining due to excessive acid, weakened mucosal barrier, or infection (often due to Helicobacter pylori), leading to erosion.
36. Bacteria Causing Gastric Ulcers
The species of bacteria primarily responsible for gastric ulcers is Helicobacter pylori. It can be detected through breath, stool, or biopsy testing.
37. Enzyme Important for Protein Digestion in Stomach
The enzyme produced in the stomach that is important for protein digestion is pepsin.
38. Substance Produced Essential for Vitamin B12 Absorption
The stomach produces intrinsic factor, which is essential for the absorption of vitamin B12 in the small intestine.
39. Stimuli Provoking Gastric Activity
Types of stimuli: 1) Psychological stimuli (thought of food), 2) Physical stimuli (stretching of the stomach), and 3) Chemical stimuli (presence of food proteins).
40. Three Phases of Gastric Activity
The three phases of gastric activity entail:
Cephalic phase: Triggered by the sight, smell, or thought of food; stimulates gastric secretion.
Gastric phase: Begins once food enters the stomach; involves the stomach's own mechanisms in response to food distention and protein.
Intestinal phase: Begins when chyme enters the small intestine; regulates gastric activity through hormonal and nervous inputs.
41. Sphincters of the Esophagus and Stomach
The upper esophageal sphincter marks the beginning and the lower esophageal sphincter marks the end of the esophagus, while the pyloric sphincter marks the end of the stomach.
42. Definition of Segmentation
Segmentation: Rhythmic contractions of circular muscles in the intestines that mix and further break down the food, also aiding absorption by increasing contact with intestinal walls.
43. Process of Food Movement Through Stomach
Food moves through the stomach in a series of churning motions, mixing with gastric juices to form chyme, which gradually moves through the pyloric sphincter into the small intestine.
44. Structure Controlling Chyme Flow to Small Intestine
The pyloric sphincter controls the flow of chyme from the stomach to the small intestine.
45. Stimuli Initiating Vomiting
Stimuli initiating vomiting may include:
Gastrointestinal irritants, high intracranial pressure, certain smells, sight and thought of vomit.
46. Sphincters of the Small Intestine
The pyloric sphincter marks the beginning of the small intestine, and the ileocecal valve marks the end, connecting to the large intestine.
47. Divisions of the Small Intestine
The small intestine consists of three divisions:
Duodenum: The initial segment where the chyme mixes with bile and pancreatic juices.
Jejunum: The middle segment where most nutrient absorption occurs.
Ileum: The final segment where more absorption occurs, particularly of bile salts and vitamin B12.
48. Structures Joining at the Hepatopancreatic Ampulla
The bile duct and the main pancreatic duct join at the hepatopancreatic ampulla (ampulla of Vater).
49. Modifications of the Small Intestine for Absorption
The small intestine is modified for absorption through:
Increase in surface area via plicae circulares (folds), villi (finger-like projections), and microvilli (tiny hair-like structures forming the brush border).
50. Structures of the Small Intestine
Plicae circulares: Circular folds in the intestinal lining that increase surface area.
Villi: Tiny, finger-like projections containing blood vessels and lacteals for nutrient absorption.
Lacteal: Lymphatic vessels within villi responsible for fat absorption.
Microvilli: Microscopic protrusions on the surface of epithelial cells that further increase surface area.
Brush border: The collective term for microvilli that contain enzymes for digestion.
51. Digestive Function of the Liver
The liver's primary digestive function is the production of bile, which emulsifies fats, aiding in their digestion and absorption.
52. Largest Gland in the Body
The liver is the largest gland in the body, weighing approximately kg ( lbs) in adults.
53. Lobes of the Liver
The liver has four lobes: right lobe, left lobe, caudate lobe, and quadrate lobe.
54. Bile Departure from the Liver
Bile leaves the liver through the common hepatic duct, which combines with the cystic duct from the gallbladder to form the common bile duct.
55. Ducts Forming the Bile Duct
The bile duct is formed by the junction of the common hepatic duct (from the liver) and the cystic duct (from the gallbladder).
56. Functional Units of the Liver
The functional units of the liver are called lobules, which consist of hepatocytes arranged in plates around a central vein.
57. Cells of the Liver
Cells of the liver are called hepatocytes.
58. Portal Triad of the Liver
The portal triad includes:
Hepatic artery: Supplies oxygenated blood to the liver.
Portal vein: Brings nutrient-rich blood from the gastrointestinal tract.
Bile duct: Transports bile produced by hepatocytes.
59. Composition and Function of Bile
Bile is composed mainly of:
Salts, bilirubin, cholesterol, and phospholipids. Its primary function is to emulsify lipids, making them easier to digest.
60. Gallbladder Location and Function
The gallbladder is located underneath the liver. Its function is to store and concentrate bile until it is needed for digestion in the small intestine.
61. Anatomy of the Pancreas
The pancreas is an elongated organ located behind the stomach. It consists of both endocrine (insulin and glucagon production) and exocrine (production of pancreatic juices) components.
62. Main Exocrine Product of the Pancreas
The main exocrine product produced by the pancreas is pancreatic juice, which contains digestive enzymes.
63. Cells Producing Pancreatic Juice
The acinar cells in the pancreas produce pancreatic juice.
64. Functions of Secretin and Cholecystokinin
Secretin: Produced in the duodenum; stimulates the pancreas to release bicarbonate to neutralize chyme.
Cholecystokinin (CCK): Also produced in the duodenum; stimulates the gallbladder to release bile and the pancreas to release enzymes. Both hormones are stimulated by the presence of fatty acids and amino acids in the chyme.
65. Time for Food Passage Through Stomach and Small Intestine
Food typically takes 2-4 hours to pass through the stomach and approximately 3-6 hours to pass through the small intestine.
66. Movement of Chyme Through Small Intestine
Chyme is moved through the small intestine primarily via peristaltic waves and through segmentation contractions.
67. Nutrient Absorption in the Body
Approximately 99% of our nutrients are absorbed in the small intestine.
68. Regions of the Colon
The colon consists of four main regions:
Cecum: The pouch connecting to the ileum.
Colon: Divided into four sections: ascending, transverse, descending, and sigmoid.
Rectum: The final section leading to the anus.
Anus: The opening at the end of the digestive tract where waste is expelled.
69. Definitions of Teniae Coli, Haustra, and Epiploic Appendages
Teniae coli: Three longitudinal bands of smooth muscle that run along the length of the colon, aiding in peristalsis.
Haustra: Pouches formed by the teniae coli that give the colon its segmented appearance.
Epiploic appendages: Small pouches of fat that hang from the colon’s surface.
70. Appendix and Infection
The appendix is prone to infection due to its closed end, which can trap bacteria and debris, leading to inflammation (appendicitis).
71. Function of Bacterial Flora in Large Intestine
Bacterial flora in the large intestine perform essential functions:
Fermentation of indigestible carbohydrates,synthesis of vitamins (like K), and supporting the immune system.
72. Waste Movement Through Large Intestine
Waste typically takes about 12-24 hours to move through the large intestine.
73. Primary Digestive Function of the Large Intestine
The primary digestive function of the large intestine is the absorption of water and the formation of feces.
74. Definition of Haustral Contractions
Haustral contractions: Slow segmenting movements in the large intestine that help mix and propel faecal material toward the rectum for elimination.
75. Description of Mass Movements
Mass movements: Strong, wave-like contractions that occur in the colon, typically seen after meals, which push feces toward the rectum for eventual expulsion.
76. Functions of Internal and External Anal Sphincters
Internal anal sphincter: Involuntary muscle that maintains closure of the anus until defecation is initiated.
External anal sphincter: Voluntary muscle that allows for conscious control over the act of defecation.
77. Defecation Process
The process of defecation involves:
Accumulation of feces in the rectum.
Stretching of rectal walls triggering signals to the CNS.
Contraction of the rectal walls while the internal sphincter relaxes.
Voluntary relaxation of the external anal sphincter allows for fecal expulsion through the anus.