The Digestive System

Question 1

What does the digestive system do?

Answer 1

• Ingests, mixes, transports and breaks down food
• Food is mixed with saliva to form a bolus
• Stomach converts the bolus into chyme
• Functions include:
  1. Ingestion: introduction of materials into the mouth
  2. Motility: involves voluntary and involuntary contractions that mix and move materials through GI tract
  3. Secretion: involves release of mucin or fluids such as acid, bile, or digestive enzymes
  4. Digestion: breaks large food items into smaller structures (mechanical and chemical forms)
  5. Absorption: involves movement of molecules across GI tract epithelium and into the blood or lymph
  6. Elimination of wastes: involves defecation of indigestible waste products

Question 2

What is the abdominopelvic cavity?

Answer 2

• Lined with moist serous membranes
• Parietal peritoneum lines inside surface of the body wall
• Visceral peritoneum covers surface of internal organs within the cavity
• Organs completely surrounded by visceral peritoneum are called intraperitoneal organs (e.g., stomach)
• Organs that lie against the posterior abdominal wall are only covered by peritoneum on their anterolateral surfaces and are called retroperitoneal organs (e.g., pancreas, rectum)

Question 3

What are mesenteries?

Answer 3

• Folds of peritoneum that support and stabilize intraperitoneal GI tract organs
• Blood vessels, lymphatic vessels, and nerves are sandwiched between the two folds and supply the digestive organs
• There are 4 types:
  1. Greater omentum extends inferiorly like an apron from the greater curvature of stomach and covers most abdominal organs
  2. Lesser omentum connects the lesser curvature of the stomach and the proximal end of the duodenum to the liver
  3. The mesentery proper suspends most of the small intestines from the posterior abdominal wall
  4. The mesocolon is a peritoneal fold that attaches parts of the large intestine to the posterior abdominal wall

Question 4

What is the GI tract?

Answer 4

- GI wall is composed of 4 tunics... from deep to superficial they are:
– Mucosa
– Submucosa
– Muscularis
– Adventitia or serosa
- 3 unpaired arteries supply the tract:
– Celiac trunk
– Superior mesenteric artery
– Inferior mesenteric artery

Question 5

What are the lymphatic structures and the nerve supply?

Answer 5

• Lacteals (lymphatic capillaries) are in villi of small intestine
• Lymph ducts transport lymph to the cisterna chyli, which
  drains into the thoracic duct
• MALT (mucosa-associated lymphatic tissue) is in appendix and in small intestine (aggregate nodules called Peyer patches)
• Nerve supply is both autonomic and visceral sensory
• Three autonomic plexuses follow the distribution of the
  unpaired artery of the same name:
  1. Celiac plexus
  2. Superior mesenteric plexus
  3. Inferior mesenteric plexus
• Generally, parasympathetic activity promotes digestive
  secretion and motility

Question 6

What is the esophagus?

Answer 6

• A tube that conducts ingested materials from the
  pharynx to the stomach
• Passes through an opening in the diaphragm called the
  esophageal hiatus just before it connects to the stomach
• Courses anterior to the vertebral bodies
• Esophageal mucosa has stratified squamous epithelium
• Submucosa is thick with many elastic fibers and mucous glands
• Muscularis contains both skeletal and smooth muscle: Transitions from skeletal muscle (voluntary control of swallowing) superiorly to smooth muscle inferiorly
  1. Superior esophageal sphincter: Ring of skeletal muscle at the top of esophagus; closes during inhalation preventing air entry
  2. Inferior esophageal sphincter: Ring of smooth muscle at bottom of esophagus; prevents materials from regurgitating from stomach

Question 7

What is the process of swallowing?

Answer 7

• 3 phases of swallowing:
  1. Voluntary phase occurs in the mouth
  2. Pharyngeal phase begins as bolus enters oropharynx and involves involuntary swallowing reflex controlled by medulla oblongata
  3. Esophageal phase consists of the 5-8 seconds when bolus passes involuntarily through esophagus to stomach

Question 8

What is the stomach’s role?

Answer 8

• In upper left quadrant of the abdomen
• Continues the mechanical and chemical digestion of bolus
• Where the bolus is eventually processed into chyme

Question 9

What are the gastric secretions?

Answer 9

• Five types of secretory cells form the gastric epithelium::
  1. Surface mucous cells
  2. Mucous neck cells
  3. Parietal cells – secrete hydrochloric acid and intrinsic factor
  4. Chief cells – secrete pepsinogen
  5. Enteroendocrine cells – secrete hormone gastrin

Question 10

What is the small intestines role?

Answer 10

• Finishes chemical digestion and is responsible
  for most nutrient absorption
• Ingested materials spend at least 12 hours in small intestine as chemical digestion and absorption are completed
• About 6 meters (20 feet) long in an unembalmed cadaver, but much shorter in a living individual due to muscle tone
• Consists of 3 segments:
  1. Duodenum
  2. Jejunum
  3. Ileum
• Internally, the mucosal and submucosal tunics are thrown into folds called the circular folds (plicae circulares)
• Microscopic finger-like projections called villi can be seen on the surface of the circular folds
• Microscopic finger-like projections called microvilli can be seen on the surface of the villi
• Intestinal glands between villi are invaginations of mucosa that release hormones such as secretin
• Submucosal glands of duodenum produce alkaline mucus

Question 11

What is duodenum? (small intestine)

Answer 11

• C-shaped
• Originates at pyloric sphincter
• Becomes continuous with jejunum at the duodenojejunal flexure
• Contains the major duodenal papilla, which is the site where bile and pancreatic secretions enter GI tract
• Minor duodenal papilla receives a small amount of pancreatic juice

Question 12

What is jejunum? (small intestine)

Answer 12

• Middle portion of the small intestine

- Is the primary region for chemical digestion and nutrient absorption

Question 13

What is ileum? (small intestine)

Answer 13

• Last segment of the small intestine
• Distal end terminates at the ileocecal valve, a sphincter that controls the entry of materials into the large intestine

Question 14

What is the large intestines role?

Answer 14

• Forms a three-sided perimeter around the small intestine
• Larger diameter than the small intestine
• Absorbs fluids and ions and compacts indigestible wastes and solidifies them into feces
• Stores feces until defecation
• Gastroileal reflex: ingestion of food increases peristaltic
  movements in ileum
• Haustral churning occurs after a relaxed haustrum fills with material stimulating reflex contractions in muscularis
• Mass movements are powerful peristaltic-like contractions involving teniae coli (occur during or just after a meal in the gastroileal reflex)

Question 15

What are the accessory digestive organs?

Answer 15

– Liver
– Gallbladder
– Pancreas
– Biliary apparatus