Lesson 13: A+P Of The GI System

Question 1

Bowel Histology - Serosa

Answer 1

• Outermost layer continuous with mesentery
• No mucous secretion

Question 2

Bowel Histology - Muscle

Answer 2

• Outer longitudinal and inner circular
• Provides mixing motion + peristalsis

Nerve plexuses found between
- Serosa + longitudinal muscle
- Between muscle layers
- Between circular muscle + submucosa

Intrinsic nervous system
- Receptors and nerve plexuses within bowel wall
- Respond to intraluminal irritants + mechanical distension

Extrinsic nervous system
- Autonomic fibers communicate with nerve plexuses in bowel wall
- Respond to systemic factors
- Sympathetic = slows peristalsis + secretion
- Parasympathetic = increases peristalsis + secretion

Question 3

Bowel Histology - Submucosa

Answer 3

Connective tissues layer
- Nerve plexus
- Large blood vessels
- Interstitial cells of Cajal (pacemaker cells to regulate motility)

Question 4

Bowel Histology - Mucosa

Answer 4

• Innermost layer
• Highly vascular
• Mucous-secreting cells
• Critical for absorption of nutrients, water, and electrolytes

Question 5

Upper Tract - Tongue

Answer 5

• Food manipulation
• Swallowing
• Taste
• Speech

Question 6

Upper Tract - Salivary Glands

Answer 6

• Lubricant + salivary amylase to begin starch digestion
• 1.5 L produced per day

Question 7

Upper Tract - Teeth

Answer 7

Initiation of mechanical digestion

Question 8

Upper Tract - Oropharynx

Answer 8

Swallowing is initiated here

Question 9

Upper Tract - Esophagus

Answer 9

• 25 cm long
• Skeletal muscle proximal and smooth muscle distal
• 2 sphincters
  — Pharyngo-esophageal
  — Esophagogastric
• Secretes mucus to protect against acid/base changes

Question 10

Upper Tract - Peritoneum

Answer 10

• Membrane that supports bowel
• Innervation, blood supply, and bowel support
• Parietal peritoneum: lines abdominal cavity
• Visceral peritoneum: wraps around bowel
• Mesentery: double layer between abdo wall and bowel
• Greater omentum: double fold hanging from stomach
  — Filled with fat

Question 11

Upper Tract - Stomach

Answer 11

• Fundus, body, and pylorus
• Reservoir with controlled emptying
• Mechanical and enzymatic digestion of proteins via pepsin
• Protection from acids
  — Mucous blanket
  — Tight junctions
  — Prostaglandins
• Secretion of intrinsic factor for B12 absorption
• Controlled emptying
• Transit time = 30 mins - 5 hours

Question 12

Upper Tract - Small Intestine

Answer 12

• 2.5 - 3.75 cm wide
• 5-8 m long
• 3L secretions/day via intestinal glands
• Low bacterial counts = minimal gas and odor production
• Movement via segmentation and peristalsis
• Villi: mucosal projections that increase absorptive surface area of bowel
  — Can become hypertrophic after extended NPO status (diarrhea)

Question 13

Small Intestine - Duodenum

Answer 13

• 20-25 cm long
• Ampulla of Vater: entry point for pancreas and common bile duct
• Bruner glands: produce mucus to neutralize acidic chyme

Functions
- Neutralize chyme
- Continue digestive process
- Absorption of iron, calcium, magnesium, and carbs

Question 14

Small Intestine - Jejunum

Answer 14

• 9’ long
• Majority of nutrients absorption happens here
• Very prominent villi

Question 15

Small Intestine - Ileum

Answer 15

• Longest portion - 12’ long and 1” wide
• Backup absorption for jejunum
• Terminal ileum
• Last 100 cm of bowel
• Only site for B12 absorption

Question 16

Lower Tract - Large Intestine

Answer 16

• 5’ long and 1.5-2” wide
• No villi in mucosal layer

Functions
- Secrete alkaline mucus to neutralize chyme
- Absorb water and electrolytes
- Synthesis of symbiotics
— Help maintain mucosal barrier from bacterial adherence
— Prevent overgrowth of pathogens and toxins
— Enhance macrophage function
— Probiotics: beneficial flora of GI tract
— Prebiotics: dietary fiber fermented by bacteria
- Maintenance of mucosal barrier
- Prevention of pathogen overgrowth
- Reduction of bacterial toxins
- Enhanced macrophage function

Question 17

Large Intestine - Ileocecal Valve

Answer 17

• Prevents reflux and bacterial migration from colon to small bowel
• Improves absorption for pts with short bowel syndrome

Question 18

Large Intestine - Cecum

Answer 18

• distal to appendix
• prone to distension and perforation

Question 19

Large Intestine - Colom

Answer 19

• Ascending, transverse, descending, and sigmoid
• Transit time to rectum = 24 hrs

Question 20

Large Intestine - Rectum

Answer 20

12 - 15 cm long

Question 21

Large Intestine - Anal Canal

Answer 21

• 3-4 cm long

Internal anal sphincter
- Allows continence at rest
- Contracts and relaxes in response to rectal distention

External anal sphincter
- The anus!
- Can be voluntarily contracted or relaxed
- Allows continence during periods of distension

Anorectal angle
- Created by puborectalis muscle
- Creates 90° angle that supports continence
- Relaxation of muscle straightened angle + facilitates elimination of stool

Question 22

Accessory Organs - Liver

Answer 22

• In right upper quadrant
• 2 lobes with multiple lobules
• Bile capillaries drain into hepatic ducts → ampulla of vater

Functions
- Metabolic support
- Synthetics of blood proteins, bile, and clotting factors
- Storage of vitamins and minerals
- Phagocytosis and detoxification

Question 23

Accessory Organs - Gallbladder

Answer 23

• Storage and concentration of bile
• Delivers bile to duodenum

Question 24

Accessory Organs - Pancreas

Answer 24

• In left upper quadrant
• Produces insulin and glucagon
• Produces enzymatic fluid to support digestion
• Secreted bicarbonate to neutralize chyme in duodenum

Question 25

Blood Supply - Arterial

Answer 25

Superior mesenteric artery → feeds small bowel and right half of colon

Inferior mesenteric artery → feeds proximal rectum and left half of colon

Question 26

Blood Supply - Venous

Answer 26

Portal vein → liver → hepatic vein → systemic circulation

Veins from proximal rectum → portal system

Veins from distal rectum → systemic circulation