Lesson 13: A+P Of The GI System Flashcards
Bowel Histology - Serosa
- Outermost layer continuous with mesentery
- No mucous secretion
Bowel Histology - Muscle
- 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
Bowel Histology - Submucosa
Connective tissues layer
- Nerve plexus
- Large blood vessels
- Interstitial cells of Cajal (pacemaker cells to regulate motility)
Bowel Histology - Mucosa
- Innermost layer
- Highly vascular
- Mucous-secreting cells
- Critical for absorption of nutrients, water, and electrolytes
Upper Tract - Tongue
- Food manipulation
- Swallowing
- Taste
- Speech
Upper Tract - Salivary Glands
- Lubricant + salivary amylase to begin starch digestion
- 1.5 L produced per day
Upper Tract - Teeth
Initiation of mechanical digestion
Upper Tract - Oropharynx
Swallowing is initiated here
Upper Tract - Esophagus
- 25 cm long
- Skeletal muscle proximal and smooth muscle distal
- 2 sphincters
— Pharyngo-esophageal
— Esophagogastric - Secretes mucus to protect against acid/base changes
Upper Tract - Peritoneum
- 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
Upper Tract - Stomach
- 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
Upper Tract - Small Intestine
- 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)
Small Intestine - Duodenum
- 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
Small Intestine - Jejunum
- 9’ long
- Majority of nutrients absorption happens here
- Very prominent villi
Small Intestine - Ileum
- Longest portion - 12’ long and 1” wide
- Backup absorption for jejunum
- Terminal ileum
- Last 100 cm of bowel
- Only site for B12 absorption
Lower Tract - Large Intestine
- 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
Large Intestine - Ileocecal Valve
- Prevents reflux and bacterial migration from colon to small bowel
- Improves absorption for pts with short bowel syndrome
Large Intestine - Cecum
- distal to appendix
- prone to distension and perforation
Large Intestine - Colom
- Ascending, transverse, descending, and sigmoid
- Transit time to rectum = 24 hrs
Large Intestine - Rectum
12 - 15 cm long
Large Intestine - Anal Canal
- 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
Accessory Organs - Liver
- 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
Accessory Organs - Gallbladder
- Storage and concentration of bile
- Delivers bile to duodenum
Accessory Organs - Pancreas
- In left upper quadrant
- Produces insulin and glucagon
- Produces enzymatic fluid to support digestion
- Secreted bicarbonate to neutralize chyme in duodenum
Blood Supply - Arterial
Superior mesenteric artery → feeds small bowel and right half of colon
Inferior mesenteric artery → feeds proximal rectum and left half of colon
Blood Supply - Venous
Portal vein → liver → hepatic vein → systemic circulation
Veins from proximal rectum → portal system
Veins from distal rectum → systemic circulation
