PSL301 GI Summary Notes

Digestive System
1. Overview
Functions:
- Supply water, electroltes and nutrients to the body
- Involves digestion, absorption, secretion, and motility
- Provide defenses to prevent both infection and autodigestion
o GI tract is essentially an open tube with an area of a tennis court
o Majority of the lymphoid tissue is at GI ~ GALT (gut associated lymphoid tissue)
- Remove waste

- Tubular GI tract
o esophagus, stomach, small and large intestine
- Accessory organs
o teeth, tongue, salivary glands, liver, gall bladder and pancreas
In more detail…
- Mouth
o Receptacle for food
o Tongue tastes & guides food
o Teeth grind food = mechanical digestion
o Mix food with saliva (from salivary glands)
o Minimal digestion of carbohydrates and lipids
- Pharynx
o Most of the epithelial cell is flexible lateraly for the delivery of food
o Swallowing reflex
o Pushes food into esophagus
o Epiglottis protects the larynx during swallowing
- Esophagus
o Moves food into the stomach with peristalsis
o The muscles become voluntary to involuntary – striated smooth muscle
o No digestion
o There are muscular sphincters to ensure no backflow
- Stomach
o Temporarily stores ingested food
o Secretes acid & enzymes for digestion (mainly protein)
o Mechanically breaks down food / mix with secretions
o Slowly

empties into small intestine through Pyloric Sphincter
Passage of chyme is regulated depending on its pH, size of particles, etc
o Has Oblique muscle > Circular muscle > Longitudinal muscles
o Inside is rugae (increasing surface area)
- Small Intestine

o Segmentation and peristalsis moves material forward
o Digestion of all types of nutrients (has most enzymes) inside
o Main site of absorption
 
o Duodenum Jejenum Illeum
o Digestion aided by secretions of liver via gall bladder and pancreas

, 
Liver makes bile salts which is secreted via Common Hepatic Duct
to be stored in Gallbladder

Stuff comes out by Common bile duct

Pancreas secretes bicarbonate, and enzymes
- Large intestine
o Segmentation
and Mass movement
o Absorption of
water / electrolytes
o Storage of fecal material for expulsion
o Has a strip of Longitudinal muscle which is actually shorter than total
length.
  
o Ascending colon Transverse colon Descending colon Sigmoid

colon Rectum
Wall of GI tract
- Four layers
- Mucosa
o Epithelial cells
o Lamina propria
o Muscularis mucosae (thin muscle that waves the vili and help expulsion)
- Submucosa
o Blood vessels and glands
o Submucosal plexus and myenteric plexus has network of nerves
- Muscularis Externa
o Circular & longitudinal muscles
o Stomach also has oblique
- Serosa
o Mesentery membranes

Specialized cells of the mucosa
- Epithelial cells: several types – secrete acid, bicarbonate, absorption of nutrients,
water, vitamins
- Enteroendocrine cells: several types secrete hormones into the blood stream
- Exocrine cells: secrete enzymes, mucus, etc
o Goblet cells: secrete mucus
o Paneth cells: secrete antimicrobial compounds
- Stem cells: because the life span is only 3~4 days, you need stem cells at the base
of crypt
o Bottom has stem cells and paneth cells
o Above is transit-amplifying cells and then up to absorptive cells & goblet
cells

Enteric Nervous System (ENS)

, - contains 100 million neurons (sensory, interneurons, parasympathetic
ganglia, sympathetic postganglionic fibers)
- Myenteric Plexus/ Auerbach’s plexus: regulates motility
- Submucosal plexus/ Meissner’s plexus: regulates secretion and absorption
o In between submucosa
- In between the two plexus is circular muscles
- Outside myenteric is longitudinal

Regulation of Digestive System
- Different kinds of stimulus: