Complete and in-depth Notes for PSL300 for EXAM prep. Kevin has combined notes from his peers and his own work to provide the most complete and comprehensive study guide for all types of students. He has achieved an overall cumulative GPA of 3.95 during his undergrad at the University of Toronto St...
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:
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