Digestive System and Gastrointestinal Tract Flashcards
Digestion
The process of breaking down polymers into substrates that can be used for catabolic reactions or anabolic reactions.
Though food can also contain vitamins.
Result of enzymatic hydrolysis.
A secondary function is defence against disease.
Digestive Reactions
Review Pg. 379, Figure 8.
Different terms for the digestive tract
Gastrointestinal tract
Alimentary canal
Gut
Tube of the digestive tract
Result of tube formed by gastrulation during embryogenesis.
Anus is derived from blastopore.
It is continuous with the outside.
Inside of the digestive tract
GI lumen
Tissue layers of the GI tract
From inside to out.
Mucosa, showing villi (epithelial tissue)
Submucosa (connective tissue)
Circular Muscularis (smooth muscle)
Longitudinal muscularis (smooth muscle)
Serosa (connective tissue)
GI Epithelium
Because it is exposed to substances from outside world, it is composed of same cells that form skin and lung tissue. By definition they are attached to a BASAL MEMBRANE.
Lumen faces the apical surface. Apical surface is coated in microvilli, and separated from the rest of the cell by tight junctions.
Basolateral surface, opposite to the apical surface.
GI smooth muscle
Smooth muscle - has smooth microscopic appearance.
Note that there are two layers in the GI tract. Circular and longitudinal.
GI motility
The rhythmic contraction of the GI tract. Depends on 5 factors:
1. Automaticity - contracts without external stimulation due to spontaneous depolarization.
2. Functional syncitium - just like heart cells, means that a depolarization in one cell will spread to others via gap junctions.
3. Enteric Nervous System - a GI tract specific nervous system
4. Hormone input - can alter GI motility
5. Parasympathetic NS - causes sphincters to relax allowing food to pass; sympathetic does the opposite.
Functions of GI motility
- Movement down the GI tract. Result of peristalsis. Note that peristalsis, first contracts behind the bolus, then around the bolus to prevent backward sliding.
- Mixing of food in the GI tract. Result of disordered contraction of smooth muscle causing churning motions.
Bolus
Food moving down the GI tract.
Enteric Nervous System
A branch of the autonomic nervous system.
Innervates the GI tract; pancreas; and gall bladder.
Helps to regulate local blood flow, gut movement, and the exchange of fluid from the gut to and from the lumen.
Can operate separately from the other two branches of the autonomic nervous system (parasympathetic/sympathetic), but both can regulate.
ENS - networks of neurons
Composed of two networks of neurons: myenteric plexus and the submucosal plexus.
Myenteric plexus - between circular and longitudinal muscle layers and regulates gut motility.
Submucosal plexus - in submucosa and helps regulate enzyme secretion, gut blood flow, and ion/water balance in the lumen. Because this is less relevant in the esophagus or anus, this plexus is not as prevalent.
GI secretions
Release of enzymes, bile, acid, is generally stimulated by food and the parasympathetic system, and inhibited by the sympathetic system.
Two types of GI secretion
Exocrine - glands that secrete into ducts that drain into the GI lumen. These glands are composed of specialized epithelial cells organized into sacs called ACINI. Primary organs include the pancreas, liver, and gallbladder, although some is performed by special cells in the gut lining itself (ex. Gastric glands; goblet cells)
Endocrine - glands that are ductless and secretions (hormones) enter capillaries –> bloodstream. Performed by specialized organs (pancreas) and specialized cells in the gut lining.
Gastric Glands
Secrete acid and pepsinogen
Goblet cells
Found along the GI tract. Secrete mucus protective layer.
Water in the GI tract
Each day gallons of water are secreted into the GI tract to facilitate digestion. It is later reabsorbed in the small intestine and the colon.
To remember regarding organs that have exocrine and endocrine function.
Exocrine and endocrine cells will tend to group such that they are microscopically distinguishable.
Ex. Pancreas
Exocrine - pancreatic acinar cells
Endocrine - islets of langerhans
Organs of the digestive system
Nasal cavity
Oral cavity
Salivary glands
Epiglottis
Trachea
Esophagus
Liver
Stomach
Pancreas
Duodenum (behind liver)
Gallbladder
Transverse colon
Ascending Colon
Jejunum
Descending Colon
Ileum
Rectum
External anal sphincter
Mouth
Three role:
Fragmentation (mastication): Incisors cut; cuspids tear; molars grind
Lubrication. Saliva.
Enzymatic digestion. Saliva.
Saliva
Contains SALIVARY AMYLASE (ptyalin).
Hydrolyzes starch. Can yield disaccharides. Note though that only monosaacharides can be absorbed by the body.
Also contains LINGUAL LIPASE. Fat digestion.
No protein digestion occurs in the mouth.
Also contains LYSOZYME, attacks bacterial walls.
Pharynx
Commonly known as the throat.
Contains the opening to two tubes: Trachea (cartilage lined tube - air to lungs) and esophagus (muscle lined tube - food to stomach).
Epiglottis
Flat cartilagionous flap that excludes food/water from the trachea.
Muscular rings that regulate movement of food through the esophagus.
Upper esophageal sphincter - top of esophagus.
Lower Esophageal sphincter (cardiac sphincter) - at the end of the esophagus.
Stomach
Large hollow organ, three functions:
- partial digestion of food
- regulated release of food into small intestine
- destruction of microorganisms
Gastric - means “related to the stomach”