The Basic Structure of the GI System Flashcards
What is the role of the GI system?
Bring nutrients into internal environment (tissues, bloodstream) to be used by the body
- Motility: transport food into and through the body, mixing luminal contents, and transport waste out
- Secretion: synthesise and release enzymes, mucus and serous fluid into the lumen
- Digestion: break nutrients into smaller pieces (mechanically and chemically)
- Absorption: bring nutrients/water from the lumen to internal environment
What are the key structures in the GI system?
- Long tube with outgrowths (outgrowths are the extra things that stem off, called accessory organs)
- Sphincters close off ends, and separate sections of tube (rings of smooth muscle that are like doors to allow different sections different environments)
- Different ‘environments’ within different organs
- Lined by epithelium
- To relate structure to function we need to understand the system at a cellular level
Describe the epithelia in the GI system
Stratified squamous: protection from abrasions
- mouth/oral cavity
- esophagus
- anus
Simple columnar: secretion and absorption
- stomach
- small intestine
- large intestine
Describe unicellular glands
They are made of single cells
Goblet cells:
- columnar
- goblet shape
- apical mucous granules
- basal nucleus
Mucus in the GI systems role is:
- Lubrication
- Protection
Mucus secreting cells is the same thing as goblet cells
Describe multicellular glands
Epithelium can invaginate to form glands (dip down into the underlying tissue):
- Simple: gland with a single duct (eg. stomach and small intestine)
- Compound: gland with 2 or more ducts (eg. salivary glands)
These have increased surface area for more section. The ducts can also produce their own secretions.
What are the four layers of the gut tube (inside to outside)?
- Mucosa
- Submucosa
- Muscular (externa/proper)
- Adventitia
What does the mucosa layer consist of?
- Epithelium
- Basement membrane
- Lamina propria (FCT)
- Sometimes has glands too
- Blood vessels and lymphatics
- Muscular mucosae
- causes tiny movements
What does the submocusa layer consist of?
- FCT
- Glands
- Blood vessels
Below it is the submucosal plexus: secretion is regulated by this plexus of nerves, which is part of the enteric nervous system (ENS).
Describe the muscularis (externa/proper) layer
The muscularis (externa/proper) consists of smooth muscle
Two main layers:
- Inner circular (cells arranged to wrap around the tube)
- Outer longitudinal (cells arranged up and down)
Really small spindle shaped cells
Myenteric plexus (part of ENS)
- Located between muscle layers
- Regulates motility
Descibe the layers of the adventita
It is the outermost layer and consists of FCT
Note: when organs are in the peritoneal cavity additional outer covering, the serosa is present (but it is not one of the four layers)
Describe the mouth and oral cavity
Need protective epithelium:
- Stratified squamous
Digestion begins:
- Mechanical
- Chemical (requires enzymes to cleave the chemical bonds)
- Lubrication needed
Through fauces, into oropharynx and laryngopharynx then into esophagus
Describe the three types of salivary glands and their secretions
3 pairs connected to oral cavity via ducts:
- Parotid: serous fluid with amylase
- underneath the ear
- Sublingual: mucus only
- underneath the tongue
- Submandibular: mixed
- under the mandible
- They are compound secretory glands
- Cells in clusters - acinus
- Acinar calls can secrete
- serous fluid and enzyme s (amylase)
- mucus
- Duct cells secrete bicarbonate (which does pH buffering (counters the acid))
Describe the esophagus
- Long tube
- Located posterior to the trachea, extends form pharynx to stomach
- Epiglottis ensures that food enters the esophagus and not the trachea
- Stratifies squamous epithelium
- Need mucus for protection
- No goblet cells, instead have glands with ducts to surface
- in submucosa
- plus in mucosa close to stomach
- Have some goblet cells as we get closer to the stomach for extra mucus to protect against stomach acid that splashes up
Describe how the muscularis externa and submucosa and mucosa change throughout parts of the esophagus
Basic tube modified to allow passage of food
Muscularis externa: move food bolas
- First 1/3: skeletal muscle
- Middle 1/3: a mixture
- Last 1/3: smooth muscle
Highly folded submucosa and mucosa
- capacity to expand for passage of bolus
How do things change from the esophagus to the stomach?
The mucosa which is stratified squamous epithelium then goes to esophagus/stomach junction and then the epithelia become simple columnar epithelium of the stomach
- Change due to different environment
