Lecture 20 - The basic structure of the GI system Flashcards
Primary function of the GI system
Brings nutrients into the internal environment so that they can be used (for energy and growth)
Specialised function for…
Motility - movement through the tract at an appropriate rate
Secretion - enzymes, acid for digestion or mucus for protection or serous fluid for lubrication
Digestion - process of breaking things down into small particles (mechanical or chemical)
Absorption
Key structures in the gastrointestinal system
Long tube with outgrowths (=accessory organs and most of these are going to empty into the tubular GI tract by a duct system )
Approximately 9m from mouth to anus
Closed of at both ends (sphincters) - thickened region of circular smooth muscle and these sphincters are really important for regulating the passage of lumens contents from one region to another and by doing so they are going to be portioning the GI tract into different functional compartments
Lined by epithelium (interface with outside world) - lumen of the tube is effectively an extension of the outside world and any surface that is exposed to the outside world must be lined with epithelium
Epithelial classifications
Simple = one layer Stratified = multiple layers
Squamous
Cuboidal.
Columnar
Goblet cells are the cells that are producing mucus
Microvilli are fingerlike extensions of plasma membrane that increases surface area of the cell for secretion and absorption
Cilia are long, mobile structures that can beat lumenal contents
Majority in the GI tract is simple columnar and stratified squamous
Epithelium of the mouth/oral cavity and the esophagus
Stratified squamous
Protection from abrasion - lots of layers to protect underlying structures like nerves, arteries and veins
Epithelium of the stomach, small intestine, large intestine
Simple columnar
Secretion and absorption
Epithelium of the anal canal
Stratified squamous
Protection from abrasion from faecal material
Unicellular glands
Only one example in the whole body = goblet cells
Columnar
Goblet shape
Apical mucus granules which gives the goblet shape
Basal nucleus - mucus granules push it to the bottom of the cell
Goblet cells
Columnar
Goblet shape
Apical mucus granules which gives the goblet shape
Basal nucleus - mucus granules push it to the bottom of the cell
Multicellular glands
Epithelium can invaginate to form glands…
Simple = gland with single duct (e.g. stomach and small intestine)
Compound = gland with 2 or more ducts (e.g. salivary glands)
Both increase surface area but the compound multicellular glands just increase it more
The four layers of the gut tube
Mucosa (innermost)
Submucosa - contains different blood vessels and some glands
Muscularis (externa/proper)
Adventitia (outermost layer) - layer of connective tissue that helps to maintain the structure of the tube and in some cases adhere it to some surrounding structures
Mucosa
The mucosa consists of the:
Epithelium
Lamina propria (FCT) - blood vessels and lymphatic vessels are located here
Muscularis mucosae - thin band of smooth muscle, specific to the mucosa, when it contracts it helps to cause localised movement of the mucosa to help enhance secretion and absorption
Submucosa
Deeper connective tissue which supports the mucosa is called the submucosa.
The submucosa contains:
Glands
Blood vessels
Has connective tissue, lots of elastin fibres and blood vessels and glands within it
Secretion is regulated by the submucosal nerve plexus, part of the enteric nervous system (ENS)
Epithelium can invaginate deep down into the submucosa to form glands
Muscularis
The muscular (externa/proper) consists of smooth muscle
Two main layers (refer to the orientation of the smooth muscle fibres)
Inner circular - smooth muscle cells are orientated around the tube circumference so these are going to be important in controlling the diameter of the tube (contraction or relax)
Outer longitudinal - smooth muscle cells are arranged lengthwise so they are uncharge of changing the length (can shorten)
Myenteric plexus
Part of the Enteric Nervous System
In between the muscle layers described above
Regulates motility
Adventitia
The adventitia is the outermost layer of the gut tube
When organs are in the peritoneal cavity:
Additional outer covering, the serosa (not one of the four layers)
Summary of the four layers of the gut tube
Four layers from innermost to outermost Mucosa Epithelium Lamina propria (FCT) Muscularis mucosae
Submucosa
Connective tissue
Glands
Muscularis
Smooth muscle
Inner circular
Outer longitudinal
Adventitia
FCT
The four basic layers of the GI tract beginning with the most luminal layer are ….
Mucosa, submucosa, muscular externa, adventitia
Peritoneum
The peritoneum is a serous membrane
Parietal layer = lines the body wall
Visceral layer = lines the organs
Between these layers is a fluid filled space
Reteroperitoneal = posterior to the peritoneum
Allows organs to move around in a nice frictionless way
Parietal layer of the peritoneum
Lines the body wall
Visceral layer of the peritoneum
Lines the organs
Reteroperitoneal
Posterior to the peritoneum
Mesentery
Double layer of visceral peritoneum that connects organ to body wall
Prevents tubular portions of our GI tract from being put into knots
Anchors the organ to the posterior body wall
Connects organ to body wall
Omenta
Double layer of visceral peritoneum that connects an organ to another organ
Greater omentum
Attaches to the stomach comes down and folds over a little and then attaches to the transverse colon
Lesser omentum
Connects the liver to the stomach
Mouth and oral cavity
Food is ingested here
Need protective epithelium - stratified squamous - this protects from abrasion
Digestion begins - requires enzymes
Lubrication
Mechanical digestion begins and it compacts the food into a little ball known as the bolus and when ready to swallow the tongue can force the bolus to the back of the throat and then through fauces, into oropharynx, then into esophagus and ultimately towards the stomach
Through fauces, into oropharynx, then into esophagus and ultimately towards the stomach
Salivary glands
3 pairs connected to oral cavity via ducts:
Paratoid - serous fluid (watery fluid containing some ions) with amylase (enzyme that is important for starting the digestion of the stomach)
Sublingual - Mucus only therefore there are lots of goblet cells
Submandibular - mixed - contributes the most to saliva
Total volume of salivary secretions = 1L per day
Epithelial layer invaginate to form compound secretory glands
Cells in clusters = acinus
Acinar cells secrete enzymes (amylase)
Duct cells secrete bicarbonate (buffering)
3 pairs of salivary glands
Paratoid - anterior and inferior to the ear
Sublingual - below tongue
Submandibular - below the mandible
Total volume of salivary secretions
1L
Cells in clusters in the salivary glands
Acinus - ones that produce the serous fluid containing the enzymes/amylase
Acinar cells in salivary glands
Acinar cells secrete enzymes (amylase)
Duct cells in the salivary glands
Duct cells secrete bicarbonate (buffering)
Esophagus
Long tube approximately 25 cm
Located posterior to the trachea, extends from the pharynx to the stomach
Epiglottis ensures that food enters the oesophagus, and not the trachea
Long muscular tube that extends from pharynx to stomach
Basic tube modified to pass food:
Highly folded submucosa and mucosa - capacity to expand
Mucosa - stratified squamous epithelium
Muscularis externa - move food bolus
Changes as you go down: First 1/3 = skeletal muscle Middle 1/3 = a mixture Last 1/3 = smooth muscle Once the process is started it can be continued involuntarily down towards the stomach hence the change to smooth muscle
Esophagus layers of the tube
Basic tube modified to pass food:
Highly folded submucosa and mucosa - capacity to expand
Mucosa - stratified squamous epithelium
Muscularis externa - move food bolus
Changes as you go down the esophagus
Changes as you go down: First 1/3 = skeletal muscle Middle 1/3 = a mixture Last 1/3 = smooth muscle Once the process is started it can be continued involuntarily down towards the stomach hence the change to smooth muscle
Esophagus and mucus
Multicellular, simple
Need mucus for lubrication and protection
No goblet cells, instead have glands with ducts to surface
The goblet cells would not really fit among the stratified squamous epithelial cells and the food abrasion would knock the cells off the part where the food is passing through
So mucus is produced elsewhere and ship it to the surface via a duct system so in the oesophagus there are mucus secreting glands in the submucosa that produce mucus and ship it up to the service via a duct which allows a greasy layer of mucus to let the food pass nicely through to the stomach
Mucus secreting glands - ducts to surface epithelium
What type of epithelium lines the esophagus?
Stratified squamous epithelium
