Lecture 20 - The basic structure of the GI system

Question 1

Primary function of the GI system

Answer 1

Brings nutrients into the internal environment so that they can be used (for energy and growth)

Question 2

Specialised function for…

Answer 2

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

Question 3

Key structures in the gastrointestinal system

Answer 3

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

Question 4

Epithelial classifications

Answer 4

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

Question 5

Epithelium of the mouth/oral cavity and the esophagus

Answer 5

Stratified squamous

Protection from abrasion - lots of layers to protect underlying structures like nerves, arteries and veins

Question 6

Epithelium of the stomach, small intestine, large intestine

Answer 6

Simple columnar

Secretion and absorption

Question 7

Epithelium of the anal canal

Answer 7

Stratified squamous

Protection from abrasion from faecal material

Question 8

Unicellular glands

Answer 8

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

Question 9

Goblet cells

Answer 9

Columnar
Goblet shape
Apical mucus granules which gives the goblet shape
Basal nucleus - mucus granules push it to the bottom of the cell

Question 10

Multicellular glands

Answer 10

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

Question 11

The four layers of the gut tube

Answer 11

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

Question 12

Mucosa

Answer 12

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

Question 13

Submucosa

Answer 13

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

Question 14

Muscularis

Answer 14

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

Question 15

Adventitia

Answer 15

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)

Question 16

Summary of the four layers of the gut tube

Answer 16

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

Question 17

The four basic layers of the GI tract beginning with the most luminal layer are ….

Answer 17

Mucosa, submucosa, muscular externa, adventitia

Question 18

Peritoneum

Answer 18

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

Question 19

Parietal layer of the peritoneum

Answer 19

Lines the body wall

Question 20

Visceral layer of the peritoneum

Answer 20

Lines the organs

Question 21

Reteroperitoneal

Answer 21

Posterior to the peritoneum

Question 22

Mesentery

Answer 22

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

Question 23

Omenta

Answer 23

Double layer of visceral peritoneum that connects an organ to another organ

Question 24

Greater omentum

Answer 24

Attaches to the stomach comes down and folds over a little and then attaches to the transverse colon

Question 25

Lesser omentum

Answer 25

Connects the liver to the stomach

Question 26

Mouth and oral cavity

Answer 26

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

Question 27

Salivary glands

Answer 27

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)

Question 28

3 pairs of salivary glands

Answer 28

Paratoid - anterior and inferior to the ear
Sublingual - below tongue
Submandibular - below the mandible

Question 29

Total volume of salivary secretions

Answer 29

1L

Question 30

Cells in clusters in the salivary glands

Answer 30

Acinus - ones that produce the serous fluid containing the enzymes/amylase

Question 31

Acinar cells in salivary glands

Answer 31

Acinar cells secrete enzymes (amylase)

Question 32

Duct cells in the salivary glands

Answer 32

Duct cells secrete bicarbonate (buffering)

Question 33

Esophagus

Answer 33

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

Question 34

Esophagus layers of the tube

Answer 34

Basic tube modified to pass food:
Highly folded submucosa and mucosa - capacity to expand
Mucosa - stratified squamous epithelium
Muscularis externa - move food bolus

Question 35

Changes as you go down the esophagus

Answer 35

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

Question 36

Esophagus and mucus

Answer 36

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

Question 37

What type of epithelium lines the esophagus?

Answer 37

Stratified squamous epithelium