Gastrointestinal Physiology Flashcards
Where does the GI tract extend from and to?
It extends from the oral cavity to the anus
What is the length of the entire GI tract and its components?
It is heavily folded and is 8-9 m long
The pharynx, oesophagus and stomach are 1m
The small bowel is 6m
The large bowel is 1.5m
What are the 6 main functions of the GI tract?
- Ingestion
- Mechanical processing
- Digestion
- Secretion
- Absorption
- Excretion
What is involved in the ingestion phase?
Material enters the digestive tract via the mouth
What is involved in mechanical processing?
Crushing and shearing, wetting and softening, mainly achieved through chewing
What is the purpose of the mechanical processing stage?
It makes material easier to propel along the GI tract by mechanically breaking them down into smaller molecules
Why is it important that food is softened during mechanical processing?
It helps to prevent blockages in the GI tract
What does digestion require?
A large surface area for the enzymes to work efficiently
What is involved in the digestion stage?
The chemical breakdown of food into small organic fragments (e.g. glucose, amino acids) for absorption by digestive epithelium
What is involved in the secretion stage?
The release of water, enzymes, acids, buffers and salts into the GI tract
What structures are involved in the secretion stage?
Glandular organs and the epithelium of the digestive tract
What is involved in the absorption stage?
The movement of organic substrates, electrolytes vitamins and water
They move across the digestive epithelium into the interstitial fluid of the digestive tract
What is involved in the excretion stage?
The removal of indigestible material and waste products from body fluids
What is the layout of the digestive tract from the mouth to the stomach?
the oral cavity leads into the pharynx
the pharynx leads into the oesophagus, which passes through the diaphragm and into the stomach
What must food pass through to enter the small bowel?
The pyloric sphincter
What are the 3 parts of the small bowel?
- duodenum
- jejunum
- ileum
What are the components of the large bowel?
- ascending colon
- transverse colon
- descending colon
- sigmoid colon
What is the peritoneum?
Layers of membrane which cover the inside of the abdominal cavity, and the organs themselves
What are the 2 types of peritoneum and the differences between them?
The parietal peritoneum lines the abdominal cavity
The visceral peritoneum covers the organs
Which organs of the GI tract are retroperitoneal?
What does this mean?
Kidney, pancreas and part of the duodenum
They are not surrounded by the peritoneum
How are retroperitoneal organs suspended?
They are plastered against the posterior abdominal wall by the peritoneum
What structures does the peritoneum form?
What are the functions of these structures?
Mesenteries
They are double layers of peritoneum which suspend the organs, support them and keep them from tangling
What other structures run in the mesenteries?
Blood vessels, lymphatics and nerves
What does the peritoneum secrete?
What is the purpose of this?
Peritoneal fluid
This provides lubrication that allows the organs to move against each other without friction
What would happen if there was friction between 2 organs?
It would cause inflammation and lead to scarring
What is the purpose of the fat pads in the mesenteries?
Where are they particularly common?
They help to cushion and protect organs
Common in the greater omentum
Where does the blood supply to the GI tract come from?
3 branches of the abdominal aorta:
- coeliac trunk
- superior mesenteric artery
- inferior mesenteric artery
Which structures are supplied by the coeliac trunk?
Foregut structures
- stomach
- gall bladder
- spleen (part of immune system)
- pancreas
What is the branch of the coeliac trunk and what does it supply?
The hepatic artery which supplies the liver with oxygenated blood
What structures are supplied by the superior mesenteric artery?
Midgut structures
- pancreas
- all of the small intestine
- the first part of the large intestine
What structures are supplied by the inferior mesenteric artery?
Hindgut structures
- the lower half of the colon and the rectum
What is the main blood supply of the anus?
It is mostly supplied by the blood supply of the skin
Where does the majority of blood from the GI tract drain into?
Where does this vessel go?
Hepatic portal vein
This goes to the liver for processing
What happens during processing in the liver, as blood enters from the hepatic portal vein?
Nutrients from the GI tract are converted into forms that can be used by the body
It also acts as a buffer to prevent glucose surges or levels falling too low
How does the liver receive oxygenated blood?
Via the hepatic artery, which branches from the coeliac artery
To what extent do the blood supplies of the GI tract cross over?
There is a small degree of cross-over between the blood supplies
What happens in an aneurysm and how does it affect blood supply to the GI tract?
Blood forces itself into the wall of the aorta, meaning it is not flowing in the correct pathway
This can lead to ischaemia in the GI tract if parts of the bowel are not receiving blood
How can an alternative blood supply be developed in an aneurysm?
Through the anastomoses of the inferior and superior mesenteric arteries
What condition is caused after an aneurysm, if there is inadequate blood supply to the large intestine?
Ischaemic colitis
This is inflammation and injury of the large intestine as a result of an inadequate blood supply
What are the 4 layers of the digestive tract, from innermost to outermost?
- mucosa
- submucosa
- muscularis externa
- serosa
What is the mucosa?
The innermost lining of the GI tract, consisting of an epithelial layer and supporting structures
What is the submucosa?
This is mainly connective tissue and a supply network
The supply network consists of blood vessels and lymphatics
What is the muscularis externa?
A muscular layer that is responsible for gut movement, such as peristalsis
What is the serosa?
This consists of the peritoneal layer and more connective tissue
How is the mucosal layer arranged and why?
It is thrown up into folds to increase the surface area
What are the two layers of the mucosa?
- lamina propria
2. mucosal epithelium
What is the role of the lamina propria and mucosal epithelium?
The epithelium forms villi to increase the surface area
The lamina propria is connective tissue that supports the epithelium
Where is the muscularis mucosae found?
What is its role?
Found between the mucosa and submucosa
It helps to maintain the shape of the epithelium
What structures are found within the submucosa?
Larger arteries, veins and nerve plexi (submucosal plexus)
It projects into the villi to allow lymphatic drainage close to the epithelium
What happens when the circular muscle layer contracts?
The gut tube becomes narrower
What happens when the longitudinal muscle layer contracts?
It runs along the length of the gut tube and contracts to make the gut tube shorter
Where is the myenteric plexus found?
Between the circular and longitudinal muscle layers
What are the two layers of the muscularis exerna?
Circular and longitudinal muscle layers
What structure is important in a sheet of epithelial cells forming an epithelial tube?
The adhesion belt with associated actin filaments
What is the first stage in forming an epithelial tube?
What is is caused by?
Invagination of the epithelial sheet
This is caused by an organised tightening of adhesion belts in selected regions of the cell sheet
What is the second stage in forming an epithelial tube?
The epithelial tube pinches off from the overlying sheet of cells
What determines how epithelial cells are orientated towards the different spaces which they line?
The basolateral and apical surfaces of the epithelial cells are different
This means that epithelial cells are polarised
What is the role of tight junctions in epithelial cells?
They join cells together and separate the basal and apical surfaces
What is significant about the formation of epithelial tubes?
This process leads to the formation of glands
How are glands structured?
They have an inside space where things are secreted from
They have an outside space where things are delivered from the bloodstream
What is significant about a polarised cell?
A polarised cell can target proteins to discrete cell domains
How would protein be distributed in polarised and non-polarised cells?
Non-polarised: the protein is distributed around the cell and its plasma membrane randomly
Polarised: the protein is targeted to one area of the cell, and the rest of the cell is free from protein
What are the types of junctions between epithelial cells, from apical to basal surface?
- tight junction
- adherens junction
- desmosome
- gap junction
What is the function of the tight junctions?
They define the apical and basolateral domains
They seal the gap between epithelial cells
What type of junction is a tight junction?
An occluding junction
What is the role of the adherens junction?
It is anchored to the actin cytoskeleton, and will connect the actin filament bundle in one cell with that in the next
What is the role of the desmosome?
It connects the intermediate filaments in one cell to those in the next cell
What type of junctions are adherens junctions and desmosomes?
cell-cell anchoring junctions
What is the role of gap junctions?
channel-forming junctions
They allow the passage of small water-soluble molecules between cells
They act as communication channels for electrical coupling
What is the role of hemidesmosomes?
They anchor intermediate filaments in a cell to the extracellular matrix (basement membrane)
What is the role of actin-linked cell-matrix adhesion molecules?
They anchor the actin filaments in a cell to the extracellular matrix
How are epithelia related to a blood supply?
Epithelia are closely associated with a capillary network
This supplies O2 and nutrients, and removes CO2 and waste
What are the basolateral and apical membranes of the epithelial cells in contact with?
The apical membrane is in contact with the luminal/mucosal fluid
The basolateral membrane is in contact with the interstitial fluid surrounding capillaries
What is the role of tight junctions?
They prevent diffusion of plasma membrane proteins between apical and basolateral domains
Why is the separation of cells into functionally discrete domains important?
It is crucial to the process of vectorial (directed) movement of solutes and water
What is meant by transepithelial movement of solutes and water?
Solutes and water can travel in 2 directions across epithelia:
absorption and secretion
What is involved in movement during absorption?
Movement from the external compartment (gut lumen) to the internal compartment (blood and lymph)
What is involved in movement during secretion?
Movement from the internal compartment (blood) into the external compartment (gut lumen)
What is meant by a ‘flux’?
What are the 2 types of flux?
A ‘flux’ describes the uni-directional movement of solutes and water
This can be an absorptive flux or a secretory flux
What is the net flux?
How is it calculated?
It is determined by the difference between the secretory and absorptive flux
J(net) = J(abs) - J(sec)
What drives a flux?
The prevailing electrochemical gradients
By which means can absorption take place?
This is movement from the lumen to the blood
It can occur via the transcellular or paracellular route
By which means can secretion take place?
This is movement from the blood into the lumen
It can occur via the transcellular or paracellular route
What is involved in the transcellular route?
This involves crossing both the apical and basolateral membranes of the epithelial cell
What is involved in the paracellular route?
This involves passing between the epithelial cells
What type of process is paracellular movement?
How does this affect the flux?
Paracellular movement is a passive process
There is no way of controlling or driving the flux
What type of process is transcellular movement?
How does this affect the flux?
Transcellular movement is driven through active transport
This ensures all molecules travel in one direction and none are wasted
What is meant by passive transport being ‘not coupled’?
Where may it occur?
The solute moves down its electrochemical gradient
It occurs through cells or through tight junctions
What molecules are involved in passive transport?
Ions and water travel via pores or channels
Transporters will facilitate the diffusion of small non-electrolytes
What route must be taken by primary or secondary active transport?
Active processes MUST use the transcellular route
What is involved in primary active transport?
Energy is used directly from ATP to drive transport
ATP forces changes in a protein, which transports a substrate against its electrochemical gradient
What is involved in secondary active transport?
Energy is used from something flowing down an electrochemical gradient, to drive another process
How are primary and secondary active transport linked?
An electrochemical gradient from substrate A is set up by primary AT
Substrate B moves against its electrochemical gradient
What is the role of the sodium-potassium pump in sugar transport?
What type of active transport is this?
Secondary active transport
The Na+/K+ pump will move Na+ ions out of the cell by primary active transport
How is glucose moved into the cell, after a low intracellular Na+ is achieved?
Na+ in the lumen moves down an electrochemical gradient into the cell
It can only do this when glucose is present
Glucose moves against its electrochemical gradient and accumulates in the cell
How is glucose released from the cell into the bloodstream?
Through a passive transporter such as GLUC-2
Where does water flow to?
By which means can it move?
It flows from areas of low osmotic pressure to high osmotic pressure
It can pass through the paracellular or transcellular pathway
What is meant by the net solute movement tending to drive the net fluid movement?
If there is an accumulation of osmotically active solutes in a compartment, water will move to equalise the osmotic gradient between the compartments
How does water cross plasma membranes?
Diffusing through the lipid or through aquaporins
Active transport of solute is followed by passive transport of water
Where does the main sensory and motor control of the GI tract come from?
Parasympathetic nerves
Which parasympathetic nerves control which parts of the GI tract?
The vagus nerve controls the foregut and midgut derivatives
The pelvic splanchnic nerves supply the hindgut derivatives
How does the parasympathetic and sympathetic nervous system affect the activity of the gut?
The parasympathetic NS increases the activity of the gut
The sympathetic nervous system inhibits activity of the gut
This allows blood to be diverted to other areas of the body where it is needed more
What does the enteric nervous system consist of?
The autonomic nervous system acts via the enteric NS
It consists of myenteric and submucosal plexi
What is the enteric nervous system and what is its role?
Much of the activity of the muscle in the gut wall is locally regulated and controlled by a complex set of nerves lying between the muscle layers and underneath the submucosa
What is the main function of the enteric nervous system?
It controls much of the normal gut motility
From which vertebral levels to the sympathetic ganglia supplying the GI tract originate from?
Supplying the stomach - T5
Supplying the large intestine - L1
From which vertebral levels do the pelvic splanchnic nerves originate from?
S2 and S4
sacral region
Which components of the GI tract are controlled by the myenteric and submucosal plexi?
The myenteric plexus controls the smooth muscle and secretory cells
The submucosal plexus controls the blood vessels and the endocrine cells
What are ‘slow waves’?
Periodic shifts (depolarisations) in the resting membrane potential of cells
This makes it easier for a small signal to initiate contraction and/or action potential
What type of signals will initiate activity only during a slow wave?
Neural and hormonal signals
What is the role of interstitial cells of Cajal in slow waves?
They act as ‘pacemakers’ to give a steady rhythm of depolarisation and repolarisation
How many times per minute do slow waves occur?
3 - 12 times per minute
What determines whether the threshold is reached in a slow wave?
What is the result of a threshold being reached?
It depends on the other signals received by the interstitial cells of Cajal
If the threshold is reached, the signal is amplified to increase activity
What do the interstitial cells of Cajal form connections with?
With each other and with smooth muscle cells
Why do interstitial cells of Cajal form connections with smooth muscle cells?
They drive processes such as peristalsis through action on smooth muscle cells
