Intro To Functions And Control Of AI tract

Question 1

What functions of the stomach associated with

Answer 1

The stomach is associated with accommodation and storage, mechanical and enzymatic
breakdown, and slow delivery of chyme to the duodenum. Food is stored in the stomach during the
first stage of digestion and remains there for around an hour unmixed. In this case, the stomach is
simply acting as a reservoir.

Question 2

What parts of the stomach allow relaxation to occur

Answer 2

The fundus and body of the stomach, that have thinner muscle tone than
other parts of the stomach, relax allowing for an increase in stomach volume (around 1.5 L but can
be bigger in some people) for food storage.

Question 3

How is muscle tone inhibited in the stomach - what are the mediators

Answer 3

The vagal reflex associated with the stomach inhibits
muscle tone through mechanoreceptors to bring about fundic relaxation. The mediators of this reflex
include VIP and NO.

Question 4

Where does the food in the stomach travel down to

Answer 4

As the food spends more time in the stomach, it begins to make its way down
the stomach towards the duodenum.

Question 5

Describe the Antral region and its function

Answer 5

The antral region of the stomach has more muscle tone
contracting and emptying the stomach (tonic contractions that also occur at the fundus and body of
the stomach). It also mixes and grinds the food with gastric secretions at the same time.

Question 6

Where do the contents of the stomach move to and how

Answer 6

The contents of the stomach move into the duodenum via the pyloric sphincter when it is relaxed (opens
when the duodenum can handle the food).

Question 7

What is an immune related function of the stomach

Answer 7

The stomach is also an acid decontaminator that protects

the digestive tract from infection.

Question 8

What are the storage sites of indigestible residues of the body

Answer 8

The colon and rectum are also storage sites of the digestive system that store indigestive residues
and faecal matter.

The stomach stores around 2-3 litres of gastric juice per 24 hrs.

Question 9

What does gastric juice contain

Answer 9

In this gastric juice is mucus,

pepsinogen, intrinsic factor and lipase that aid and help with digestion and absorption of food.

Question 10

How is mucus secreted in the stomach and function

Answer 10

Mucus is secreted by goblet and mucus neck cells in the stomach and acts as a lubricant to protect
the walls of the stomach (as well as the colon), especially from gastric acid (can cause trauma if
unprotected).

Question 11

What breaks down fats and what into

Answer 11

Lipase converts triglycerides to fatty acids and glycerol with around 30% of the
digestion of fats occurring in the stomach.

Question 12

How is Pepsin activated and where is it secreted

Answer 12

Pepsin is secreted by chief or peptic cells in the stomach

as pepsinogen where it is then activated (via the action of HCl acid) for protein digestion.

Question 13

Function and place hcl is secreted

Answer 13

Hydrochloric acid is secreted by parietal cells and is an important defence mechanism of the
digestive tract.

Question 14

Function and place intrinsic factors are secreted

Answer 14

Intrinsic factor is secreted by parietal cells and is key for vitamin B12 absorption.

Question 15

What is the effect of Paracrine secretions how do they act and where are secreted

Answer 15

There are paracrine secretions of the digestive tract that are often labelled local hormones. These are
secreted by gut mucosa but unlike hormones, act locally on adjacent cells via interstitial fluid. An
example of this is somatostatin that inhibits gastrin (promotes acid secretion via two pathways to be
discussed below) release in the stomach (acts in a paracrine fashion).

Question 16

What are the exocrine secretions on the digestive tract and what are there functions

Answer 16

There are also a number of
exocrine secretions that occur in the digestive tract. Salivary glands secrete mucus involved in
lubrication for mastication (chewing) and speech, as well as lipase. Gastric glands secrete
hydrochloric acid, pepsin and mucus as well. The pancreas secretes bicarbonate ions (secretion
under the influence of secretin) and enzymes including amylase (hydrolyses starch into sugars), lipase
and carboxypeptidase (proteases that hydrolyse peptide bonds at the C-terminal of a protein) in the
digestive tract. The liver secretes bile salts and bile acids into the digestive tract (important for fat
emulsification). These secretions from numerous glands enter the gut lumen via ducts and are
involved in digestion, lubrication and protection.

Question 17

What are the endocrine secretions of the digestive tract and their function

Answer 17

The endocrine secretions of the digestive tract are hormones. These are synthesised by ductless
glands and enter the bloodstream to travel to their target tissues where they bind to specific
receptors to elicit their effects. Gastrin is an endocrine secretion of the stomach secreted by G-cells
in the antrum. Secretin that is involved in controlling the secretion of bicarbonate ions is secreted by
duodenal mucosa. This is to neutralise the acidic chyme entering the duodenum and prevent it from
causing any damage. Pancreozymin-cholecystokinin (CCK) is also secreted by duodenal mucosa under
the influence of lipids. It plays a major role in decreasing food intake. Insulin is secreted by beta cells
of the pancreas.

Question 18

What do exocrine and endocrine and Paracrine secretions allow

Answer 18

The exocrine, endocrine and paracrine secretions allow active digestion to take
place and control it as well as gastric motility and energy homeostasis.

Question 19

For food to be used in the body where must digested nutrients travel - what is this called and where does this mostly take place

Answer 19

For food to be of use to the body, the nutrients that are digested in the digestive tract (e.g. glucose,
amino acids) must be transported across the intestinal epithelium into the blood or lymph (in the
case of fats and lipids). This process is known as absorption and mainly occurs in the small intestine.

Question 20

Where does fluid absorption take place

Answer 20

Fluid absorption occurs in the small intestine as well as the colon with the colon absorbing 90% of
water leaving around 200ml of semi-solid faecal matter.

Question 21

What do disorders for fluid secretion and absorption cause

Answer 21

Disorders of fluid secretion and absorption

as well as motility contribute to diarrhoea.

Question 22

Where does storage in the gi occur

Answer 22

Storage in the GI occurs in the proximal stomach and the descending colon.

Question 23

What is motility in the gi described as - what does this allow

Answer 23

Motility is the
movements of the muscular wall (mostly smooth muscle except for the extreme ends of the
oesophagus and rectum) of the GI tract and allows for unidirectional movement from these regions
of storage (law of the gut). This movement can result in movement of substance from one region to
another in mass evacuation, mechanical degradation (e.g. occurs in the gastric antrum), mixing of
lumen contents (e.g. occurs in the small intestine), and the transport of nutrients, water, urea and
electrolytes. These processes can collectively be termed digestion and absorption.
Food spends varying amounts of time in different regions of the gut.

Question 24

What is the function of the esophagus sphincters

Answer 24

Starting between the upper and
lower oesophageal sphincters, food spends around 20s in this region. The function of this region is to
propel food on to the next region that is the stomach.

Question 25

Time span of food in stomach and main functions

Answer 25

The stomach contains the fundus and gastric
body that are regions of storage, and the gastric antrum the mixes the contents of the stomach. Food
can spend anywhere between 5 and 300 minutes in this region.

Question 26

Next region after the stomach - function and time span

Answer 26

The next region begins at the pyloric
sphincter and ends at the ileocecal valve spanning the small intestine. The purpose of this region is
to pump food and mix it followed by absorption and desiccation of the food. This process takes
around 90-180 minutes.

Question 27

What signals between small and large intestine - and function

Answer 27

The ileocecal valve signals the beginning of the large intestine that is a site
of storage and then propulsion at the internal and external sphincters. Contents can remain in this
region between 16 and 48 hrs with the propulsion phase taking between 5 and 30 minutes.

Question 28

What is excretion and what ways can this occur

Answer 28

Excretion is the way the body removes unwanted products of metabolism as well as drugs. This can
leave in the form of saliva, bile, faeces and vomit. Indigestible food residues (e.g. tomato skin) leave
the body in the form of faeces.

Question 29

How is the intestine specialized to burden if environment

Answer 29

The gastrointestinal tract plays a key role in defence. The gut epithelium is an interface with the
atmosphere alongside the skin and airways and is therefore exposed to threats of the external
environment. The intestine is exposed to the heaviest burden of environmental antigens meaning it
has the largest mucosal surface in the body. Any breach in this barrier and toxins can enter the blood.
The gut also has a very large lymphoepithelial organ that is key to protecting it.

Question 30

What are the defence mechanisms of the gut

Answer 30

There are a number
of defence mechanisms of the gut. Sight, smell and taste alerts one to harmful food substances.
Vomit is a reflex to expel harmful substances. Hydrochloric acid in the stomach kills most harmful
bacteria. Mucous secretions help trap harmful substances and protect the gut lining from the acidic
environment. Natural flora bacteria prevent the colonisation of harmful bacteria. Lymphoid tissue in
the GI is able to aggregate (e.g. Peyer’s patches) and
mount a response to food-borne antigens.

Question 31

Where are peyers latches located

Answer 31

Peyer’s
patches are located in the lamina propria layer of the
mucosa (lies beneath the epithelium) and extend into
the submucosa of the ileum.
The gut also has a number of metabolic functions (to be
discussed in greater detail in coming lectures).

Question 32

Main functions of the liver

Answer 32

The liver
is a major metabolic organ in the abdominal cavity and
is involved in carbohydrate, nitrogen and lipoprotein
metabolism as well as the production of bile and the
excretion of bilirubin.

Question 33

How is the gut innervated

Answer 33

The gut is innervated autonomically through the sympathetic and parasympathetic nervous systems.

Question 34

How does the parasympathetic innervate the gut

Answer 34

The parasympathetic system stimulates digestive behaviour and the gall bladder, relaxes the rectum,
and stimulates salivation.

Question 35

How does the sympathetic innervate the gut

Answer 35

The sympathetic system inhibits digestive activity, stimulates glucose
release from the liver, inhibits salivation, contracts the rectum and relaxes the bladder.

Question 36

What is the guts own ‘brain called “ - what does it do

Answer 36

The gut is
encircled by nerves from the oesophagus to the stomach. Some of these nerves lead to the brain
whilst some end at the spinal cord. The gut is said to have its own brain known as the enteric
nervous system. It can initiate responses to different gut stimuli without requiring the higher
function of the brain (108 neurons).

Question 37

How is the gut able to move food - how is this mediated

Answer 37

The gut moves the bolus of fluid along it by ascending

contraction and descending relaxation. These actions are mediated by excitatory reflexes.

Question 38

How is ascending contraction mediated and relaxation

Answer 38

Ascending
contraction is in fact mediated by acetylcholine and substance P/K and descending relaxation is
mediated by VIP and NOS. The enteric nervous system involves different nerve types transmitting
nerves between the gut, the spinal cord and the brain.

Question 39

Where do afferent to nerves carry impulses from and to

Answer 39

Afferent nerves carry impulses from sensory

stimuli in the gut towards the CNS and brain whilst

Question 40

Where do efferent nerve carry impulses away and to

Answer 40

efferent nerves carry impulses away from the CNS

and brain.

Question 41

Where are splanchnic nerves found

Answer 41

Splanchnic nerves are in pairs and carry sensory fibres from organs as well as efferent
fibres of the ANS. They all carry sympathetic fibres except for the pelvic splanchnic which carry
parasympathetic fibres.

Question 42

What are the nerves that innervate the enteric nervous system known as

Answer 42

The nerves that innervate the enteric nervous system from the brain are
known as vagus nerves with the sensory nerves initiating this response also vagal nerves.

Question 43

What enables food storage in the stomach

Answer 43

The autonomic nervous system enables food storage in the stomach.

Question 44

What facilitates the receptive relaxation of the fundus if the body if the stomach

Answer 44

Receptive relaxation of the
fundus and body of the stomach is facilitated by the ANS through parasympathetic nerves and vagus
inhibitory fibres.

Question 45

What facilitates the accommodation of the stomach

Answer 45

The accommodation of the stomach is in fact facilitated by receptive relaxation,
adaptive relaxation and feedback relaxation (see control of food intake lecture for more details).

Question 46

What is released due to presence if lipids and what is the effect

Answer 46

CCK
is released as part of one feedback relaxation pathway caused by the presence of lipids. CCK causes a
decrease in food intake due to the fact lipids cannot proceed into the duodenum without
emulsification.

Question 47

What does cck promote the contraction of

Answer 47

CCK also promotes the contraction of the gall bladder and secretion of bile salts to
allow for emulsification of the fats.

Question 48

Describe movement of food in duodenum and what causes this

Answer 48

The autonomic system is also vital to movement of food into the duodenum. Ripples of contraction
moves food towards the antrum of the stomach that has a thicker muscle layer. The pyloric sphincter
is usually relaxing however, it closes upon arrival of the peristaltic wave as it detects the size of food
particles.

Question 49

What is the prime function of the pyloric sphincter

Answer 49

The pyloric sphincter in fact acts as a sieve and small partly digested food is squirted into 
the duodenum (up to 1-2 mm).

Question 50

What allows mixing and grinding if food

Answer 50

Repulsion of antral contents backwards towards the body allows for
mixing and grinding into smaller pieces until the duodenum is able to handle these particles.

Question 51

How is endocrine control of gut mediated - how do they act

Answer 51

There are different controls in the gut used to regulate it. Endocrine control is mediated by hormones
carried in blood and they are carried from their site of their production to their target site. All
hormones produced in the gut are peptides. The hormones then binds to target cells inducing an
effect.

Question 52

Describe action of gastrin

Answer 52

Gastrin is an endocrine hormone produced in G cells of the stomach and is secreted due to
distension of the stomach (can be released upon expectation of food). This causes neural stimulation
that promotes gastrin releasing peptide release (GRP). GRP acts on G cells causing them to release
gastrin into circulation. It can then promote histamine secretion from ECL cells (enterochromaffin-like
cells) that is a neuroendocrine cell found in the gastric glands of gastric mucosa. The histamine
released then interacts with receptors on parietal cells causing them to release HCl acid. Histamine
release from ECL cells can also be initiated by acetylcholine release from neurons with the
acetylcholine being able to also directly interact with parietal cells and cause HCl acid release.
Gastrin can also bind directly to the surface of parietal cells also promoting release of HCl acid. The
result is hypersecretion of acid.

Question 53

Describe the main Paracrine control mechanism of the gut

Answer 53

Paracrine control is another level of control of the gut. Hypersecretion of acid caused by release of
gastrin can be damaging to the gut despite its mucosal layer. Hypersecretion of acid causes the
release of somatostatin from D cells with the somatostatin going on to inhibit the release of gastrin
from G cells. This mechanism does not operate without hypersecretion of HCl acid.

Question 54

Describe the neural reflex and how it contributes to control gut

Answer 54

Neural reflexes also contribute to control of the gut. Swallowing at the oesophagus causes the
stimulation of vagal reflexes the relay information to the brainstem. This would allow coordinated
contraction and relaxation so the food can be massaged down the oesophagus into the stomach.

Question 55

What mediates the relaxation of the stomach to accommodate food

Answer 55

Once the food is in the stomach, relaxation is mediated by VIP and NO for accommodation. There is
also efferent innervation of the gut that is also essential to gut control.

Question 56

What is the vago-vagal reflex

Answer 56

The vago-vagal reflex is a type
of reflex in which both the afferent and efferent axons are in the vagal trunk. This reflex circuit
operates within the gut and is a pathway mediated by the brainstem. Gut stimuli causes a response
via the NTS (nucleus tractus solitarius) and DMVN (dorsal motor vagal nucleus) of the brain and
causes receptive relaxation of the stomach during swallowing. It also promotes motility and acid
secretion as acetylcholine that is the mediator causes both these effects (ACh causing acid secretion
mentioned in section on how gastrin causes hypersecretion of acid).

Question 57

What happens when the vagaries innervation is interrupted

Answer 57

Interruption of the vagal
innervation causes a rise in intra-gastric pressure and is a potential cause of vomiting due to the
inability of the proximal stomach smooth muscle to undergo receptive relaxation. The NTS is the
main site of the termination of the vagal afferents and the DMVN is the main site of the origin of the
vagal efferents.

Question 58

What makes up the enteric nervous - what is each function

Answer 58

The enteric nervous (local reflex) involves two nerve fibres that are intrinsic to the gut. The 
Myenteric plexus (Auerbach’s plexus) facilitates motor function whilst the submucosal plexus 
(Meissner’s plexus) facilitates intestinal secretion (closer to the gut lumen than the myenteric 
plexus).

Question 59

What do the auerbachs plexus and the meissners plexus reflexes regulate - how are they connected to cns

Answer 59

These reflexes regulate the gut function within the wall of the gut. They are connected to the
CNS via parasympathetic and sympathetic fibres but can function autonomously without these
connections. The effects mentioned are mediated entirely by the enteric nervous system (ENS) that is
the third component of the ANS (has a similar number of neurons as the

Question 60

What Neurotransmitters are involved in the enteric nervous system - where are they present

Answer 60

The
neurotransmitters involved in this nervous system are most importantly acetylcholine and
noradrenaline, but also include NO, 5-HT, GABA and ATP. Both plexuses are present in the wall of the
colon and with the myenteric plexus receiving inputs from both the SNS and PNS (submucosal plexus
only has parasympathetic fibres).

Question 61

What is pns activity associated with - what is this called

Answer 61

PNS activity is associated with contraction of the proximal colon
whilst SNS activity is associated with decreased colonic movement (PNS increases gastric motility
whilst SNS decreases gastric motility). This reflex in the colon is known as the colo-colonic reflex
(tends to be promoted upon the ingestion of food).

Question 62

What is the rate of emptying of the duodenum reliant on

Answer 62

The rate of emptying of the stomach into the duodenum is dependent upon the material’s ability to
be absorbed meaning it also has some level of control of the gut. Carbohydrates are absorbed the
quickest so empty quickly into the duodenum. Proteins empty slowly with fats emptying even slower.
This is because fatty acids in the duodenum increase the contraction of the pyloric sphincter
decreases the lumen size into the duodenum.

Question 63

What is peristalsis and where is it the slowest - how is it initiated

Answer 63

Peristalsis also controls gut function. It is slower in the large intestine compared to the small intestine
and is characterised by waves of propulsive contraction that results in the mass movement of
contents of the gut towards the anus. Distension initiates peristalsis with vagal inhibitory and
excitatory fibres controlling the movement.

Question 64

What is the mechanism of peristalsis

Answer 64

They do this by innervating the circular (reduce the
diameter of the lumen when they contract) and longitudinal muscles (reduce the length of the lumen
when they contract) of the gut. This means the circular muscles relax (via VIP and NO) just in front of
the bolus but contract (via substance P and ACh) just behind it. The longitudinal muscles will contract
in front of the bolus and relax behind it. Once the bolus has passed, the respective muscles relax to
normal size. This is because there are constant peristaltic waves passing over the stomach. During
these waves, the oesophageal sphincter is contracted (not contracted associated with reflux
problems) to prevent acidic chyme from flowing into the not very well protected oesophagus (does
not have much of a mucous layer). Barret oesophagus can be caused by overexposure of the
oesophagus to acidic chyme. This is a potential cause for oesophageal cancers.