Transport Along And Across GI Tract

Question 1

What causes the emptying of the gastric reservoir

Answer 1

Emptying of the gastric reservoir (the stomach) into the antral pump is caused by tonic contractions
and peristaltic waves in the region of the gastric corpus.

Question 2

What generates the tonic contractions

Answer 2

The propulsive force generated by tonic

contractions is mediated by electric activity.

Question 3

How does the stomach deliver digests ri lower levels of the gi tract

Answer 3

The stomach has a mechanism of generating rhythmic

contractions to deliver digesta to the lower levels of the GI tract.

Question 4

What allows the grinding of food to occur

Answer 4

This propulsive force together with
the backflow from the closed pyloric sphincter allow grinding to occur in the antral region of the
stomach.

Question 5

What does the stomach do when all woke acid to act on food

Answer 5

The proximal stomach relaxes to store food at low pressure whilst it is acted upon by acid, enzymes
and mechanically.

Question 6

Why is the emptying of the stomach regulated

Answer 6

The emptying phase is then carefully regulated to ensure adequate acidification 
and neutralisation, correct action of enzymes and mechanical breakdown, and to avoid swamping of 
the duodenum (can cause duodenal ulcers due to acidic chyme).

Question 7

What disorders are associated with emptying of stomach

Answer 7

Disorders during this phase that are
associated with impaired gastric motility include gastric stasis (leads to gastroparesis, a long term
condition where the stomach cannot empty itself of food normally causing food to pass out slowly).

Question 8

Why is the emptying of the stomach also Important

Answer 8

This phase is also important to prevent colonisation as well as prevent bacteria from the small
intestine moving up into the stomach.

Question 9

What is gastric emptying specifically dependant on

Answer 9

Gastric emptying is also dependant on the stomach’s ability to
differentiate between different types of meals ingested and their components. Fatty, hypertonic and
acidic chyme in the duodenum decrease the force and rate of gastric emptying.
Gastric emptying of liquid, semi-solid and solid meals take different amounts of time.

Question 10

Describe stomach emptying liquid vs solid food - what effects rate if emptying liquids

Answer 10

The percentage
of food remaining in the stomach is much larger after a solid meal rather than a liquid meal. Liquid
food passes into the duodenum in spurts immediately after entering the stomach. The rate of
emptying of liquids is influence by nutrient content (liquids with higher nutrient contents take longer
to be emptied). Semi-solid and solid food have a lag phase before emptying commences with the
duration of lag time related to particle size (trituration of large particles to smaller ones). Solids are
broken down into 1-2 mm sizes with large indigestible materials remaining. These are removed by
vomiting or migrating motor complex (MMC that occurs after a certain time period) that allows the
stomach to be cleansed.

Question 11

What inhibits motor events and gastric emptying

Answer 11

Fatty foods liquify at body temperature and float on top of a liquid layer. Fats are inhibitors of motor
events and gastric emptying. In fact, the type of food in general influences gastric emptying with
carbohydrates emptying faster than proteins that empty faster than fatty foods that empty faster
than indigestible solids.

Question 12

What feature of the duodenum effects the gastric emptying

Answer 12

The osmotic pressure of duodenal contents also affects gastric emptying with
hyperosmolar chyme decreasing the rate of gastric emptying. Over distension of the stomach causing
vagal innervation also decreases gastric motility.

Question 13

What hormones inhibit gastric emptying

Answer 13

Hormones such as somatostatin, secretin, CCK and
GIP all inhibit emptying as well. Injury to intestinal wall as well as infections all contribute to
decreased motility.

Question 14

What allowed the initiation of tonic contractions in the gut

Answer 14

The gut is able to initiate tonic contractions due to the rhythmic activity of pacemaker cells within 
the interstitial cells of cajal (ICC) located within the walls of the stomach, small intestine and large 
intestine. These ICCs generate recurring migrating ripples (3 waves/min) known as intrinsic basic 
electric rhythm (BER).

Question 15

What does rhythmic activity of pacemaker cells include

Answer 15

This rhythm consists of depolarisations (generated by sodium/calcium entry)
followed by repolarisations (generated by potassium efflux).

Question 16

What excites and decreases fundic motion

Answer 16

The fundus is under vagal excitatory control that causes the BER generated from these cells to affect
contraction of smooth muscle. Fundic motor activity is decreased by CCK, secretin, glucagon, VIP,
somatostatin, duodenal distension and acid as well as GRP.

Question 17

What does motilin do

Answer 17

Motilin on the other hand increases 
fundic contractions (secreted during MMC).

Question 18

What controls movement through the small intestines

Answer 18

Movement through the small intestine is controlled by

various hormonal and nervous factors that influence peristalsis as well as mixing.

Question 19

What increases motility in the small instestines

Answer 19

Localised
distension as well as CCK, gastrin, motilin (especially increasing colonic motility), insulin and
serotonin all increase motility whilst secretin and glucagon decrease motility.
Gastric emptying is also under the influence of feedback control

Question 20

What causes lprolongation of relaxation of fundus

Answer 20

Antral over-distension causes the
vago-vagal reflex to prolong relaxation of the fundus (via inhibitory signals) whilst distension of the
reservoir stimulates antral contractions.

Question 21

What does duodenal over-dissension cause

Answer 21

Duodenal over-distension also causes the vago-vagal reflex

with chemical stimulation (content of chyme) causing the release of various hormones (e.g. CCK).

Question 22

What does the pyloric sphincter base its contractions on

Answer 22

The pyloric sphincter bases its contraction in response to antral or duodenal rhythm and signals. The
descending inhibitory reflex caused by contraction of the middle antrum causes pyloric relaxation via
NO and VIP. The ascending excitatory reflex from the duodenal stimuli (e.g. presence of acidic chyme
or fatty acids that is sensed by the enteric nervous system) causes pyloric contractions. Pyloric
activity is therefore governed by antral inhibitory and duodenal excitatory reflexes.

Question 23

What is gastric emptying mainly governed by

Answer 23

Gastric emptying as a whole is in fact mainly governed by the presence of fatty, hypertonic and acidic
chyme in the duodenum. It can affect the rate of gastric emptying through duodenal
enteroendocrine cells that secrete enterogastrones like secretin, CCK and GIP.

Question 24

What can affect the rate of gastric emptying

Answer 24

The rate of emptying
can also be affected by chemoreceptors and stretch receptors that target enteric neurons in short
reflexes (intrinsic control) or CNS centres (increase SNS and decrease PNS) in long reflexes (extrinsic
control) to decrease the contractile force in the stomach.

Question 25

What is segmentation

Answer 25

Segmentation (the mixing of | contents) involves stationary contractions and relaxations.

Question 26

What does peristalsis do

Answer 26

Peristalsis generates a propulsive force | that starts in wave forms beginning from the stomach.

Question 27

What is the mmc complex

Answer 27

The migrating motor complex is a sweeping | movement that cleanses the gut and is associated with removing undigested material.

Question 28

There are in | fact different phases associated with gut motility name them

Answer 28

Phase I is a quiescence (quiet) phase, phase II involves irregular propulsive contractions and phase III is associated with a burst of uninterrupted phasic contractions (peristaltic rush). Segmentation activity originates from the ICC pacemaker cells. It involves divisions and subdivisions of chyme that brings it into contact with intestinal walls where it is mixed with luminal contents as well as pancreatic secretions (see divisions right). Segmentation activity causes slow migration of chyme towards the ileum (contraction of circular muscles slows down the progression of chyme through the GI tract making it available for absorption) with the frequency of this type of contraction decreasing between the duodenum, jejunum and ileum (from 10-12 contractions/min to 8-9 contractions/min).

Question 29

When is segmentation activity most important

Answer 29

Segmentation activity is most important in the fed state to prevent unstirred layer formation

Question 30

What regulates peristalsis- bolus describe

Answer 30

Peristalsis is regulated by a combination of excitatory and inhibitory motor neurons. It causes mass movement along the GI tract in waves. Circular muscles contract behind each wave and relax in front of it whilst longitudinal muscles contract in front of the bolus and relax behind it. Once the bolus has passed, the surrounding structures return to their original shapes (see right how the different muscles work in tandem). Peristaltic contractions spread the food out as well allowing digestive enzymes to mix within it but primarily propel the food (opposite with segmentation). Peristaltic contractions are in fact governed by a circuit.

Question 31

What do the receptors in intestinal walls detect and do

Answer 31

Different receptors in the intestinal walls detect chemicals like CCK, glucose (for osmolarity), long chain fatty acids and amino acids. The receptors then initiate sensory neurons to stimulate the vagal centre to produce vago-vasal reflexes that govern contractile patterns of peristalsis. This circuit does not need to involve the brain with the sensory fibres interacting directly with the ENS that elicits the response (through integrating and program circuits that activate specific motor neurons).

Question 32

What is mmc and when does it occur

Answer 32

MMC is a highly organised motor activity involving a cyclical sequence of events. It occurs between meals when the stomach and intestines are empty starting in the lower portion of the stomach. It inly involves phase III motility that involves a burst of high frequency contractions with large amplitudes that migrate along the length of the intestine and die out. The interval between this activity is between 90 and 120 minutes. The MMC removes indigestible residues through these large contractions as well as wide opening of the pyloric sphincter.

Question 33

What is another function of mmc- not digestion related

Answer 33

It also functions to remove dead epithelial cells by abrasion as well as prevent the overgrowth of bacteria and colonisation of pathogenic bacteria.

Question 34

What initiates mmc

Answer 34

The control mechanisms of MMC are not fully known however the contractions are initiated by the vagus nerve in the upper tract and coordinated by the ENS as well as ICC. There is some evidence that cyclical secretion of motilin is related to MMC with motilin secretion inhibited during feeding.

Question 35

What processes occur in the large intestines

Answer 35

Bacterial fermentation, storage of waste and indigestible materials as well as absorption of water and ions all occur in the large intestine. Intensive mixing also occurs as well as slow movement of waste and indigestible material aborally (away from the mouth).

Question 36

What happens to fibre and indigestible nutrient in large intestines

Answer 36

Fermenting chambers allow for the hydrolysis of fibre and | indigestible nutrients. Faeces also form in the large intestine.

Question 37

What are the three types of motility that occcur in the large intestine

Answer 37

There are mainly 3 types of motility | that occur in the large intestine segmental , peristalsis , mass movement

Question 38

What is segmental motility in the large intestine

Answer 38

Segmental or haustral contractions (1st type of motility) mix contents of the large intestine and potentially facilitated by taenia coli longitudinal smooth muscle (3 bands of smooth muscles). The contraction of the taenia coli produces the bulges in the colon. Epiploic appendages are fat-filled sacs attached to the taenia coli and have an unknown function. They are mainly found in the transverse and sigmoid sections of the colon and are not present at all on the rectum.

Question 39

What is epopolic appendagitis

Answer 39

Epiploic appendagitis is a painful process that occurs when the appendages become inflamed (can be mistaken for appendicitis).

Question 40

Describe persistaliss in the large intestine

Answer 40

Peristalsis is the second type of motility associated with the large intestine. This is slow in the large intestine in comparison to the small intestine and moves contents towards the anus. Distension of the large intestine initiates this type of contraction.

Question 41

Describe mass movement in the large intestines

Answer 41

The final type of movement in the large intestine is mass movement. This is a powerful contraction of the mid-transverse colon that moves colon contents into the rectum (colonic evacuation).

Question 42

What can disorders of motility and fluid secretion cause

Answer 42

Disorders of motility, fluid secretion and absorption can cause diarrhoea and constipation. Diarrhoea is the frequent (greater than 3 times a day) discharge of liquid faeces and constipation is the difficulty in emptying the bowel (hard faeces). Marked deviation of the rough times the food spends in the gut (see previous lecture for exact timings) can indicate one of these problems. In order for us to benefit from the food ingested, absorption must occur. The different structures involved in absorption must be adapted for this process.

Question 43

How is the small intestines adapted for absorption

Answer 43

The small intestine has epithelial folds with villi and microvilli that give it a large internal surface area of 200 m2. The villi are highly vascularised to give them large amounts of blood supply. The epithelia of the villi extend down into crypts located in the lamina propria that are the site of cells involved in host defence and signalling. Crypt cells also contain stem cells that replenish the epithelial cells further up the villi. Materials absorbed into these cells can be transported through the cell into the blood (transcellularly) or through gap junctions between two cells (paracellularly) in a passive but selective and regulated process. Different components of food are digested and absorbed in different ways.

Question 44

How are carbohydrates able to be absorbed

Answer 44

Carbohydrates are complex molecules that can only be absorbed in their monosaccharide form. Amylases and other enzymes breakdown complex carbohydrates into monosaccharides ready for absorption. Within the gut lumen is salivary and pancreatic amylase whilst specific brush border enzymes at the brush border of the gut convert disaccharides and simpler glucose compounds into monosaccharides.

Question 45

What does sucrose , glucoamykase and lactose break down into and how are they absorbed

Answer 45

Sucrose is broken down into glucose and fructose by sucrase whilst glucoamylase breaks down glucose oligomers into glucose. Lactose is broken down into glucose and galactose Absorption of glucose into blood is dependent on chemical gradients. There is a lower concentration of sodium ions inside villus cells than the gut lumen and an even lower concentration of sodium ions in the bloodstream (maintained by sodium/potassium pump). This causes a net flow of sodium ions from the gut lumen to blood. Sodium enter villus cells from the gut via SGLT1, a sodium/glucose co- transporter. This means the movement of sodium in moves glucose into villi as well (1 glucose every two sodium ions). This transporter can also transport galactose as galactose occupies the site glucose uses to enter villi.

Question 46

Name inhibitors of glucose transport

Answer 46

Galactose is therefore a competitive inhibitor of glucose transport. Phlorizin is also an inhibitor of glucose transport and simply blocks glucose absorption through its glucose unit occupying the glucose site on SGLT1.

Question 47

How does glucose enter the blood stream

Answer 47

Glucose enters the blood from villi through GLUT2 transporters (transports galactose and some fructose basolaterally). Since the concentration of glucose in the blood is lower than in villi, glucose moves into the blood passively. Fructose does not need any transporter to enter the bloodstream but is rather passively transported (slow). It can also move into the blood via GLUT5 (specific to fructose) in facilitated diffusion.

Question 48

What happens when the duodenum detects lipids

Answer 48

When the duodenum detects lipids in it, it causes the gall bladder to contract (under the influence of CCK). The bile released from the gall bladder contains bile salts that facilitate the emulsification (prevent small droplets of fat combining and make them amphipathic) and allow pancreatic lipase to access the core of the lipids (initially globules of lipids). This increases the surface area upon which lipase can act increasing the rate of digestion. Lipase action is in fact enhanced by colipase.