Transport along the alimentary tract

Question 1

Describe Motor Behaviour in the Alimentary Tract

Answer 1

Motor activity in the alimentary tract= the way in which food moves along the gut
Motor activity speed must be regulated to allow enzyme action and secretory actions to function
Delivery of chyme to the duodenum is regulated so that it occurs at a manageable pace
Diarrhoea is caused by increased motility, constipation stems from reduced motility

Question 2

Describe Gastric Accommodation

Answer 2

When fasting, our stomach shrinks. When we eat, oesophageal mechanoreceptors detect distension caused by a bolus
This leads to cholinergic activity, which causes oesophageal contraction and fundus relaxation.
When the bolus reaches the stomach, it’s detected by mechanoreceptors, triggering the vagovagal reflex
The vagovagal reflex results in VIP/NO mediated fundus relaxation for accommodation
Over time, antral contraction occurs, pushing chyme through the pyloric sphincter into the duodenum.

Question 3

Why must gastric accommodation and emptying be carefully handled?

Answer 3

To ensure we don’t swamp the duodenum:
Allows chyme to be adequately neutralised so that it is broken down but does not cause peptic ulcers
It also allows enzymatic action and mechanical degradation to occur, breaking down the food correctly

Question 4

How do the duodenum and pyloric sphincter work together to ensure the duodenum isn’t swamped?

Answer 4

Duodenum contents control the pyloric sphincter, ensuring the duodenum is not swamped
Fatty foods stimulate CCK release from the duodenal mucosa. This closes the pyloric sphincter, slowing gastric emptying so that hay time for the fat emulsification by bile salts in the duodenum
Acidic, hyperosmolar chyme stimulates secretin release. This stimulates the liver/pancreas to secrete bicarbonate, neutralising and diluting the chyme.
When the duodenum is ready to receive more chyme, the antrum contracts and the pylorus relaxes, facilitating gastric emptying

Question 5

How does overeating and infection affect gastric motility and emptying?

Answer 5

Overeating causes overdistension of the antrum.
Overdistension triggers vagovagal reflexes and hormone release, leading to pyloric contraction/closure whilst excess food is ground up in the contracted antrum
Closing of the pyloric sphincter slows gastric emptying.

Injury or bacterial infection in the intestinal wall also affects motility

Question 6

What is gastroparesis?

Answer 6

Gastroparesis is a chronic condition in which the stomach cannot empty itself of food in the normal way, causing food to pass through it slowly.

Question 7

How does the physical state of the food eaten affect gastric emptying?

Answer 7

Liquids have no lag phase, so empty into the duodenum instantly
Solids must be ground up by the antrum before emptying so have a lag phase
Indigestible foods stay in the stomach until the migrating motor complex (MMC)/vomiting remove them
Fatty foods must be emulsified, so have a log lag phase

Question 8

What is reciprocal vagal control?

Answer 8

For a bolus to move forwards, the gut area in front must relax and the one behind contracts. This= reciprocal control
This reciprocal control comes from vagal innervation to the smooth muscle in the gut wall
Vagal excitatory fibres release ACh and SubP, causing contraction behind the bolus
Vagal inhibitory fibres release VIP and NO, causing relaxation ahead of the bolus
When the fundus relaxes, it causes the stomach body to contract, forcing food into the antrum for grinding

Question 9

Describe Myogenic Control of Gastric Motor Activity

Answer 9

The basic electrical rhythm (BER) of the stomach determines the Hz of peristaltic contractions
Interstitial cells of Cajal (ICCs) are specialised pacemaker cells in the gut wall. They undergo rhythmic depolarisation and repolarisation in gut smooth muscle cells, generating the BER
Normally, the BER isn’t strong enough to initiate contractions- hormones/Ach/SubP are needed to help

Question 10

Describe Control of Motility in the Small Intestine

Answer 10

Hormonal and neuronal inputs initiate peristalsis and mixing in the small intestine using ACh, NO and VIP
Localised distension in the duodenum causes increased motility. CCK, gastrin and motilin are all secreted
Since the receptors in the small intestine are different to stomach receptors, the hormone effects are different.

Acidic chyme decreases intestinal motility by stimulating secretin secretion

Question 11

Gastric emptying is regulated by…

Answer 11

Gastric emptying is regulated by negative feedback
Contractions of the middle antrum stimulate a descending inhibitory reflex to the pylorus, where VIP and NO mediate relaxation
Acidic/fatty chyme in duodenum stimulate an ascending excitatory reflex to the pylorus. Here ACh and SubP cause contraction to prevent duodeno-gastric reflux
These duodenal stimuli also stimulate secretin and CCK release, which cause pyloric contraction whilst acid is neutralised and the fat is emulsified.

Question 12

What are the 3 types of movement in the intestine?

Answer 12

Intestinal motility is composed of 3 types of contraction:

Peristalsis, segmentation and MMC

Question 13

Describe segmentation

Answer 13

Segmentation is a mixing contraction
It consists of stationary contraction and relaxation, dividing chyme and bringing it into contact with the intestinal wall for absorption.
Slower than peristalsis
It also slows chyme migration towards the ileum.
Segmental contractions are myogenic- they originate in the ICCs

Question 14

Describe The Migrating Motor Complex (MMC)

Answer 14

The MMC is highly organised, recurring motor activity that originates in the distal stomach when it’s empty
Occurs in 3 phases- the peristaltic rush (phase III) is of interest:
High Hz and amplitude contractions along the entire length of the intestine clear out any indigestible contents. Abrasion removes dead epithelial cells
It clears out bacteria, preventing overgrowth and infection. It prevents the migration of colonic bacteria up to the small intestine.

Question 15

Describe the mechanism of the MMC

Answer 15

The mechanism by which the MMC occurs is not very well understood
Smooth muscle cells in the stomach create slow waves of contraction coordinated by ICCs and ENS, via the vagus nerve
Evidence suggests motilin secretion by M cells initiates the MMC. Motilin isn’t secreted when we eat, inhibiting the MMC

Question 16

Describe motility and contraction of the colon

Answer 16

Hay mixing and slow movement of waste and indigestible material. In the colon, hay 3 contraction types:
Haustral/Segmental: Haustra are sacs of colon from arrangement of the taenia coli smooth muscle
Contraction of the taneia coli bunches up haustra, mixing colon contents
Peristalsis: contractile waves moving colon contents to the anus. Initiated by distension of the intestinal wall activating mechanoreceptors
Mass Movement: powerful contractions in the mid-transverse colon that sweep contents to the rectum

Question 17

How is digestion and absorption mediated in the intestine?

Answer 17

Digestion and absorption in the intestine are mediated by enterocytes
These contain 2 parts: crypts for secretion, and villi which increase SA for absorption
All nutrients, water and electrolytes that enter the small intestine are absorbed

Question 18

What are the main mechanisms in which enterocyte transport occurs?

Answer 18

Transport through enterocytes occurs via 2 main mechanisms:
Transcellular: substances are transported through the cells in either actively or passively.
Paracellular: substances are transported passively through tight junctions between the cells. It’s highly selective, tightly regulated. Often uses transport proteins

Question 19

How are carbohydrates digested?

Answer 19

Carbohydrates can only be absorbed as monosacs.
Salivary amylases, then pancreatic amylases begin initial carb breakdown into disaccharides
Brush border enzymes in the small intestine then convert the disacs to monosacs:
Sucrase converts sucrose to glucose and fructose
Glucoamylase converts glucose oligomers to glucose
Lactase converts lactose to glucose and galactose.
The monosacs are then absorbed thru the enterocytes

Question 20

How are carbohydrates absorbed into the blood?

Answer 20

Na is pumped out of the enterocyte by basolateral (away from lumen) Na/K ATPase, creating a diff gradient
Na diffuses into the cell via an apical (cell membrane towards lumen) SGLT-1 transporter
This provides energy for the secondary a.transport of glucose and galactose into the enterocyte via SGLT-1.
Basolateral GLUT-2 transporters then transport the glucose and galactose into the blood

Question 21

What is the difference between glucose/galactose absorption and fructose absorption?

Answer 21

Glucose and galactose are absorbed by apical SGLT-1 and basolateral GLUT-2 transporters
Apical GLUT-5 and basolateral GLUT-2 absorb fructose from the intestinal lumen
This is all passive transport

Question 22

How are proteins digested before absorption?

Answer 22

Pepsin in the stomach breaks proteins down into peptides

Peptides are further broken down to di and tri-peptides in the duodenum by pancreatic proteases

Question 23

How are proteins absorbed?

Answer 23

Proteins are absorbed in one of 2 ways:
1)Aa.s are transported into enterocytes by a Na coupled carrier
Diff amino acids have diff carriers. The aa.s are transported across the basolateral membrane into the blood via active transport
AA.s are then carried in the circulation to the liver, where they are recycled
2)Some di and tri-peptides are transported using an inwardly directed H+ gradient

Question 24

What kind of lipids do we ingest?

Answer 24

The majority of the lipids we ingest are triglycerides.

Others inc phospholipids, cholesterol and fat soluble vitamins (A,D,E and K)

Question 25

How are lipids digested?

Answer 25

Triglycerides must be broken down into smaller subunits to be absorbed. This is done by salivary, then by pancreatic lipases in the small intestine.

Lipases are hydrophilic, so can only act on a small area round the outside of fat droplets.
To fix this problem, fat globules are emulsified into smaller droplets by bile salts in the duodenum.
This increases SA with which lipases can interact

Question 26

How are fat globules emulsified?

Answer 26

Fatty substances in the duodenum mean CCK stimulates bile salt secretion from the gallbladder via the cystic duct
The cystic and pancreatic duct joins to form the common bile duct. This duct enters the duodenum at the ampulla of Vater.

Question 27

The pancreas also secretes lipase and co lipase. What is the function of each?

Answer 27

The pancreas also secretes lipase and co-lipase.
Co-lipase is amphipathic. This means hydrophilic lipase binds to the hydrophilic regions of the co-lipase

The hydrophobic regions of the co-lipase then bind to the hydrophobic lipid droplets. This breaks TG down into glycerol and fatty acids.

Question 28

Once lipids have been digested, how are they absorbed?

Answer 28

For fatty acids and lipids to be absorbed, they must be made into micelles. Micelles from when bile salts create a phospholipid layer round the digested lipid.
The phospholipid micelle layer can then fuse with the enterocyte membrane. Short fatty acid chains are then carried by albumin to the liver, via the portal vein

Question 29

How are other longer chain lipids absorbed?

Answer 29

All other lipids are reformed into chylomicrons in the sER of the enterocyte. Chylomicrons form when the lipid is combined w apolipoproteins and glucose
Chylomicrons then enter the lymphatic, and then venous system via lacteals (lymphatic vessels in the small intestine).
Lacteal absorption here causes chyle from the small intestine to look very milky
From the venous system, the lipids are carried as lipoproteins in the blood.

Question 30

Describe Disorders of Lipid Digestion and Absorption

Answer 30

Cholelithiasis, pancreatitis, Crohn’s disease and liver diseases can all lead to fat malabsorption, which presents as steatorrhea
Steatorrhea is fatty diarrhoea – it is often described as being a sticky, fatty, smelly poo that is very loose