Structure and FUnction of GI tract I

Question 1

What is chyme

Answer 1

mixture of food and secretions

Question 2

What organs are found within the GI tract

Answer 2

mouth - ingestion of food
{pharynx - swallowing of food
Oesophagus - transport of food to stomach
Stomach - breakdown of food
SI - digestion and absorption
LI - Formation and storage of faeces

Question 3

What are the accessory organs of the GI tract

Answer 3

Salivary glands - secretion of lubricating fluid
Liver - secretion of bile, storage of nutrients
Gall bladder - storage and concentration of bile
Pancreas - secretion of enzymes and hormones

Question 4

Explain the process of digestion

Answer 4

Starts at ingestion of food in oral cavity
Within the oral cavity, chewing and secretion of saliva by salivary glands takes place
Food then swallowed and transported to stomach by oesophagus
Food digested in stomach by mixing food with acids and enzymes to create chyme
Chyme moved into SI via the Pylorus
Most digestion occurs in SI
Unabsorbed chyme passes in LI where it becomes faeces

Question 5

What regulates what flows through the pylorus

Answer 5

Pyloric valve

Thick band of smooth muscle which allows amounts of chyme into the SI at any one time

Question 6

What are the layers of the GI tract

Answer 6

Mucosa - epithelium, underlying connective tissue and smooth muscle
Submucosa - dense irregular connective tissue
Muscularis - two layers of smooth muscle
Serosa - simple squamous epithelium and connective tissue

Question 7

What are the functions carried out by the digestive system

Answer 7

Digestion - chemical and mechanical breakdown of food into smaller pieces that can be moved across the intestinal epithelium
Absorption - movement of substances from the lumen of the GI tract to the ECF
Secretion - movement of water and ions from ECF to digestive tract lumen; release of substances synthesised by GI cells into lumen or ECF
Motility - movement of material in the GI tract as a result of muscle contraction

Question 8

How does mass balance pose a challenge to the digestive system

Answer 8

Maintaining fluid input with output; people ingest around 2 litres of fluid per day, in addition exocrine glands and cells secrete 7 litres or so of enzymes, mucus, electrolytes and water in the GI tract lumen
If this was not reabsorbed then body would dehydrate

Question 9

How does autodigestion affect the digestive system

Answer 9

Autodigestion is when digestive enzymes break down the cells of the GI tract itself, causing peptic ulcers to develop

Question 10

What are digestive enzymes

Answer 10

Breakdown polymeric macromolecules into smaller building blocks
Secreted by exocrine glands or epithelial cells in the stomach and SI
Help facilitate absorption of nutrients into the body

Question 11

How are digestive enzymes classified

Answer 11

Based on target substances

• amylases = carbohydrates
• proteases = proteins
• lipases = fats
• nuclease = nucleic acids

Question 12

What are zymogens

Answer 12

Enzymes secreted in an inactive form

Must be activated in the GI lumen before they carry out digestion

Question 13

What occurs in the mouth during digestion

Answer 13

Secretion of saliva to soften and lubricate food
Mastication mixes food to increase its surface area
Salivary amylase breaks down starch

Question 14

What are the three salivacry glands

Answer 14

Parotid - back of the mouth
Sublingual - under the tongue
Submandibular - under floor of the mouth

Question 15

What is saliva

Answer 15

Complex fluid that contains water, ions, mucus and proteins

Question 16

How does saliva production occur

Answer 16

Initial fluid secreted by acinar cells resembles extracellular fluid composition
Asfluid passes through the duct on its wa to the oral cavity, epithlial cells reabsorb NaCl and secrete K+ and bicarbonate ion until the ion ratio is more like intracellular fluid

Question 17

Function of saliva

Answer 17

Soften and moisten food
DIgestion of starch with salivary amylase
Taste
Defense via lysozymes and salivary immunoglobulins

Question 18

Why is colipase needed

Answer 18

Lipase is unable to penetrate the bile salts surrounding the droplets, therfore digestion also requires colipase

Question 19

What is swallowing and how does it occur

Answer 19

Reflex action that pushes bolus of food or liquid down into oesophagus
As swallowing begins, the soft palate elevates to close of the nasopharynx; muscle contraction move the larynx up and forward which closes off the trachea
as bolus moves down towards the oesophagus, epiglottis fold down completing closure of the upper airway and preventing food and liquid from entering the airway

Question 20

What triggers swallowing

Answer 20

pressure created when the tongue pushes the bolus against the soft palate and the back of the mouth activates sensory enurons that run the glossopharyngeal nerve to the swallowing centre in the medualla oblongata

Question 21

What is peristalsis

Answer 21

Contraction of the muscles of the oesophagus to move food down to the stomach

Question 22

Why is the hydrophobic nature of lipids a complication in digestion

Answer 22

Aqueous chyme leaving the stomach contains large fat droplets with less surface area, decreasing rate of digestion
So bile is secreted to form smaller droplets with larger surface area

Question 23

What does colipase do

Answer 23

displace bile salts allowing lipase access to the fat droplets

Question 24

How are lipophilic fats absorbed

Answer 24

Simple diffusion
Once absorbed into enterocytes they move to the smooth endoplasmic reticulum where they recombine into triglycerides
Triglycerides link with cholesterol and protein to form chylomicrons
Exit via exocytosis and then absorbed into lacteals
Pass through lymphatic system then enter venous blood

Question 25

How are shorter fatty acids absorbed

Answer 25

directly cross basement membrane and enter blood

Question 26

How is carbohydrate digested

Answer 26

Amylase breaks long glucose polymers into smaller glucose chains and into maltose

Starch digestion starts with salivary amylase and continues with pancreatic amylase

Maltose and other disaccharides are broken down by intestinal brush-border enzymes known as disaccharidases

Question 27

How does intestinal glucose and galactose absorption occur

Answer 27

uses apical Na+-glucose SGLT symporter and basolateral GLUT2 transporter
Glucose enters on SGLT symporter with Na+ and exits membrane by GLUT2 transporter

Question 28

How is metabolism of enterocytes different to other cells

Answer 28

Glutamine is main source of energy

Allows absorbed glucose to pass unchanged into blood stream

Question 29

What are endopeptidases

Answer 29

Commonly known as proteases
Attack peptide bonds in amino aid chains to break them into smaller fragments
secreted as zymogens

Question 30

What are exopeptidases

Answer 30

Release single amino acids by acting on the terminal amino acid on the protein

Question 31

How are proteins absorbed

Answer 31

Most free AA are carried by NA+-dependant cotransport proteins
Few AA are transported by H+ dependant cotransporter
Dipeptides and tripeptides are carried into enterocytes on the oligopeptide transporter PepT1 that uses H+ dependant cotransporter

Question 32

What happens to dipeptides and tripeptides once they are absorbed into cell

Answer 32

Most digested into AA and transported across basolateral membrane into circulation
Those that are undigested are transported across BL membrane on H+ dependant exchanger

Question 33

How are vitamins absorbed

Answer 33

Fat soluble vitamins like A,D,E and K are absorbed in the SI with fats
Water soluble vitamins B (except B12) and C are absorbed by mediated transport
B12 contains cobalt and is absorbed by a intestinal transporter found in the ileum and only is recognised when it is complexed with protein intrinsic factor

Question 34

How is dietary iron absorbed

Answer 34

Absorbed by apical transporter on the enterocyte
Fe2+ Actively absorbed by apical cotransport with H+ on a protein called DMT1
Inside cell enzyme converts iron to Fe2+ and both pools of ionised iron leave the cell on transported called ferroportin

Question 35

How is iron uptake regulated

Answer 35

Hepcidin; when body stores of iron are high, liver secretes hepcidin which binds to ferroportin and destroys the transporter decreasing iron uptake

Question 36

How does ion and water absorption occur

Answer 36

Absorption of nutrients moves solute from the lumen of the intestine to the ECF creating an osmotic gradient - water can follow
Enterocytes in SI and colonocytes absorb Na+ using apical Na+ channels, Na+-Cl- symporter, Na+-H+ exchanger and
basolateral primary transporter is Na+-K+-ATPase
Chloride uptake via apical Cl- HCO3- exchanger and basolateral Cl channel

Question 37

What happens in the large intestine

Answer 37

1.5 litres of unabsorbed chyme remain
Bacteria present in the colon break down any undigested complex carbs and protein through fermentation
End products include lactate and short chain fatty acids (which are absorbed by simple diffusion[colonocytes are preferred source of energy])