GIT structure & function

Question 1

function of digestive system

Answer 1

also known as the gastrointestinal system
function : mechancial (teeth) and chemical (enzymatic) breakdown of food
–> break up food into simple, absorbable units

Question 2

anatomy of digestive system

Answer 2

divided into 2 major components:
1) The gastrointestinal tract (GIT)
2) Accessory glands and organs

Question 3

The gastrointestinal tract (GIT)

Answer 3

• 4.5 m tube running through out body with openings at
  either end.
• Consists of multiple organs joined in series

Question 4

Accessory glands and organs

Answer 4

• Located outside the GIT. Secrete substances into the
  lumen to aid digestion

Question 5

The 4 layers of the GIT wall

Answer 5

• Mucosa- inner lining. Role in absorption
  and secretion
• Submucosa- middle layer with connective
  tissue with blood and lymph vessels
• Muscularis externa- outer wall
  comprised of 2 (or 3 in stomach) layers
  of smooth muscle: 1) circular and
  2) longitudinal
  –> plus 3) oblique in stomach
• Serosa- a connective tissue

Question 6

serosa role

Answer 6

a layer of the GIT wall
role : structure/support
–> ensure different regions of GIT are anchored in place to one another and also other regions of the body

Question 7

submucosal plexus

Answer 7

part of the submucosa layer of the GIT wall
–> control secretion (because its close to the epithelial)

Question 8

myenteric plexus role

Answer 8

in the muscularis externa
role: control muscle contraction

Question 9

process of the GIT

Answer 9

1. secretion
   movement of material from cells into lumen or ECF
2. digestion
   chemical and mechanical breakdown of food into absorbale units
3. absorption
   movement of material from GI lumen to ECF
4. motility
   movement of material through the GI tract as a result of muscle contraction
   –> ensures digestion occurs in one direction

Question 10

secretion in GIT

Answer 10

epithelial cells can secrete material into ECF, affect neighbouring cells or enter the blood

Question 11

digestion in GIT

Answer 11

digesting food (chemical and mechanical) and breaking it into absorbale units in the lumen of the digestive tract

FOOD –> food + food + food

Question 12

absorption in GIT

Answer 12

absorbable units from digestion in the lumen of the digestive tract is moved across the GIT wall, the interstitial fluid and into the blood

Question 13

motility in GIT

Answer 13

muscle contractions so absorbale units from digestion can move through GIT tract
–> ensures digestion occurs in one direction

Question 14

Oral digestion including salivation

Answer 14

Mastication (chewing) - mechanical digestion by lips, tongue,
teeth to generate a bolus that can be swallowednical digestion by lips, tongue, teeth to generate a bolus that can be swallowed

Salivation- soften and moisten food, taste, chemical digestion of carbohydrates, defense.

Question 15

Phases of digestion ; cephalic

Answer 15

Three phases- cephalic, gastric and intestinal phases
Cephalic phase: Thinking, smelling, seeing, tasting food can stimulate parietal cells to produce gastric acid
o Prepares the stomach for the arrival of food
Vagus nerve innervates stomach wall

Question 16

saliva contentes

Answer 16

contains water, ions, mucus, enzymes (amylase, lipase,
lysozyme) and proteins eg. immunoglobulins

amylase : breaks down starch/carbs => carbohydrate digestion occurs in the mouth
lysosome : role of defense => affects the intergrity of bacterial wall
immunoglobulins : also part of defense in mouth

Question 17

salivary glands

Answer 17

o Sublingual (under tongue)
o Parotid (near hinge of jaw)
o Submandibular (under jawbone)
=> produces saliva

Question 18

Deglutition meaning

Answer 18

= swallowing
* Movement of bolus or liquid from mouth to esophagus.

• Sequence of peristaltic contraction and relaxation (circular and longitudinal muscles) pushes bolus down esophagus to the stomach

Question 19

deglutition process

Answer 19

1. food chewed into small pieces by teeth + mixed with saliva
2. tongue rolls food into bolus and otngue rises to push bolus towards pharynx
3. soft palate moves up to prevent bolus entering nasal cavity
4. epiglottis also lowers to cover opening ot trachea so bolus does not go down trachea
5. bolus enters esophagus

Question 20

peristaltic contraction in deglutition

Answer 20

alternating contractions + relaxations = produce wave-like movements called peristalsis
* circular muscles = wraps around the tube => contracts to decrease diameter of tube
* longtitudinal = runs alongside tube => contracts to decrease length of tube muscles

during contraction :
circular muscles contract
longtitudinal muscles relax

during relaxation :
circular muscles relax
longtitudinal muscles contract

bolus moves towards the stomach

Question 21

when bolus arrives at stomach

Answer 21

1. bolus first arrives at lower esophageal spincter
   == spincter has very hihg muscle tenstion
   –> lots of contracted circular muscle to prevent acid from coming up esophagus
2. spincter tension relaxes when food swallowed allowing bolus to enter stomach

3 processes:
* Chemical and mechanical break down of food into chyme
* Regulates passage of chyme to small intestine
–> chyme can be stored so small intestine has enough time to digest and absorb the chyme in it
* Very acidic environment- destroy bacteria and other
pathogens.
–> stomach acid

Question 22

stomach structure

Answer 22

1. cardia
2. fundus
3. body/corpus
4. antrum
5. pylorus

Question 23

parietal cells

Answer 23

produces gastric acid (HCl)
–> HCl activates pepsin, kills bacteria, denatures proteins for pepsin to act on
stimulated by ACh, gastrin, histamine

Question 24

functions of gastric acid

Answer 24

o kills bacteria and other pathogens
o Inhibits salivary amylase
o Denatures proteins (allow 3D structure to unfold and allow better access for protein to act on it and digest it)
o Promotes the release and activation of the enzyme pepsin

Question 25

production of gastric acid in the stomach

Answer 25

1. carbonic anhydrase (CA) catalyses the hydration of CO2 by combining CO2 + H20 to form HCO3- 2. CA forms HCO3- + H+ (bicarbonate and a proton) 3. H+ pumped out by proton pump and move H+ into lumen by H+/K+ ATPase and K+ enters the parietal cells 4. HCO3- transported out of cell into interstitial space and CL- enters the cell via the Cl-/HCO3- exchanger 5. Cl- and K+ are transported out of the cell via channels into the lumen of stomach 6. Cl- and H+ combine to form HCl (gastric acid)

Question 26

Phases of digestion ; gastric

Answer 26

Bolus arrives in stomach and the stomach stretch’s : stimulated by distension (stretch receptors) and chemoreceptors (presence of peptides, pH) Stretch receptors promote gastric acid production chemoreceptor sense presence of peptides = (can affect pH) secretion oceu0 70-12 Acidify and homogenise bolus into chyme

Question 27

Control of gastric acid secretion

Answer 27

Acetylcholine, gastrin and histamine can stimulate parietal cells : ACh released by the nervous system/ vagus nerve can stimulate parietal cells to secrete gastric acid G cells : produce gastrin to stimulate gastric acid production by binding on to receptors on parietal cells Gastrin can also bind to the receptors on ECL cells to promote histamine to further promote gastric acid production ECL cells produce histamine which can also act on parietal cellsto promote gastric acid production D cells : negative regulators Too much acid in stomach will cause d cells to release somatostatin to inhibit HCl production -> acts on G and parietal cells

Question 28

ECL cells

Answer 28

Produces histamine Histamine can stimulate gastric acid production Stimulated by ACh and gastrin

Question 29

Chief cells

Answer 29

Releases pepsin(ogen) = digests proteins and gastric lipase =digests fats Stimulated by ACh and acid secretion

Question 30

D cells

Answer 30

Produces somatostatin Which inhibits gastric acid secretion Stimulated by acid in the stomach

Question 31

G cells

Answer 31

Produces gastrin Stimulates gastric acid production Stimulated by ACh, peptides and amino acids (both from our diet)

Question 32

Molecules that control gastric acid secretion and their receptors

Answer 32

ACh = stimulated by nerves = M3R (muscarinic M3 receptor) Gastrin = G cells = CCK2R (cholecystokinin) Histamine = ECL cells = H2R (histamine H2 receptor) Somatostatin = D cells = SSTR2 (somatostatin receptor 2) — decreases acid production

Question 33

pepsinogen

Answer 33

pepsinogen is a zymogen (inactive enzyme/ precursor to pepsin) --> stored as the inactive fom in vesicles of the chief cells gets activated and turns into pepsin when the pH is low

Question 34

exo and endo proteases

Answer 34

pepsin is an endoprotease that hydrolyses internal peptide bonds breaks down proteins into peptides

Question 35

mechanical digestion

Answer 35

once bolus arrives activates stretch receptors --> promote churning of bolus in stomach for mechanical digestion * Stomach has three layers of muscle + (oblique) * Churning action of the stomach- mixes food with acid and digestive enzymes * Primarily under neural control, but also stimulated by distension

Question 36

stomach protection

Answer 36

2 types of mucus cells * mucous surface cells : mucus * mucous neck cells : bicarbonate mucus (consists of proteins and sugars) provides a physical barrier between lumen and epithelium bicarbonate buffers gastric acid to prevent damage to epithelium and provides chemical buffer pepsinogen is only activated in low pH so bicarbonate increases pH, and inactivating it