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MEDS2001 > GIT structure & function > Flashcards

GIT structure & function Flashcards

1
Q

function of digestive system

A

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

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2
Q

anatomy of digestive system

A

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

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3
Q

The gastrointestinal tract (GIT)

A
  • 4.5 m tube running through out body with openings at
    either end.
  • Consists of multiple organs joined in series
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4
Q

Accessory glands and organs

A
  • Located outside the GIT. Secrete substances into the
    lumen to aid digestion
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5
Q

The 4 layers of the GIT wall

A
  • 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
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6
Q

serosa role

A

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

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7
Q

submucosal plexus

A

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

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8
Q

myenteric plexus role

A

in the muscularis externa
role: control muscle contraction

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9
Q

process of the GIT

A
  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
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10
Q

secretion in GIT

A

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

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11
Q

digestion in GIT

A

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

FOOD –> food + food + food

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12
Q

absorption in GIT

A

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

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13
Q

motility in GIT

A

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

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14
Q

Oral digestion including salivation

A

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.

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15
Q

Phases of digestion ; cephalic

A

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

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16
Q

saliva contentes

A

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

17
Q

salivary glands

A

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

18
Q

Deglutition meaning

A

= 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
19
Q

deglutition process

A
  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
20
Q

peristaltic contraction in deglutition

A

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

21
Q

when bolus arrives at stomach

A
  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

22
Q

stomach structure

A
  1. cardia
  2. fundus
  3. body/corpus
  4. antrum
  5. pylorus
23
Q

parietal cells

A

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

24
Q

functions of gastric acid

A

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

25
Q

production of gastric acid in the stomach

A
  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)
26
Q

Phases of digestion ; gastric

A

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

27
Q

Control of gastric acid secretion

A

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

28
Q

ECL cells

A

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

29
Q

Chief cells

A

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

30
Q

D cells

A

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

31
Q

G cells

A

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

32
Q

Molecules that control gastric acid secretion and their receptors

A

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

33
Q

pepsinogen

A

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

34
Q

exo and endo proteases

A

pepsin is an endoprotease that hydrolyses internal peptide bonds

breaks down proteins into peptides

35
Q

mechanical digestion

A

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
36
Q

stomach protection

A

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

MEDS2001 (26 decks)

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