GIT structure & function Flashcards
function of digestive system
also known as the gastrointestinal system
function : mechancial (teeth) and chemical (enzymatic) breakdown of food
–> break up food into simple, absorbable units
anatomy of digestive system
divided into 2 major components:
1) The gastrointestinal tract (GIT)
2) Accessory glands and organs
The gastrointestinal tract (GIT)
- 4.5 m tube running through out body with openings at
either end. - Consists of multiple organs joined in series
Accessory glands and organs
- Located outside the GIT. Secrete substances into the
lumen to aid digestion
The 4 layers of the GIT wall
- 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
serosa role
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
submucosal plexus
part of the submucosa layer of the GIT wall
–> control secretion (because its close to the epithelial)
myenteric plexus role
in the muscularis externa
role: control muscle contraction
process of the GIT
- secretion
movement of material from cells into lumen or ECF - digestion
chemical and mechanical breakdown of food into absorbale units - absorption
movement of material from GI lumen to ECF - motility
movement of material through the GI tract as a result of muscle contraction
–> ensures digestion occurs in one direction
secretion in GIT
epithelial cells can secrete material into ECF, affect neighbouring cells or enter the blood
digestion in GIT
digesting food (chemical and mechanical) and breaking it into absorbale units in the lumen of the digestive tract
FOOD –> food + food + food
absorption in GIT
absorbable units from digestion in the lumen of the digestive tract is moved across the GIT wall, the interstitial fluid and into the blood
motility in GIT
muscle contractions so absorbale units from digestion can move through GIT tract
–> ensures digestion occurs in one direction
Oral digestion including salivation
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.
Phases of digestion ; cephalic
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
saliva contentes
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
salivary glands
o Sublingual (under tongue)
o Parotid (near hinge of jaw)
o Submandibular (under jawbone)
=> produces saliva
Deglutition meaning
= 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
deglutition process
- food chewed into small pieces by teeth + mixed with saliva
- tongue rolls food into bolus and otngue rises to push bolus towards pharynx
- soft palate moves up to prevent bolus entering nasal cavity
- epiglottis also lowers to cover opening ot trachea so bolus does not go down trachea
- bolus enters esophagus
peristaltic contraction in deglutition
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
when bolus arrives at stomach
- 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 - 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
stomach structure
- cardia
- fundus
- body/corpus
- antrum
- pylorus
parietal cells
produces gastric acid (HCl)
–> HCl activates pepsin, kills bacteria, denatures proteins for pepsin to act on
stimulated by ACh, gastrin, histamine
functions of gastric acid
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
production of gastric acid in the stomach
- carbonic anhydrase (CA) catalyses the hydration of CO2 by combining CO2 + H20 to form HCO3-
- CA forms HCO3- + H+ (bicarbonate and a proton)
- H+ pumped out by proton pump and move H+ into lumen by H+/K+ ATPase and K+ enters the parietal cells
- HCO3- transported out of cell into interstitial space and CL- enters the cell via the Cl-/HCO3- exchanger
- Cl- and K+ are transported out of the cell via channels into the lumen of stomach
- Cl- and H+ combine to form HCl (gastric acid)
Phases of digestion ; gastric
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
Control of gastric acid secretion
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
ECL cells
Produces histamine
Histamine can stimulate gastric acid production
Stimulated by ACh and gastrin
Chief cells
Releases pepsin(ogen) = digests proteins and gastric lipase =digests fats
Stimulated by ACh and acid secretion
D cells
Produces somatostatin
Which inhibits gastric acid secretion
Stimulated by acid in the stomach
G cells
Produces gastrin
Stimulates gastric acid production
Stimulated by ACh, peptides and amino acids (both from our diet)
Molecules that control gastric acid secretion and their receptors
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
pepsinogen
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
exo and endo proteases
pepsin is an endoprotease that hydrolyses internal peptide bonds
breaks down proteins into peptides
mechanical digestion
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
stomach protection
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
