Functions Of The Components Of The GI System Flashcards
Where is saliva produced?
The salivary glands
What are the names of the salivary glands?
- Sublingual
- Submandibular
- Parotid
- Buccal
What stimulates the production of saliva?
Smell, thought, and presence of food
What does saliva contain?
Mainly water, but also small quantities of;
- Amylase
- Mucus
- Lysozyme
What is the function of salivary amylase?
Converts polysaccharides to maltose
What is the function of salivary mucus?
To allow lubrication
What is the function of salivary lysozyme?
It lyses certain bacteria
What is the first stage in breakdown of food?
Chewing
What is the purpose of the chewing of food?
It allows easier passage into the absorption
Does absorption take place in the mouth?
Some drugs can be absorbed through the buccal mucosa, but no nutrient absorption takes place
Give an example of a drug that can be absorbed through the buccal mucosa?
Midazolam
What happens when a food bolus is moved to the back of the pharynx?
The swallowing centre in the cerebral medulla is activated
What is the result of the activation of the swallowing centre?
Closure of the vocal cords and epiglottis
Why is the closure of the vocal cords and epiglottis important in swallowing?
Faciltates the safe pasage of the bolus into the oesophagus
How is reflux of food particles back into the mouth prevented?
Sphincters at the lower and upper end of the oesophagus
What is the ‘normal’ state of the upper oesophageal sphincter?
Tonically contracted
What happens when the upper oesophageal sphincter needs to let food through?
It relaxes
What controls the upper oesophageal sphincter?
Cranial nerves V, IX, and X
What stimulates peristalsis within the oesophagus?
Swallowing a food bolus
What is the role of the inhibitory neurones of the oesophagus in swallowing?
- Induce lower oesophageal sphincter relaxation
- Co-ordinate proximal-to-distal peristaltic contraction
What condition is caused by a failure of the lower oesophageal sphincter to relax?
Achalasia
How is achalasia treated?
Usually needs surgery to divide the muscles at the lower oesophageal sphincter to allow passage of food into the stomach
Can achalasia occur in children?
It is rare, but can still occur
Does any digestion or absorption of food take place in the oesophagus?
No
What are the functions of the stomach?
- Storage of food
- Secretion of digestive juices
- Mixing up the contents of the stomach into chyme
What unique property does the smooth muscle of the stomach wall have?
‘Plasticity’
What does the property of ‘plasticity’ allow?
In fully grown children and young adults, the stomach can hold 50-1000ml whilst the pressure or tone exerted on the contents remains the same
What gives the stomach the property of ‘plasticity’?
The folds of the stomach mucosa, or rugae, stretch out as the stomach becomes fuller
What happens once the stomach becomes fully distended?
The person will experience discomfort, and usually this will prompt them to stop eating
What produces contraction of the stomach?
The cells in the fundus
How do the cells in the fundus produce contraction of the stomach?
They generate slow-wave potentials that move down the length of the stomach at the pylorus
At what rate do the cells in the fundus of the stomach generate slow-wave potentials?
3/minute
Describe the pattern of contractions produced by potentials from the fundus of the stomach?
Rhythmic
What muscles do not contract as a result of the potentials produced by the fundus of the stomach?
Circular muscles
What happens to the strength of contractions as they travel from fundus to antrum?
Increase in strength
Why do contractions of the stomach increase in strength from the fundus to the antrum?
The muscle layers in the fundus of the stomach are significantly thinner than the antrum
What are antral peristaltic contractions responsible for?
Mixing of food
What happens to stomach contents as the peristaltic wave meets the pylorus?
The pylorus opens enough to allow very watery chyme through
What happens to thicker chyme when it reaches the pylorus?
It is not able to pass through, and so it is propelled backwards against the pylorus and back into the antrum for further mixing
What volume of gastric secretions are produced by a well-hydrated typical child?
10-20mk/kg per day
What does gastric fluid contain?
Water and a combination of secretions
Where do the secretions forming gastric fluid arise from?
The gastric pits
What cells are responsible for producing secretions for gastric fluid?
- Mucous neck cells
- Chief cells
- Parietal cells
- G cells
- Enterochromaffin-like cells
- D cells
What do mucous neck cells secrete?
- Bicarbonate
- Mucus
What is the purpose of the secretion of mucus by the stomach?
Provides a protective barrier against damage from mechanical trauma from peristalsis, auto digestion, and acid
What do chief cells produce?
Pepsiogen
What is pepsinogen?
A precursor of pepsin
What is the function of pepsin?
Protein digestion to amino acids
What do parietal cells produce?
- Hydrochloric acid
- Intrinsic factor
What is the function of hydrochloric acid in the stomach?
- Activates pepsinogen to it’s active form, pepsin
- Helps break down connective tissue fibres within food contents
- Destroys microorganisms within food
What is the function of intrinsic factor in the stomach?
Important for absorption of vitamin B12
Where is vitamin B12 absorbed?
Terminal ileum
Where are G cells found?
In the pyloric area
What do G cells secrete?
Gastrin
What is gastrin?
A hormone
What is the role of gastrin?
Integral in stimulation of parental and chief cells to produce their enzymes
What do enterochromaffin-like cells produce?
Histamine
What is the role of histamine in the stomach?
It stimulates gastric acid secretions by the parietal cells
What do D cells produce?
Somatostatin
What is the function of somatostatin?
Inhibition of acid secretion
What is the gastric pH at birth?
Neutral range (6-8)
Why is the gastric pH in the neutral range at birth?
Due to the presence of alkaline amniotic fluids within the stomach
What happens to the gastric pH within a day of birth?
It falls to 1-3
Is gastric acidity well maintained in neonates?
No
What happens to gastric pH following a milk feed in neonates?
It is increased towards neutral
When does the production of stomach acidity reach adult capacity?
Age 3 years
Is the stomach involved in absorption?
Relatively little, but does absorb a few products of digestion
What produces of digestion can be absorbed by the stomach?
- Glucose
- Simple sugars
- Amino acids
- Some fat-soluble substances
Give a fat soluble substance that notably can be absorbed by the stomach
Ethanol
What determines the absorption of some drugs in the stomach?
The gastric pH
Give an example of a drug that gastric pH affects the absorption of
Aspirin
How does gastric pH affect the absorption of aspirin?
At low pH, aspirin is absorbed almost as rapidly as water, but as the pH of the stomach rises, aspirin is absorbed more slowly
What happens to water in the stomach?
It moves freely from gastric contents across gastric mucosa into the blood
What is the net contribution of the stomach to the absorption of water?
Small
Why is the net contribution of the stomach to water absorption small?
Because water moves just as easily from the blood across the gastric mucosa to the lumen of the stomach
When is the absorption of water and alcohol from the stomach be slowed?
If the stomach contains foodstuffs and especially fats
Why is the absorption of water and alcohol from slowed if the stomach contains foodstuffs, especially fats?
Because gastric emptying is delayed by fats, and most water in any situation is absorbed from the small intestine
What does the rate of emptying of the stomach depend on?
Physical and chemical composition of the meal
Do fluids or solids empty the stomach more quickly?
Fluids
Do carbohydrates or protein empty the stomach more rapidly?
Carbohydrates
