Week 4- Overview of gastrointestinal tract + Upper GI physiology Flashcards
what organs are included in the gastrointestinal tract?
- mouth
- oesophagus
- stomach
- small intestine
- large intestine
what glands are included in the gastrointestinal tract?
- salivary glands in the mouth
- pancreas
- liver
what is the lumen of the gut?
the part of the gut where the food goes down
the lumen is said to be …. to the body?why
external
as the conditions for digestion is tolerated in the gut but not the body
what are the conditions for digestion in the gut? why cant they occur in the body?
- pH of the stomach as low as 2.0 compared to body 6.8-8
- pancreas has digestive enzymes that could destroy out tissue
- microorganism in lower intestine
- food are foreign particles need to broken down so body doesn’t attack them
what are the basic processes in the digestive system?
- motility
- secretion
- digestion
- absorption
what are the two types of motility in the digestive system and what are the actions causing them?
voluntary- skeletal muscles due to chewing, swallowing and defaecation
involuntary- smooth muscle contraction that mix and move forward content in the gut
what is the main type of secretion in the DS?
-digestive juices secreted by exocrine glands into lumen of the gut
what is part of the digestive juices?
-contains water, electrolytes, and specifc prodcuts for digestion or absorption e.g. mucus, enzyme bile salts
what need to happen for secretion to occur?
secretion cells need to extract large volumes of water and raw material which uses a lot of energy AT for raw material and secretion is reabsorbed into blood
what secretes gastrointestinal hormones?
endocrine glands
what do gastrointestinal hormones regulate?
motility and exocrine glands secretion
what is digestion?
biochemical breakdown of complex proteins,
carbohydrates and fats by enzymes
what is maltose hydrolysed by and to make what?
broken down by maltase to make glucose
where does protein digestion start?
in the stomach
what is pepsinogen?
it is in gastric juice and is converted to pepsin at low pH
What is pepsin?
it breaks down proteins into peptides
what is trypsinogen?
in pancreatic juice and is converted to trypsin at basic pH
what is trypsin?
breaks down proteins to peptides
what is peptidase?
found in the epithelial cell that lines the small intestine breaks down peptides into amino acids that can be absorbed into the blood capillary
what form is a dietary fat in?
triglyceride
what do fats start to break down?
in the duodenum
what 2 things make lipids breakdown into smaller molecules?
bile salts from the liver emulsifies large globules of fat making them smaller and lipase from the pancreas digests triglycerides to monoglycerides and fatty acids
how are lipoproteins also know as chylomicrons made?
the monoglycerides and fatty acids diffuse into the epithelial cells where they combine with proteins
what are the 3 types of polysaccharides?
starch
glycogen
cellulose
what is starch broken down by?
salivary amylase and then pancreatic amylase to
maltose
what is glycogen broken down into?
glucose
Is cellulose and other indigestible CHO broken down by gut enzymes?
no
what are the 3 main enzymes to break down disaccharides?
maltase
sucrase
lactase
what does maltase break down?
maltose to glucose
what does sucrase break down?
sucrose to glucose and fructose
what does lactase break down?
lactose into glucose and galactose
how does glucose get into the blood?
it is actively transported into the epithelial cells of villi and then onto the blood
what is the mucosa?
it lines the lumen and the surface is highly folded for larger surface area for absorption
what are the different layer of the digestive tract wall?
mucosa
submucosa
Muscularis externa
Serosa
what helps regulate the digestive functions?
- Autonomic smooth muscle function
- Intrinsic nerve plexuses
- Extrinsic nerves
- Gastrointestinal hormones
how does the autnomic smooth muscle help regulate digestive function?
Interstitial cells of Cajal –
– pacemaker cells that instigate cyclic slow-wave activity
– located between circular and longitudinal muscle, allow whole sheets of
smooth muscle cells to contract together when threshold reached
-threshold is reached when enough food is present
what is the guts own nervous system called?
enteric nervous system
what does the Intrinsic nerve plexuses
primarily coordinate local activities
how does the sympathetic system affect digestion?
slows down digestion
how does the sympathetic and parasympathetic nerves influence motility and secretion?
by modifying activity of intrinsic nerve plexuses – altering secretion of GI hormones – directly acting on smooth muscle and glands
what is the effect of the parasympathetic nervous system on the digestive system?
(vagus nerve)
Increases smooth muscle motility
Promote GI hormone and enzyme secretion
what are the 3 main types of sensory receptors in the tract?
– Chemoreceptors
– Mechanoreceptors
– Osmoreceptors
what is the effect of stimulation of the sensory receptors?
results in neural reflexes or secretion of
hormones which alter activity of effector cells (smooth
muscle cells, exocrine and endocrine gland cells)
how do hormones enter the blood?
Endocrine gland cells in mucosa of specific regions of tract release hormones into blood upon stimulation
what is the tongue?
a skeletal muscle important for chewing swallowing and speech and theres approx 10,000 taste buds in the mouth and throat
how many receptors are on each taste bud?
50 which are long spindly, modified epithelial cells
what is salt stimulated by and signalled by?
- chemical salts especially NaCl
- through Na+ channels
what is sour stimulated by and signalled by?
- Acids with a free H+ ion
- hydrogen potassium channels
what is sweet stimulated by and signalled by?
-Configuration of glucose
-Activates GPCR, cAMP,
phosphorylation and
blockage of K+ channels
what is bitter stimulated by and signalled by?
- they are complex and detect chemically diverse substances
- 50-100 receptors each responding to different bitter flavours
what is umani (savoury) stimulated by and signalled by?
-Amino acids, especially
glutamate and MSG
-Glutamate binds to GPCR
where is saliva secreted from?
3 types of salivary glands
- parotid gland under the ear
- sublingual gland under the tongue
- submandibular gland near the jaw
how much saliva is secreted a day and what is its composition?
1/2L a day
water, electrolytes, and proteins (amylase, mucos, lysozyme)
what is the functions of saliva?
• Digestion of carbohydrates by salivary amylase
• Swallowing facilitated by moistening food
• Lubrication provided by mucus
• Antibacterial action – Lysozyme destroys bacteria – Saliva rinses away material that could serve as food
source for bacteria
• Solvent for molecules that stimulate taste buds
• Aids speech by facilitating movements of lips and
tongue
• Helps keep mouth and teeth clean
• Rich in bicarbonate buffers
how is the secretion of saliva controlled?
- its a simple reflex pressure receptors and chemoreceptors in the mouth
- sends signals to the salivary centre in the medulla in the brain
- this them switches on the autonomic nerves to switch salivary glands on to secrete saliva
where does swallowing occur?
in the pharynx
what is the oesophagus?
straight muscular tube extending between pharynx and stomach
what is the top and the bottom of the oesophagus caller?
- Pharyngoesophageal sphincter
* Gastroesophageal sphincter
what is the role of the Gastroesophageal sphincter?
-controls food going into the stomach and prevents acid reflux back into the oesophagus once it has been swallowed
how is food moved down the oesophagus?
- the swallowing centre in the brain controls the peristaltic wave that pushes food through the oesophaus
- mucus that is produced here keeps the bolus moist to keep it moving
what are the 3 main sections of the stomach?
- fundus mainly air
- body stores the food
- antrum mixing and where the enzymatic breakdown occurs
what is the 3 main functions of the stomach?
- store food in the body
- secrete acid and enzymes that begin protein digestion
- mixing of the food
what are the 4 aspects of gastric motility?
- filling of the stomach whereby receptive relaxation occurs allowing extra volume of food without a rise in stomach pressure which is mediated by the vagus nerve
- storage in the body of the stomach
- mixing in the antrum of stomach
- emptying is slow and controlled by many different factors
what are 5 factors that affect gastric emptying?
-Amount of chyme (broken down food) stomach influences strength of
contraction
-fat in the duodenum= as fat slows down absorption because small intestine is the only place fat digests-lipase
-amount of acid in the duodenum as it shoudl be neutral in there or it will damage the duodenal mucosa
-Osmolarity of the duodenum =as osmolarity rises, gastric emptying inhibited
-Distension of the duodenum = large volume of chyme inhibits
gastric emptying
what do the 5 factors that affect gastric emptying trigger to occur?
– Neural response through intrinsic nerve plexuses (short
reflex) and autonomic nerves (long reflex), collectively
called enterogastric reflex
– Hormonal response with release of hormones
(enterogastrones) from duodenal mucosa which inhibit
gastric emptying – Secretin (produced by S cells) – Cholecystokinin (CCK, produced by I cells)
what are some additional factors that influence gastric motility?
- emotions= sadness and fear tends to decrease motility snd anger and aggression tends to increase motility.
- intense pain= tends to inhibit motility
what is the 2 main distinct areas of gastric mucosa that secrete gastric juice?
– Oxyntic mucosa which lines body and fundus
– Pyloric gland area (PGA) which lines the antrum
what are the 3 types of cells that are found in the gastric pits?
– Mucous cells
• Line gastric pits and entrance of glands
• Secrete thin, watery mucus protects from acid
– Chief cells
• Secrete enzyme precursor, pepsinogen
– Parietal (oxyntic) cells
• Secrete HCl and intrinsic factor
what do enterochromaffin cells secrete?
hormones
what is the function of HCL?
-aids breakdown of connective tissue and muscle fibers
-denatures protein by uncoiling
-activates pepsinogen by cleaving it to become pepsin and
provides acid medium for
optimal pepsin activity
-Along with salivary
lysozyme, kills most of the
microorganisms ingested
with food
what is purpose of the gastric mucosal barrier?
Enables stomach to contain acid without
injuring itself and producing ulcers
what are some components of gastric mucosal barrier that protect it from acid?
- the membranes of the gastric mucosal cells are impermeable to H+ so that HCL from penetrate into the cells
- cells are joined by tight junctions that prvent HCL penerating in
- a mucosa coating over the gastric mucosa serves as a physical barrier to acid peneration
- the HCO-3 rich mucus also serves as a chemical barrier that netrilises acid in the vicinity if the mucosa.even when luminal pH is 2, the mucus ph is 7
what are the other components of the gastric juice?
-pepsinogen stored in the zymogen granules of the chief cells, HCl in stomach converts it to pepsin, which is autocatalytic, Acts optimally in acid, breaking specific peptide bonds
-mucus prtects mucosa against mechanical injury, protects stomach wall from digestion by pepsin, alkaline neutrilising HCL in vicinity of gastric lining
-Intrinsic factor secreted by parietal cells, Essential for vit B12 absorption, allows binding to receptor in terminal
ileum and absorption by receptor-mediated endocytosis
what helps regulate gastric secretion?
hormones that are released by the gastric endocrine glands
– Gastrin, released by protein presence and Ach
from G cells, stimulates HCl and pepsinogen
– Somatostatin, released from D cells inhibits HCl,
pepsinogen and gastrin
what are he 3 phases of gastric secretion?
- Cephalic phase
- Gastric phase
- Intestinal phase
what is the cephalic phase?
-– Increased secretion of HCl and pepsinogen that occurs in response
to stimuli acting in the head before food reaches stomach
what is gastric phase?
– Begins when food actually reaches the stomach
– Presence of protein increases gastric secretions – Presence of caffeine or alcohol
what is the intestinal phase?
– Inhibitory phase – acid, fat, hypertonicity, distension
– Helps shut off flow of gastric juices as chyme begins to empty into
small intestine
what is digestion in the stomach?
– Carbohydrate digestion from salivary amylase continues in
body of stomach as little mixing here
– Protein digestion by pepsin begins in antrum as mixed with
digestive juices in this region
what is absorption in the stomach?
– Alcohol – lipid-soluble, diffuses slowly through mucosa into
blood
– Aspirin and other weak acids – un-ionised, lipid-soluble so
absorbed quickly
