E1 GIT physiology Flashcards
1
Q
2 major divisions of the GI system
A
GIT
accessory glands
2
Q
simply describe the GIT
A
several organs joined in series to form a passageway for conduction of food and digestion products
3
Q
major organs of GIT
A
mouth
pharynx
oesophagus
stomach
small intestine
large intestine
rectum
4
Q
simply describe accessory glands of GI system
A
secrete various fluids and enzymes into the lumen of the tract for the digestive process
5
Q
state 3 accessory glands in the GI system
A
liver
gallbladder
pancreas
6
Q
4 main functions of the GI system
A
ingestion and propulsion
digestion
absorption
elimination
7
Q
describe the function of the GI system: ingestion and propulsion
A
- food intake via eating and drinking
- mixes and moves the content along the alimentary tract
8
Q
describe the function of the GI system: digestion
A
- mechanical breakdown of food (mastication) and chemical digestion into small molecules by digestive enzymes
9
Q
describe the function of the GI system: absorption
A
- digested food products are transported in the bloodstream
- fluids containing enzymes and other substances are transported into the lumen of the GIT via a process called secretion
10
Q
describe the function of the GI system: elimination
A
- excretion of food substances not digested and / or absorbed
11
Q
functions of the oral cavity in the GIT
A
- sensory analysis of food material before swallowing
- mechanical processing via the action of the teeth, tongue and palatal surfaces
- lubrication by mixing food material with mucus and salivary gland secretion
- limited digestion of carbohydrates and lipids
12
Q
functions of the tongue in the GIT
A
- mechanical processing by compression, abrasion and distortion
- manipulation to assist in chewing and prepare material for swallowing
- sensory analysis by touch, temperature and taste receptors
- secretion of muffins and lingual lipase
13
Q
functions of the teeth in the GIT
A
- mechanical breakdown of food materials
- this process also saturates the salivary secretions and enzymes within the food material for better digestion
14
Q
functions of the pharynx in the GIT
A
- passageway of food material to the oesophagus
- respiratory function for air movement into the lung
- during swallowing, closure of the nasopharynx and larynx occurs to maintain proper direction of food
15
Q
what are the 3 pairs of salivary glands in the GIT?
A
- parotid
- sublingual
- submandibular
16
Q
describe parotid salivary glands
A
produce serous secretions containing a large amount of salivary amylase, which breaks down carbohydrate complexes
17
Q
describe sublingual salivary glands
A
produce a mucous secretion that serves as both a buffer and lubricant
18
Q
describe submandibular salivary glands
A
secrete a mixture of buffers, glycoproteins called muffins and salivary amylase
19
Q
how much saliva is made and secreted each day?
A
1-1.5 litres
20
Q
function of saliva
A
- lubricates mouth to prevent friction between the mucosa of the oral cavity and food
- moistens the food for easy swallowing
- initiates lipid and carbohydrate complex digestion
- promotes oral hygiene (contains bicarbonate which neutralises acids in food to reduce dental cavities)
21
Q
what control is saliva produced under?
A
parasympathetic
22
Q
describe production of primary saliva
A
acinar cells secrete amylase and electrolytes with similar tonicity to plasma to produce primary saliva
23
Q
describe production of secondary saliva from primary saliva
A
- duct cells modify secretion
- actively reabsorb sodium and chloride ions
- secrete potassium ions and hydrogen carbonate ions
- this forms secondary saliva
24
Q
is secondary saliva hypertonic, hypotonic or isotonic? explain why
A
- hypotonic (lower concentration of electrolytes than cell it comes from)
- sodium and chloride ion reabsorption is greater then potassium and hydrogen carbonate ion secretion
- also, the ducts are relatively impermeable to water
25
state some stimuli for saliva secretion
smell
taste
touch
mastication
nausea etc.
26
what nervous system is salivary secretion controlled by?
autonomic
parasympathetic and sympathetic
27
sympathetic innervation can control salivary secretion via what receptors?
alpha and beta adrenoceptors
28
parasympathetic innervation can control salivary secretion via what receptors?
M3 muscarinic receptors
29
state the effects of sympathetic innervation to control salivary secretion
- decreased production of saliva by acinar cells
- increased protein secretion
- decreased blood flow to the salivary glands
30
state the effects of parasympathetic innervation to control salivary secretion
- increased secretion of saliva by acinar cells
- increased hydrogen carbonate ion secretion
- increased blood flow to the salivary glands
- contraction of myoepithelium to increase the rate of expulsion of saliva
31
describe how increased parasympathetic stimulation changes quantity and composition of saliva secretion
- increased flow of saliva
- waterier in composition
32
describe structure of oesophagus
- fibromuscular tube approx 25cm in length
- upper third = mainly skeletal muscle
- middle third = mix of skeletal and smooth muscle
- lower third = mainly smooth muscle
33
function of oesophagus
transports food from the pharynx to the stomach
34
function of upper oesophageal sphincter
- controls the movement of food from the pharynx into the oesophagus
- rhythmic waves of peristalsis, which begin in the upper oesophagus, propel the bolus of food towards the stomach
35
function of the lower oesophageal sphincter
- relaxes to prevent stomach acids from backing up into the oesophagus
36
how does lower oesophageal sphincter work? what can happen if it doesn't work properly?
- muscular diaphragm surrounds sphincter which helps close off the sphincter when no food is being swallowed
- when the lower oesophageal sphincter doesn't fully close, the stomach contents can reflux, causing heartburn or GORD
37
what can reflux of stomach acid / contents cause?
heartburn
GORD
38
4 major anatomical regions of the stomach
fundus
body
antrum
pylorus
39
describe the fundus region of the stomach
- upper portion
- extends above the lower oesophageal sphincter
40
describe the body region of the stomach
- middle region
- accounts for the bulk of the stomach's volume
41
describe the antrum region of the stomach
- lower region
- narrower and smaller in volume
42
describe the pylorus region of the stomach
- narrowing where the stomach joins the small intestine
43
which 2 anatomical regions of the stomach contain gastric pits?
- fundus and body
- these are populated by cells that secrete the products found in gastric juice
44
2 regions of the stomach based on differences in motility
the orad region
the caudad region
45
describe the orad region of the stomach
- proximal
- contains the fundus and the proximal portion of the body
- thin walled and easily expands to accommodate increase in volume following a meal (50ml -> 1L when we eat to accommodate large volume)
46
describe the caudad region of the stomach
- distal
- contains the distal portion of the body and the antrum
- thick walled to generate much stronger contractions for gastric mixing and emptying (causes homogenous, thorough mixing of stomach contents to form chyme)
47
describe gastric motility and stomach movement when it comes to storage
- presence of food stretch the stomach and induce a vago-vagal reflex from the stomach to the brain stem and back to the stomach wall
- result in receptive relaxation to accommodate as much as 1.5L of food
48
describe gastric motility and stomach movement when it comes to mixing and digestion
- food is churned with gastric juices to form a thick, milky material (chyme) which is ejected about every 20 seconds into the duodenum
- the peristaltic waves (propulsion) pushes the chyme toward the pyloric sphincter
- because the pyloric sphincter remains closed most of the time, the chyme is forced to move backward (retropulsion)
49
what is the rate of peristaltic waves?
about 3-5 minutes
50
describe what is happening in this image
- propulsion and retropulsion are the 2 main methods of stomach contractile movements
- methods mix chyme with all the gastric juices secreted in the stomach and helps with mechanical digestion of stomach contents
51
what does a higher volume of food in the stomach lead to?
quicker gastric emptying into the small intestine
52
how must gastric emptying be regulated? what happens if it doesn't get regulated?
- must happen gradually
- if not, quick gastric emptying can cause bloating and other discomforts
53
describe the effects of increased gastric food volume
- increased stretch in the stomach wall
- increased pyloric pump activity
54
describe the release and effects of gastrin
- released by the astral mucosa induces secretion of acidic gastric juices
- enhances the activity of pyloric pump
55
gastric factors of regulating gastric emptying
- gastric food volume
- gastrin
56
duodenal factors of regulating gastric emptying
inhibitory enterogastric nervous reflexes
hormonal control
57
what are inhibitory enterogastric nervous reflexes mediated by?
- enteric nervous system, directly from duodenum to stomach
- extrinsic nerves to sympathetic ganglia
- vagus nerves to the brain stem
58
state some factors able to initiate inhibitory enterogastric nervous reflexes
duodenal irritation
distension
acidity
osmolality of the chyme
protein content of the chyme
59
describe the duodenal factor towards gastric emptying regulation of hormonal control
fat or acidity of chyme induce the release of CCK (cholecystokinin), secretin and GIP (glucose-dependent insulinotropic peptide)
60
what kinds of cells are contained within gastric pits?
- surface mucous cells
- mucous neck cells
- chief cells
- parietal cells
- endocrine cells (eg. G cells)
61
describe the surface mucous cells in gastric pits
secrete mucus in an alkaline fluid (containing bicarbonate)
62
describe the mucous neck cells in gastric pits
secrete mucus
63
describe the chief cells in gastric pits
secrete pepsinogen (precursor for pepsin)
64
describe the parietal cells in gastric pits
secrete HCl and intrinsic factor (necessary for the absorption of vitamin B12)
65
describe the endocrine cells in gastric pits
eg. G cells
secrete the hormone gastrin directly into the bloodstream (important role in secreting gastric acid)
66
describe the pH of the stomach and why it is like this
- pH of stomach contents can be as low as 2
- acidity necessary for converting pepsinogen into its active form, pepsin
- acidity also helps with protecting the body against certain infections
67
how is gastric acid generated?
- inside parietal cells
- by the enzyme: carbonic anhydrase (CA)
- bicarbonate is then pumped out and hydrogen ion reacts with chloride ion to form hydrochloric acid for stomach
- hydrogen and chloride ions are transported into the lumen while bicarbonate is transported into the interstitial fluid
68
describe the function of the proton pump is gastric acid generation
- uses ATP and transports hydrogen ions out of the parietal cell into the lumen of the stomach
- these are in exchange for potassium ions which move from the stomach lumen into the cell
69
what do PPIs often target regarding gastric acid?
ATPase to block these pathways in the parietal cells
70
parietal cell acid output is directly or indirectly controlled by:
1. acetylcholine
2. histamine
3. gastrin
4. prostaglandins E2 and I2
5. somatostatin
71
what is acetylcholine?
a stimulatory neurotransmitter
72
what is histamine?
a stimulatory local hormone
73
what is gastrin?
a stimulatory peptide hormone
74
what are prostaglandins E2 and I2?
inhibitory local hormones
75
what is somatostatin?
inhibitory peptide hormone
76
2 types of ulcer
gastric
duodenal
77
what are the phases of gastric acid secretion?
cephalic phase
gastric phase
intestinal phase
78
describe the cephalic phase of gastric acid secretion
1. the taste, smell or thought of food or tactile sensations of food in the mouth stimulate the medulla oblongata (green arrows)
2. vagus nerves carry parasympathetic action potentials to the stomach (pink arrow) where enteric plexus neurons are activated
3. postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells
4. gastrin is carried through the circulation back to the stomach (purple arrow) where it stimulates secretion, along the histamine
79
describe the gastric phase of gastric acid secretion
1. distension of the stomach stimulates mechanoreceptors (stretch receptors) and activates a parasympathetic reflex (action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata (green arrow))
2. medulla oblongaata increases APs in the vagus nerves that stimulate secretions by parietal and chief cells and stimulate gastric and histamine secretion by endocrine cells (pink arrow)
3. distension of the stomach also activates local reflexes that increase stomach secretions (orange arrow)
4. gastrin is carried through the circulation back to the stomach (purple arrow) where it stimulates secretion, along with histamine
80
describe the intestinal phase of gastric acid secretion
1. chyme in the duodenum with a pH less than 2 or containing fat digestion products (lipids) inhibits gastric secretions by 3 mechanisms (steps 2-4)
2. chemoreceptors in the duodenum are stimulated by low pH or lipids; APs generated by the chemoreceptors are carried by the vagus nerves to the medulla oblongata (green arrow) where they inhibit parasympathetic action potentials (pink arrow) thereby decreasing gastric secretions
3. local reflexes activated by protons or lipids also inhibit gastric secretion (orange arrows)
4. secretin and cholecystokinin produced by the duodenum (brown arrows) decrease gastric secretions in the stomach
81
what does the stomach frequently contain that may cause harm to gastric mucosa?
HCl
proteolytic enzymes
82
state 4 potential mechanisms for how the stomach protects itself from self-digestion
1. luminal membranes of gastric mucosal cells are impermeable to protons; cells are interconnected with tight junctions, therefore, no proton movement between cells
2. negative feedback: pH < 2 inhibits G cells directly and via somatostatin release from D cells
3. mucous released form surface mucous cells
4. rapid repair of damaged mucosa (cells lining stomach turnover in 3 days)
83
main functions of small intestine
- mechanical and chemical digestion of nutrients
- absorption of nutrients
84
3 regions of the small intestine
duodenum
jejunum
ileum
85
how long is the duodenum?
25cm
86
how long is the jejunum?
2m
87
how long is the ileum?
3m
88
describe the duodenum
- the part of the intestine exiting from the stomach
- performs chemical digestion and some absorption
89
describe the jejunum
where most absorption occurs
90
describe the ileum
- the terminal and longest region of the small intestine
- absorption also occurs here
91
what is chemical digestion mostly mediated by?
- enzymes produced by the pancreas
- exocrine secretion of digestive enzymes and bicarbonate via pancreatic duct merging into the common bile duct
92
what does the motility of the small intestine serve to do?
- mix the chyme with digestive enzymes and pancreatic secretions
- expose the nutrients to the intestinal mucosa for absorption
- propel the unabsorbed chyme along the small intestine into the large intestine
93
how often to contractions relating to mechanical digestion occur? why do they occur? why may contraction increase or decrease?
- occur every 90 minutes
- to clear the small intestine of residual chyme
- parasympathetic stimulation increases contraction of intestinal smooth muscle
- sympathetic activity decreases contraction
94
what do parasympathetic nerves do to motility of small intestine?
increase
95
what do sympathetic nerves do to motility of small intestine?
- decrease
- directly (via beta adrenoceptors)
- indirectly by decreasing ACh release (via alpha 2 receptors)
96
what are the 2 patterns of small intestine contractions?
segmentation contraction
peristaltic contraction
97
describe segmentation contraction of the small intestine
- mix the chyme and expose it to pancreatic enzymes and secretions
- chyme separated into 2 parts then the small intestine relaxes to mix the chyme again (ensures chyme and juices are mixed)
98
describe peristaltic contraction of the small intestine
- propel the chyme along the small intestine towards the large intestine
- food bolus in the intestinal lumen is sensed by enterochromaffin cells of the intestinal mucosa (these release serotonin which binds to receptors on intrinsic primary afferent neurons which initiates the peristaltic reflex)
99
when chyme passes into the small intestine, what is it mixed with?
- pancreatic juice (secreted from pancreas)
- bile (secreted from liver)
- intestinal juice (secreted from lining of small intestine)
100
composition of pancreatic juice
- water
- mineral salts
- enzymes (amylase, lipase, nucleases)
- inactive enzyme precursors (trypsinogen and chymotrypsinogen)
101
what is the secretion of pancreatic juice stimulated by and where are these things produced? what stimulates their production?
- stimulated by secretin and CCK
- produced by endocrine cells in the walls of the duodenum
- presence of acid chyme in the duodenum stimulates their production
102
digestion of proteins
- polypeptides to tripeptides, dipeptides and amino acids
- trypsinogen and chymotrypsinogen activated by enterokinase into the active trypsin and chymotrypsin
103
digestion of carbohydrates
- pancreatic amylase converts all digestible polysaccharides (starches) to disaccharides
104
digestion of fats
lipase converts fats to fatty acids and glycerol
105
describe the secretion and storage of bile
- secreted by the liver
- stored in the gallbladder under the stimulation by secretin and CCK
106
describe the pumping of bile after storage
- pumped by bile duct into small intestine
- duodenum specifically
107
composition of bile
water
mineral salts
mucus
bile salts
bile pigments, bilirubin
cholesterol
108
functions of bile
emulsify fats
eliminate waste products
digest proteins by enzymatic activity
regulate the digestion process
109
describe the large intestine's role of absorption of water and electrolytes
- complete absorption of water by osmosis
- sodium ions absorbed by the action of Na+/K+ pump
- mineral salts, vitamins and some drugs are also absorbed
110
describe mass movement in large intestine
peristalsis 1-3 times a day in the ileum forces chyme into the caecum
111
describe microbial activity in the large intestine
region heavily colonised by certain types of bacteria which synthesise vitamin K and folic acid
112
describe defecation in the large intestine
of every 500ml of food residue that enters the caecum each day, about 150ml becomes faeces
