1
Q
the stomach 3
A
- The bolus of food has now entered the stomach, a J-shaped chamber.
- Its structure is arbitrarily divided up into three areas.
- It is separated from the small intestine by a barrier called the pyloric sphincter.
2
Q
The three areas of the stomach:
A
- The fundus
- The body
- The antrum
3
Q
the fundus
A
This is the part of the stomach that lies above the esophageal opening
4
Q
the body
A
This is the main part of the stomach
5
Q
the antrum
A
This is the very muscular lower section of the stomach.
6
Q
The stomach performs three main functions:
A
- To store ingested food until it is ready to be released to the small intestine
- To secrete HCl and enzymes necessary for protein digestion.
- To mechanically mix the food with gastric secretions to produce the thick fluid known as chyme.
7
Q
gastric filling 5
A
- When empty, the stomach volume is about 50 ml.
- It can easily expand to accommodate a meal of about 1000 ml and even expand to about 4000 ml.
- The stomach has deep folds that get smaller and flatten out as the stomach expands.
- This allows expansion without an increase in tension or pressure.
- This occurs by receptive relaxation
8
Q
gastric storage 4
A
- Pacemaker cells in the fundus generate slow-wave potentials that travel down the length of the stomach at a rate of about three per minute.
- As discussed previously, these slow-wave potentials do not necessarily reach threshold, it depends on the level of smooth muscle excitability.
- When threshold is reached, a peristaltic wave sweeps over the fundus down towards the pyloric sphincter.
- Most food is stored in the body of the stomach and gradually moved into the muscular antrum where mixing occurs
9
Q
gastric mixing (+ what this process is called) 4
A
- With each peristaltic wave, the chyme is pushed towards the pyloric sphincter.
- However, the pyloric sphincter is usually in an almost closed position so that only fluids can pass through.
- When the peristaltic wave pushes the chyme to the pyloric sphincter and it cannot pass through it, the chyme folds back upon itself only to be propelled forward and folded back over and over.
- This process is called retropulstion and ensures the chyme is thoroughly mixed until the particles are small enough for emptying
10
Q
gastric emptying 4
A
- With each peristaltic wave some chyme is pushed through the pyloric sphincter.
- The volume of chyme in the antrum is about 30 ml, a few ml of which will be pushed through with each peristaltic wave.
- When very strong waves occur, a greater volume of chyme will pass through before the sphincter tightens again.
- The intensity of the peristaltic waves is under the influence of various signals
11
Q
what is the main factor influencing the strength of contraction in the stomach? 3
A
- the main factor influencing the strength of contraction is the amount of chyme in the stomach and its fluidity.
- The greater the volume of chyme, the more distension and therefore more contractions.
- The faster chyme becomes a liquid, the faster it will pass through the pyloric sphincter.
12
Q
duodenum and gastric emptying 3
A
- The duodenum also influences the rate of gastric emptying.
- Unless the duodenum is ready to receive chyme, it will send signals to reduce gastric emptying.
- There are neuronal responses mediated by both the intrinsic nerve plexus and the autonomic nerves, collectively called the enterogastric reflex.
13
Q
hormone response and gastric emptying 2
A
- There are also hormone responses as several hormones are released from the duodenal mucosa.
- The two most important hormones are secretin and cholecystokinin (CCK).
14
Q
There are four duodenum stimuli that affect gastric emptying
A
- fat
- acid
- hypertonicity
- distention
15
Q
fat and gastric emptying 3
A
- This is the most potent stimulus for inhibiting gastric emptying.
- It takes a long time for fat to be digested and absorbed in the lumen of the small intestine so the presence of fat reduces gastric emptying to allow more time to process it.
- High fat meals may remain in the stomach for up to six hours whereas high protein or carbohydrate meals may empty in about three hours
16
Q
acid and gastric emptying 3
A
- Because the stomach secretes a lot of HCl and it is mixed in with the chyme, the duodenum must neutralize it to prevent damage to the tissue and digestive enzymes.
- It does so by secreting, mainly from the pancreas, Na HCO3.
- High levels of acid in the duodenum will reduce gastric emptying
17
Q
hypertonicity and gastric emptying 4
A
- As proteins and carbohydrates are broken down into smaller molecules, this can have a dramatic increase in osmolarity.
- Since water moves freely across the duodenal wall, it moves into the lumen by osmosis.
- If digestion is faster than absorption, the increase in osmolarity will bring enough water in to distend the duodenum and potentially decrease plasma volume.
- Because of this, increased osmolarity in the duodenum will reflexively inhibit gastric emptying
18
Q
distention and gastric emptying
A
Quite simply, the more the duodenum is distended, the slower the rate of gastric emptying
19
Q
the steps which occur during vomiting: 5
A
- It begins with a deep inspiration and closure of the glottis (to prevent gastric contents going into the lungs) and uvula (to prevent gastric contents going into the nasal cavity).
- The diaphragm then contracts downward and the abdominal muscles contract inwards.
- The flaccid stomach is compressed, its contents are forced upwards, through the relaxed esophageal sphincters, and out through the mouth.
- This will repeat until the stomach is empty.
- Because the autonomic system is involved, vomiting is usually preceded by profuse sweating, salivation, increased heart rate, and the sensation of nausea
20
Q
situations that can induce vomiting 6
A
- Touch stimulation to the back of the throat
- Irritation or distension of the stomach or duodenum
- Elevated intracranial pressure, think about vomiting after a head injury
- Rotation or acceleration of the head, think motion sickness
- Chemical agents or drugs that interact with the chemoreceptor trigger zone next to the vomiting
centre - Psychogenic vomiting (triggered by stress)
21
Q
vomiting as a protective mechanism
A
Vomiting can be a protective mechanism to remove noxious substances from the digestive tract
22
Q
Within the gastric mucosa of the stomach there are two distinct areas with respect to secretion (+ what they are):
A
- The oxyntic mucosa: line the fundus, the body, and the pyloric gland area, which lines the antrum.
- Gastric pits: pockets formed by the in-foldings of the gastric mucosa on the luminal surface of the stomach. At the bottom of the gastric pits are the gastric glands.
23
Q
within the oxyntic mucosa, there are three types of secretory cells whose exocrine secretions collectively are known as gastric digestive juice:
A
- mucous cells
- chief cells
- parietal cells
24
Q
mucous cells
A
Mucous cells that line the pits and the entrance to the glands secrete a watery mucus
25
2. chief cells
Chief cells, the more numerous cells in the gastric glands, secrete pepsinogen
26
parietal cells 3
- Parietal cells, also in the gastric glands, secrete HCl and intrinsic factor.
- Between the gastric pits, the mucosa is covered by surface epithelial cells that secrete a viscous, alkaline mucus that makes a thick cover (a few millimeters) over the mucosa.
- Within the pyloric gland area, the gastric glands secrete mucus and a small amount of pepsinogen, but no HCl.
27
hydrochloric acid
The parietal cells actively secrete HCl into the gastric pits, which empty into the lumen of the stomach and can decrease the pH to as low as 2
28
HCl serves four functions within the stomach:
- Activates pepsinogen to the active form pepsin.
- Helps break down connective tissues and muscle fibres.
- Denatures proteins.
- Kills most microorganisms ingested with food.
29
mechanism of HCl secretion 5
- Within the parietal cells, H2O is broken down into H+ and OH-.
- The H+ is then secreted into the lumen by the H+ - K+ ATPase by active transport.
- The HCO3- that is simultaneously brought into the parietal cell passively leaks back into the lumen.
- The generated OH- combines with a H+ from H2CO3 to remake H2O.
- Since parietal cells have lots of carbonic anhydrase, the H2O combines with CO2 (either from metabolic activity or the plasma) to form H2CO3, which partially dissociates into H+ and HCO3-, thereby regenerating the H+ that was secreted.
- The HCO3- is moved into the plasma by a Cl-- HCO3- exchanger. This exchanger creates a buildup of Cl within the parietal cells, which will then move through channels, down its electrochemical gradient, into the gastric lumen
30
pepsinogen
Within chief cells, pepsinogen is stored in secretory vesicles, which are released into the stomach lumen upon the appropriate stimulus
31
how pepsinogen is converted to its active form, pepsin, within the lumen of the stomach 4
- Once released, HCl cleaves off a small part of the protein to release the active form of the
enzyme, pepsin.
- Pepsin itself can then cleave more pepsinogen to form even more pepsin.
- The active pepsin then starts protein digestion by splitting certain amino acid linkages to release smaller amino acid chains.
- This activity of pepsin is dependent upon the acidic environment of the stomach.
32
why is pepsin stored in the chief cells as pepsinogen?
Since pepsin is able to digest protein, it must be stored in an inactive form to prevent it from digesting the proteins within the chief cells in which it is formedm (thus, so it is harmless to the cell).
33
mucus
the mucus secreted by the surface epithelial cells forms a thick cover that serves as a protective barrier against several forms of potential injury
34
mucus as a protective barrier 3
- Mucus acts as a lubricant and protects the stomach lining from mechanical injury.
- It neutralizes HCl near the stomach lining, creating a pH of around 7 in the mucus, while the stomach lumen has a pH of 2.
- Protects against self-digestion by inactivating pepsin, as it needs an acidic environment to be active.
35
intrinsic factor 3
- Intrinsic factor is secreted by the parietal cells, and is important for the absorption of vitamin B12.
- Vitamin B12 is essential for the normal function of red blood cells and can only be absorbed by combining with intrinsic factor.
- The vitamin B12-intrinsic factor complex interacts with a receptor in the latter part of the small intestine causing it to undergo receptor-mediated endocytosis
36
3 other secretory cells
1. G cells
2. Enterochromaffin-like Cells
3. D Cells
37
G cells 3
- These are endocrine cells located in the pyloric gland area and they secrete the hormone gastrin.
- Its release is stimulated by protein in the stomach and ACh from the intrinsic nerve plexus.
- Once in the blood, it travels to the stomach body and fundus where it stimulates chief and parietal cells to increase their secretions.
38
Enterochromaffin-like Cells 2
- These are found among the chief and parietal cells in the oxyntic mucosa and secrete histamine, which acts as a paracrine to increase HCl secretion.
- Histamine release is stimulated by gastrin and ACh
39
D cells 2
- These are found near the pylorus and in the duodenum.
- In response to acid, they secrete somatostatin, which acts as a paracrine to inhibit the secretions of parietal cells, G cells, and ECL cells
40
3 phases of Gastric secretions:
1. Gastric Phase
2. Intestinal Phase
3. cephalic phase (discussed in section 2)
41
Gastric Phase 4
- Stimuli in the stomach, namely protein, distension, caffeine, and alcohol, all increase gastric secretion by overlapping efferent pathways.
- Protein is the most potent stimulus and stimulates chemoreceptors that activate the intrinsic nerve plexus, which then stimulates the secretory cells.
- Protein also directly stimulates the release of gastrin which further enhances secretions as already discussed.
- Distension, caffeine, and alcohol all stimulate acidic secretion even if no food is present in the stomach.
42
intestinal phase 2
- This refers to factors originating in the small intestine that affect gastric secretion.
- In general, they are all inhibitory to help turn off the flow of gastric juices as chyme starts moving into the small intestine
43
how does the nervous system mediate the increase of gastric secretions in the cephalic phase 4
- Tasting, smelling, chewing, swallowing, or even thinking about food increases gastric secretions.
- This happens through the parasympathetic nervous system, specifically vagal nerve activity.
- Vagal stimulation increases acetylcholine, which boosts HCl and pepsinogen secretion.
- Vagal stimulation of G cells releases gastrin, further enhancing secretion of HCl and pepsinogen
44
gastric digestion
In the body of the stomach, most of the food stays in a semi-solid state as the peristaltic contractions are not strong enough for mixing
45
gastric digestion to proteins
Food here is not mixed with gastric juices so very little protein digestion occurs until the food moves into the antrum, where it is thoroughly mixed with gastric juices
46
gastric digestion to carbohydrates 2
- The food is mixed with salivary amylase.
- Even though acid inhibits amylase activity, the interior of the food mass is not exposed to acid.
47
gastric absorption 2
- No food or water is absorbed through the wall of the stomach into the blood.
- However, there are two categories of substances that can cross the stomach wall
48
gastric absorption to alcohol 3
- Ethanol is somewhat lipid soluble so it can diffuse across the stomach wall.
- That said, it is absorbed faster across the intestinal wall where the surface area for absorption is much greater.
- Delaying alcohol absorption can be achieved by eating high fat foods before consuming alcohol as this slows down gastric emptying and thus slows the absorption of alcohol.
49
gastric absorption to apsirin 2
- Weak acids such as acetylsalicylic acid are completely unionized in the acidic environment of the stomach.
- In the unionized form, they are lipid soluble and can cross the plasma membrane of the epithelial cells lining the stomach.
