Section 3

Question 1

What are the three areas of the stomach?

Answer 1

The fundus: This is the upper part of the stomach, located above the esophageal opening.

The body: This is the main part of the stomach.

The antrum: This is the muscular lower section of the stomach.

Question 2

What separates the stomach from the small intestine?

Answer 2

The stomach is separated from the small intestine by a barrier called the pyloric sphincter.

Question 3

What are the three main functions of the stomach?

Answer 3

Storage: The stomach stores ingested food until it is ready to be released into the small intestine.

Secretion: It secretes hydrochloric acid (HCl) and enzymes necessary for protein digestion.

Mechanical Mixing: The stomach mechanically mixes food with gastric secretions to produce a thick fluid known as chyme.

Question 4

What is gastric filling? What is this called in other words?

Answer 4

Gastric filling refers to the stomach’s ability to expand to accommodate food.

The stomach has deep folds that get smaller and flatten out as the stomach expands. This allows for expansion without an increase in tension or pressure.

This occurs by the vagus nerve, and is called receptive relaxation

Question 5

How does gastric storage occur?

Answer 5

Pacemaker cells in the fundus generate slow-wave potentials. These do not always reach threshold, depends on the level of smooth muscle excitability.

When they do reach it, peristaltic waves are initiated that move food towards the pyloric sphincter.

Most food is stored in the body of the stomach before gradual movement into the antrum for mixing.

Question 6

What is gastric mixing?

Answer 6

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

Question 7

Describe gastric emptying.

Answer 7

Gastric emptying is the process where peristaltic waves push chyme through the pyloric sphincter. The intensity of contractions is influenced by the volume and fluidity of chyme. More chyme and increased fluidity result in stronger contractions and faster emptying.

Question 8

How does the duodenum influence the rate of gastric emptying?

Answer 8

The duodenum influences gastric emptying rate by sending signals to reduce it unless ready to receive chyme.

Neuronal responses, collectively called the enterogastric reflex, mediated by the intrinsic nerve plexus and autonomic nerves, play a role. Additionally, several hormones, such as secretin and cholecystokinin (CCK), released from the duodenal mucosa, affect gastric emptying.

Question 9

How does fat affect gastric emptying?

Answer 9

Fat is the most potent stimulus for inhibiting gastric emptying. It takes a long time for fat to be digested and absorbed in the small intestine, so its presence reduces gastric emptying to allow more time for processing. High-fat meals may remain in the stomach for up to six hours.

Question 10

What role does acid play in gastric emptying?

Answer 10

High levels of acid in the duodenum, resulting from the mixing of gastric acid with chyme, can be damaging. To neutralize this acid, mainly from the pancreas, the duodenum secretes NaHCO3, which can reduce gastric emptying.

Question 11

How does hypertonicity affect gastric emptying?

Answer 11

As proteins and carbohydrates are broken down into smaller molecules, osmolarity increases. Water moves into the duodenum by osmosis. If digestion is faster than absorption, increased osmolarity can distend the duodenum, potentially decreasing plasma volume and reflexively inhibiting gastric emptying.

Question 12

What effect does distention have on gastric emptying?

Answer 12

Increased distention of the duodenum slows the rate of gastric emptying. The more the duodenum is distended, the slower the gastric emptying process.

Question 13

What are the steps involved in vomiting (emesis)?

Answer 13

1. Vomiting begins with a deep inspiration and closure of the glottis and uvula to prevent gastric contents from entering the lungs and nasal cavity, respectively.
2. Next, the diaphragm contracts downward, and abdominal muscles contract inward. This compresses the flaccid stomach, forcing its contents upwards through the relaxed esophageal sphincters and out through the mouth.
3. This process repeats until the stomach is empty.

Question 14

What physiological symptoms often precede vomiting?

Answer 14

Vomiting is usually preceded by profuse sweating, salivation, increased heart rate, and the sensation of nausea. These symptoms are mediated by the autonomic nervous system .

Question 15

What are some situations that may induce vomiting?

Answer 15

• Touch stimulation to the back of the throat
• Irritation or distension of the stomach or duodenum
• Elevated intracranial pressure (e.g., after a head injury)
• Rotation or acceleration of the head (motion sickness)
• Chemical agents or drugs acting on the chemoreceptor trigger zone
• Psychogenic factors

Vomiting can serve as a protective mechanism to remove noxious substances from the digestive tract, but excessive vomiting can disrupt acid/base balance and lead to physiological consequences.

Question 16

What are the two distinct areas within the gastric mucosa of the stomach with respect to secretion?

Answer 16

Oxyntic mucosa: Lines the fundus and the body of the stomach.

Pyloric gland area: Lines the antrum of the stomach.

Question 17

What are gastric pits, and what lies at the bottom of these pits?

Answer 17

Gastric pits are pockets formed by the in-foldings of the gastric mucosa.

At the bottom of these pits lie the gastric glands.

Question 18

What are the three types of secretory cells found within the oxyntic mucosa, and what are their secretions collectively known as?

Answer 18

Mucous cells: Secrete watery mucus.
Chief cells: Secrete pepsinogen.
Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.

Collectively, their secretions form gastric digestive juice.

Question 19

What do surface epithelial cells between gastric pits secrete, and what is their function?

Answer 19

Surface epithelial cells secrete a viscous, alkaline mucus.

This mucus forms a thick cover over the mucosa, providing protection.

Question 20

What do gastric glands in the pyloric gland area secrete, and what is the difference compared to the oxyntic mucosa?

Answer 20

Gastric glands in the pyloric gland area secrete mucus and a small amount of pepsinogen, but no hydrochloric acid (HCl).

Question 21

What are the four functions of HCl in the stomach?

Answer 21

1. Activates pepsinogen to pepsin.
2. Assists in breaking down connective tissues and muscle fibers.
3. Denatures proteins.
4. Kills most microorganisms ingested with food.

Question 22

Describe the mechanism of HCl secretion

Answer 22

Parietal cells in the gastric pits actively secrete HCl into the stomach lumen.

HCl secretion involves breaking down water (H2O) into H⁺ and OH⁻.

H⁺ ions are transported into the lumen by the H⁺ - K⁺ ATPase pump.
Simultaneously, bicarbonate ions (HCO3⁻) passively leak back into the lumen.

OH⁻ ions combine with H⁺ from carbonic acid (H2CO3) to form water (H2O).

Carbonic anhydrase in parietal cells facilitates the conversion of H2O and CO2 into H2CO3.

H2CO3 partially dissociates into H⁺ and HCO3⁻, regenerating the H⁺ ions for secretion.

HCO3⁻ ions are moved into the plasma by a Cl⁻ - HCO3⁻ exchanger.
This creates a buildup of Cl⁻ within parietal cells, which then moves through channels into the gastric lumen.

Question 23

Why is pepsin stored in chief cells as pepsinogen?

Answer 23

Pepsin is stored in an inactive form (pepsinogen) within chief cells to prevent premature digestion of proteins within the cells themselves. This ensures that it remains harmless to the cell until it is released into the gastric lumen and activated by HCl.

Question 24

How is pepsinogen converted into its active form, pepsin, within the lumen of the stomach?

Answer 24

Once released into the stomach lumen, HCl cleaves off a small part of the pepsinogen protein, activating it into pepsin.

Pepsin itself can then further cleave more pepsinogen to form additional 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.

Question 25

What are the functions of the mucus secreted by the surface epithelial cells in the stomach?

Answer 25

1. The mucus acts as a lubricant and protects the gastric mucosa against mechanical injury. 2. It protects the stomach wall from the acidic environment by neutralizing HCl near the lining. Despite the low pH (around 2) in the stomach lumen, the mucus layer has a pH of approximately 7. 3. The mucus layer prevents self-digestion of the stomach by inactivating pepsin near the stomach wall, as pepsin requires an acidic environment for activity.

Question 26

What is the role of intrinsic factor secreted by parietal cells in the stomach?

Answer 26

Intrinsic factor is essential for the absorption of vitamin B12. Vitamin B12 is crucial for the normal function of red blood cells and can only be absorbed by forming a complex with intrinsic factor. This complex interacts with a receptor in the latter part of the small intestine, leading to receptor-mediated endocytosis for absorption.

Question 27

What are G cells, and what hormone do they secrete?

Answer 27

G cells are endocrine cells located in the pyloric gland area of the stomach. They secrete the hormone gastrin. Gastrin release is stimulated by protein in the stomach and acetylcholine (ACh) from the intrinsic nerve plexus. Upon entering the bloodstream, gastrin travels to the stomach body and fundus, where it stimulates chief and parietal cells to increase their secretions.

Question 28

Where are enterochromaffin-like cells located, and what do they secrete?

Answer 28

Enterochromaffin-like cells are found among the chief and parietal cells in the oxyntic mucosa of the stomach. They secrete histamine, which acts as a paracrine to increase hydrochloric acid (HCl) secretion. Histamine release is stimulated by gastrin and acetylcholine (ACh).

Question 29

What are D cells, and what is their function?

Answer 29

D cells 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 enterochromaffin-like cells.

Question 30

What stimuli increase gastric secretion in the Gastric Phase?

Answer 30

Stimuli in the stomach, including protein, distension, caffeine, and alcohol, increase gastric secretion during the Gastric Phase. Protein is the most potent stimulus, activating chemoreceptors that stimulate the intrinsic nerve plexus and directly triggering the release of gastrin (enhancing secretions). Distension, caffeine, and alcohol all stimulate acidic secretion even if no food is present in the stomach.

Question 31

Describe the Intestinal Phase of gastric secretion.

Answer 31

The Intestinal Phase refers to factors originating in the small intestine that inhibit gastric secretion. These inhibitory factors help reduce gastric juice flow as chyme moves into the small intestine.

Question 32

How does the nervous system mediate the increase of gastric secretions in the cephalic phase?

Answer 32

The cephalic phase of gastric secretion occurs in response to stimuli acting in the head before food enters the stomach. Activities such as tasting, smelling, chewing, and even thinking about food lead to an increase in gastric secretions. This response is mediated by the parasympathetic nervous system, particularly through vagal nerve activity. Vagal stimulation of the intrinsic plexuses promotes increased secretion of acetylcholine, which triggers the secretion of HCl and pepsinogen. Additionally, vagal stimulation of the G cells leads to the release of gastrin, further enhancing the secretion of HCl and pepsinogen.

Question 33

How does gastric digestion occur in the body of the stomach?

Answer 33

Gastric Digestion Overview: Within the body of the stomach, the majority of food remains in a semi-solid state due to the relatively weak peristaltic contractions, limiting thorough mixing. Protein Digestion: In this region, where food isn't extensively mixed with gastric juices, protein digestion is minimal until it reaches the antrum, where it undergoes thorough mixing with gastric juices. Carbohydrate Digestion: Despite the inhibitory effect of stomach acid on amylase, the carbohydrate-digesting enzyme, salivary amylase, mixes with food. The interior of the food mass remains shielded from acid exposure, enabling some degree of carbohydrate digestion.

Question 34

What substances can cross the stomach wall, and how does this process occur? (gastric absorption)

Answer 34

Gastric Absorption Overview: Generally, food and water are not absorbed through the stomach wall into the bloodstream. However, certain substances can permeate this barrier. Alcohol Absorption: Ethanol, being somewhat lipid-soluble, can diffuse across the stomach wall. Nonetheless, it's absorbed more efficiently across the intestinal wall due to its larger surface area. Delaying alcohol absorption is feasible by consuming high-fat foods before drinking, which slows gastric emptying and consequently alcohol absorption. Aspirin Absorption: Weak acids like acetylsalicylic acid remain unionized in the acidic stomach environment. In this form, they're lipid-soluble and can pass through the plasma membrane of the stomach's epithelial cells, facilitating absorption.