Gastric/Stomach Physiology

The stomach performs three main functions:

1. The stomach’s most important function is to store ingested food until it can be
emptied into the small intestine at a rate appropriate for optimal digestion and
absorption. It takes hours to digest and absorb a meal. Because the small intestine is
the primary site for this digestion and absorption, it is important that the stomach store
the food and meter it into the duodenum at a rate that does not exceed the small
intestine’s capacities.
2. The stomach secretes hydrochloric acid (HCl) and enzymes that begin protein
digestion.
3. Through the stomach’s mixing movements, the ingested food is pulverized and mixed
with gastric secretions to produce a thick liquid mixture known as chyme.

Gastric motility is complex and subject to multiple regulatory inputs. The four aspects of
gastric motility are (1) filling, (2) storage, (3) mixing, and (4) emptying.

1. Gastric Filling Involves Receptive Relaxation

- When empty, the stomach has a volume of about 50 mL, but it can expand up to
20-fold to a capacity of about 1 liter (1000 mL) during a meal.
- The interior of the stomach is thrown into deep folds. During a meal, the folds get
smaller and nearly flatten out as the stomach relaxes slightly with each mouthful.
This vagally mediated response, called receptive relaxation, allows the stomach
to accommodate the meal with little change in intragastric pressure.
- If more than a liter of food is consumed, however, the stomach becomes
overdistended, intragastric pressure rises, and the person experiences discomfort.

2. Gastric Storage Takes Place in The Body of The Stomach.

- A peristaltic wave spreads over the fundus and body to the antrum and pyloric
sphincter. Because the muscle layers are thin in the fundus and body, the
peristaltic contractions in this region are weak. When the waves reach the antrum,
they become stronger and more vigorous because the muscle there is thicker.
- Because only feeble mixing movements occur in the body and fundus, food
delivered to the stomach from the esophagus is stored in the relatively quiet body
without being mixed. The fundus usually does not store food but contains only a
pocket of gas. Food is gradually fed from the body into the antrum, where mixing
does take place.

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

3. Gastric Mixing Takes Place in the Antrum of The Stomach.
- The strong antral peristaltic contractions mix the food with gastric secretions to
produce chyme. Each antral peristaltic wave propels chyme distally toward the
pyloric sphincter. Tonic contraction of the pyloric sphincter normally keeps it
almost closed (but not completely). The opening is large enough for water and
other fluids to pass through with ease.
- As the peristaltic wave reaches the pyloric sphincter and closes it tightly, the large
particles are forced backward toward the body of the stomach. The bulk of the
antral chyme that is forced backward is again propelled forward and then tumbled
back as the next peristaltic wave advances. This churning action is called
retropulsion, which thoroughly shears and grinds the chyme until the particles are
small enough for emptying, mixing the contents in the process.

4. Gastric Emptying is Largely Controlled by Factors in The Duodenum.

- In addition to mix gastric contents, the antral peristaltic contractions are the
driving force for gastric emptying. The amount of chyme that escapes into the
duodenum with each peristaltic wave before the pyloric sphincter tightly closes
depends largely on the strength of antral peristalsis.
- As the chyme enters the small intestine, the chyme stimulates the cells of the
duodenual wall to release the hormone secretin into the blood. When secretin
reaches the pancreas, it stimulates the pancreas to release its bicarbonate-rich
juices. Thus, whenever the duodenum signals that acidic chyme is present, the
pancreas responds by sending bicarbonate to neutralize it. When the need has
been met, the cells of the duodenal wall are no longer stimulated to release
secretin, the hormone no longer flows through the blood, and the pancreas no
longer receives the message and stops sending pancreatic juice. Nerves also
regulate pancreatic secretions.

Factors in the Stomach that Influence the Rate of Gastric Emptying

The main gastric factor that influences the strength of contraction is the amount and
volume of chyme in the stomach. Furthermore, the degree of fluidity of the chyme
influences gastric emptying. The sooner the appropriate degree of fluidity can be
achieved, the more rapidly the contents are ready to be evacuated.

Factors in the Duodenum that Influence the Rate of Gastric Emptying

The duodenum must be ready to receive the chyme and can delay gastric emptying by
reducing the strength of antral peristalsis until the duodenum is ready to accommodate

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

more chyme. The four most important duodenal factors that influence gastric emptying
are fat, acid, hypertonicity, and distension. The presence of one or more of these stimuli
in the duodenum activates appropriate duodenal receptors, triggering neural and
hormonal responses that put brakes on antral peristaltic activity, there by slowing the rate
of gastric emptying:
 The neural response is mediated through both the intrinsic plexuses (short reflex) and
the autonomic nerves (long reflex). Together these constitute the enterogastric
reflex.
 The hormonal response involves the release from the small intestine mucosa into the
blood of several hormones collectively known as enterogastrones. The blood carries
these hormones to the stomach, where they inhibit antral contractions to reduce
gastric emptying. The two most important enterogastrones are secretin and
cholecystokinin (CCK). Because it was a secretory product that entered the blood, it
was termed secretin. The name cholecystokinin derives from this same hormone also
causing contraction of the bile-containing gallbladder (chole means “bile”; cysto
means “bladder”; and kinin means “contraction”). Secretin and CCK are major GI
hormones that perform other important functions in addition to serving as
enterogastrones.

Let us examine why it is important that each of these stimuli in the duodenum (fat, acid,
hypertonicity, and distension) delays gastric emptying:
 Fat. Among the different nutrients that we consume, fat is most effective in delaying
gastric emptying. This effect is important because fat digestion and absorption take
more time than for the other nutrients and take place only in the small intestine lumen.
Triglycerides strongly stimulate duodenal release of CCK. This hormone inhibits
antral contractions and also induces contraction of the pyloric sphincter, which both
slow gastric emptying. This delay in emptying ensures that the small intestine has
enough time to digest and absorb the fat already there before more fat enters from the
stomach. That fat is the most potent inhibitor of gastric emptying is evident when you
compare the rate of emptying of a high-fat meal (after 6 hours, some of a bacon-and-
eggs meal may still be in the stomach) with that of a protein and carbohydrate meal (a
meal of lean meat and potatoes may empty in 3 hours).

 Acid. Because the stomach secretes HCl, highly acidic chyme empties into the
duodenum, where it is neutralized by sodium bicarbonate (NaHCO 3) secreted into the
duodenum primarily from the pancreas. Unneutralized acid may damage the duodenal

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

 mucosa and inactivate the pancreatic digestive enzymes secreted into the duodenum.
Appropriately, unneutralized acid in the duodenum induces the release of secretin.
 Hypertonicity. Because water is freely diffusible across the duodenal wall, it enters
the duodenal lumen from the plasma as the duodenal osmolarity rises. Large volumes
of water entering the intestine from the plasma lead to intestinal distension, and, more
important, circulatory disturbances result because of the reduction in plasma volume.
To prevent these effects, gastric emptying is reflexly inhibited when the osmolarity of
the duodenal contents starts to rise. Thus, the amount of food entering the duodenum
for further digestion into a multitude of additional osmotically active particles is
reduced until absorption processes have had an opportunity to catch up.

 Distension. Too much chyme in the duodenum inhibits the emptying of even more
gastric contents, giving the distended duodenum time to cope with the excess volume
of chyme it already contains before it gets any more.

Vomiting
Vomiting, or emesis, the forceful expulsion of gastric contents out through the mouth, is
not accomplished by reverse peristalsis in the stomach, as might be predicted. Actually,
the stomach, the esophagus, and associated sphincters are all relaxed during vomiting.
The major force for expulsion comes, surprisingly, from contraction of the respiratory
muscles—namely, the diaphragm (the major inspiratory muscle) and the abdominal
muscles (the muscles of active expiration)
The complex act of vomiting is coordinated by a vomiting center in the medulla of the
brain stem. Vomiting is usually preceded by profuse salivation, sweating, rapid heart rate,
and sensation of nausea. Vomiting begins with a deep inspiration and closure of the
glottis. The contracting diaphragm descends downward on the stomach while
simultaneous contraction of the abdominal muscles compresses the abdominal cavity,
increasing the intra-abdominal pressure and forcing the abdominal Viscera upward. As
the flaccid stomach is squeezed between the diaphragm from above and the compressed
abdominal cavity from below, the gastric contents are forced upward through the relaxed
sphincters and esophagus and out through the mouth. The glottis is closed, so vomited
material does not enter the trachea. Also, the uvula is raised to close off the nasal cavity.
The vomiting cycle may be repeated several times until the stomach is emptied.

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

Gastric Juice
In the stomach, gastric glands secrete gastric juice, a mixture of water, enzymes, and
hydrochloric acid. The acid is so strong that it causes the sensation of heartburn if it
happens to reflux into the esophagus. The strong acidity of the stomach prevents bacterial
growth and kills most bacteria that enter the body with food. It would destroy the cells of

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

the stomach as well, but for their natural defenses. To protect themselves from gastric
juice, the cells of the stomach wall secrete mucus, thick, slippery, white substance that
coats the cells, protecting them from the acid, enzymes, and disease-causing bacteria that
might otherwise cause harm.
Each day, the stomach secretes about 2 liters of gastric juice. The cells that secrete gastric
juice are located in the gastric mucosa, which is divided into two distinct areas:
(1) the oxyntic mucosa, which lines the body and fundus, and
(2) the pyloric gland area (PGA), which lines the antrum.
The luminal surface of
the stomach is pitted with
deep pockets formed by
infoldings of the gastric
mucosa. The first parts of
these invaginations are
called gastric pits, at the
base of which lie the
gastric glands. A variety
of secretory cells line
these invaginations, some
exocrine and some
endocrine or paracrine

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

Three types of gastric exocrine secretory cells are found in the walls of the pits and
glands in the oxyntic mucosa:
■ Mucous cells line the gastric pits and the entrance of the glands. They secrete a thin,
watery mucus. (Mucous is the adjective; mucus is the noun.)
■ The deeper parts of the gastric glands are lined by chief and parietal cells. The more
numerous chief cells secrete the enzyme precursor pepsinogen.
■ The parietal (or oxyntic) cells secrete HCl and intrinsic factor. When stimulated,
parietal cells form deep invaginations called canaliculi (singular canaliculus) along the
luminal (or apical) membrane, which increase the membrane surface area bearing
transport proteins that actively secrete HCl into the lumen of the gastric pits.

Functions of HCl
Although HCl does not actually digest anything (that is, it does not break apart nutrient
chemical bonds), it performs these specific functions that aid digestion:
1. HCl activates the enzyme precursor pepsinogen to an active enzyme, pepsin, and
provides an acid environment optimal for pepsin action.
2. It aids in the breakdown of connective tissue and muscle fibers, reducing large food
particles into smaller particles.
3. It denatures protein—that is, it uncoils proteins from their highly folded final form,
thus exposing more of the peptide bonds for enzymatic attack.
4. Along with salivary lysozyme, HCl kills most of the microorganisms ingested with
food, although some escape and then grow and multiply in the large intestine.

Pepsinogen is activated to pepsin, which begins protein digestion.

The major digestive constituent of gastric secretion is pepsinogen, an inactive enzymatic
molecule produced by the chief cells. Because pepsin can digest protein, it must be stored
and secreted in an inactive form so that it do not digest the proteins of the cells in which
it is formed.
Pepsinogen, once activated to the enzyme pepsin, begins protein digestion. Pepsinogen is
stored in the chief cells’ cytoplasm within secretory vesicles known as zymogen
granules, from which it is released by exocytosis on appropriate stimulation. When
pepsinogen is secreted into the gastric lumen, HCl cleaves off a small fragment of the
molecule, converting it to the active form of pepsin. Once formed, pepsin acts on other
pepsinogen molecules to produce more pepsin, a mechanism called Autocatalytic
process (autocatalytic means “self-activating”).

Source : Sherwood Human Physiology From Cells to Systems 9th Edition

Pepsin initiates protein digestion by splitting certain amino acid linkages in proteins to
yield peptide fragments (small amino acid chains); it works most effectively in the acid
environment provided by HCl.

Mucus is protective.
The surface of the gastric mucosa is covered by a layer of mucus derived from the surface
epithelial cells and mucous cells. This mucus is a protective barrier against several forms
of potential injury to the gastric mucosa:
 Through its lubricating properties, mucus protects the gastric mucosa against
mechanical injury.
 It helps protect the stomach wall from self-digestion because pepsin is inhibited when
it comes in contact with the layer of mucus coating the stomach lining.
 Being alkaline, mucus helps protect against acid injury by neutralizing HCl in the
vicinity of the gastric lining, but it does not interfere with the function of HCl in the
lumen. Whereas the pH in the lumen may be as low as 2, the pH in the layer of mucus
adjacent to the mucosal cell surface is about 7.

Source : Sherwood Human Physiology From Cells to Systems 9th Edition