The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the brain, stomach, and small intestine activate or inhibit gastric juice production. This is why the three phases of gastric secretion are called the cephalic, gastric, and intestinal phases. However, once gastric secretion begins, all three phases can occur simultaneously.
The Three Phases of Gastric Secretion - Gastric secretion occurs in three phases: cephalic, gastric, and intestinal. During each phase, the secretion of gastric juice can be stimulated or inhibited.
The cephalic phase (reflex phase) of gastric secretion, which is relatively brief, takes place before food enters the stomach. The smell, taste, sight, or thought of food triggers this phase.
EXAMPLE
When you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion.
This enhanced secretion is a conditioned reflex, meaning it occurs only if you like or want a particular food. Depression and loss of appetite can suppress the cephalic reflex.
The gastric phase of secretion lasts 3 to 4 hours and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach.
EXAMPLE
When the sushi reaches the stomach, it creates distention that activates the stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice.
Partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn induces parietal cells to increase their production of HCl, which is needed to create an acidic environment for the conversion of pepsinogen to pepsin, and protein digestion. Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, it should be noted that the stomach does have a natural means of avoiding excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the stomach react by suspending HCl secretion and increasing mucous secretions.
The intestinal phase of gastric secretion has both excitatory and inhibitory elements. The duodenum has a major role in regulating the stomach and its emptying. When partially digested food fills the duodenum, intestinal mucosal cells release a hormone called intestinal (enteric) gastrin, which further excites gastric juice secretion. This stimulatory activity is brief, however, because when the intestine distends with chyme, the enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the pyloric sphincter, which blocks additional chyme from entering the duodenum.
terms to know
Cephalic Phase
(also, reflex phase) The initial phase of gastric secretion that occurs before food enters the stomach.
Gastric Phase
The phase of gastric secretion that begins when food enters the stomach.
Intestinal Phase
The phase of gastric secretion that begins when chyme enters the intestine.
2. The Mucosal Barrier
The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice. Gastric enzymes that can digest protein can also digest the stomach itself. The stomach is protected from self-digestion by the mucosal barrier. This barrier has several components. First, the stomach wall is covered by a thick coating of bicarbonate-rich mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid. Second, the epithelial cells of the stomach's mucosa meet at tight junctions, which block gastric juice from penetrating the underlying tissue layers. Finally, stem cells located where gastric glands join the gastric pits quickly replace damaged epithelial mucosal cells when the epithelial cells are shed.
did you know
In fact, the surface epithelium of the stomach is completely replaced every 3 to 6 days.
IN CONTEXT
HOMEOSTATIC IMBALANCES Ulcers: When the Mucosal Barrier Breaks Down
As effective as the mucosal barrier is, it is not a “fail-safe” mechanism. Sometimes, gastric juice eats away at the superficial lining of the stomach mucosa, creating erosions, which mostly heal on their own. Deeper and larger erosions are called ulcers.
Why does the mucosal barrier break down? A number of factors can interfere with its ability to protect the stomach lining. The majority of all ulcers are caused by either excessive intake of non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin, or Helicobacter pylori infection.
Antacids help relieve symptoms of ulcers such as “burning” pain and indigestion. When ulcers are caused by NSAID use, switching to other classes of pain relievers allows healing. When caused by H. pylori infection, antibiotics are effective.
A potential complication of ulcers is perforation: perforated ulcers create a hole in the stomach wall, resulting in peritonitis (inflammation of the peritoneum), which you learned about in a previous lesson. These ulcers must be repaired surgically.
Term Pronunciation Table
Term
Pronunciation
Audio File
Mucosal Barrier
mu·co·sal bar·ri·er
terms to know
Mucosal Barrier
The protective barrier that prevents gastric juice from destroying the stomach itself.
Ulcer
A sore that develops when stomach acid damages the lining of the digestive tract.
3. Digestive Functions of the Stomach
The stomach participates in virtually all digestive activities with the exception of ingestion and defecation. Although almost all absorption takes place in the small intestine, the stomach does absorb some drugs, such as alcohol and aspirin.
3a. Mechanical Digestion
Within a few moments after food enters your stomach, mixing waves begin to occur at intervals of approximately 20 seconds. A mixing wave is a unique type of peristalsis that mixes and softens the food with gastric juices to create chyme. The initial mixing waves are relatively gentle, but these are followed by more intense waves, starting at the body of the stomach and increasing in force as they reach the pylorus. It is fair to say that long before your sushi exits through the pyloric sphincter, it bears little resemblance to the sushi you ate.
The pylorus, which holds around 30 mL (1 fluid ounce) of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, but not fully, closed pyloric sphincter. In a process called gastric emptying, rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater amount of chyme at one time would overwhelm the capacity of the small intestine to handle it. The rest of the chyme is pushed back into the body of the stomach, where it continues mixing. This process is repeated when the next mixing waves force more chyme into the duodenum.
Gastric emptying is regulated by both the stomach and the duodenum. The presence of chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents additional chyme from being released by the stomach before the duodenum is ready to process it.
3b. Chemical Digestion
The fundus plays an important role because it stores both undigested food and gases that are released during the process of chemical digestion. Food may sit in the fundus of the stomach for a while before being mixed with the chyme. While the food is in the fundus, the digestive activities of salivary amylase continue until the food begins mixing with the acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity of which inactivates salivary amylase and activates lingual lipase. Lingual lipase then begins breaking down triglycerides into free fatty acids, and mono- and diglycerides.
The breakdown of protein begins in the stomach through the actions of HCl and the enzyme pepsin.
Its numerous digestive functions notwithstanding, there is only one stomach function necessary to life: the production of intrinsic factor. The intestinal absorption of vitamin B₁₂, which is necessary for both the production of mature red blood cells and normal neurological functioning, cannot occur without intrinsic factor. People who undergo total gastrectomy (stomach removal)—for life-threatening stomach cancer, for example—can survive with minimal digestive dysfunction if they receive vitamin B₁₂ injections.
The contents of the stomach are completely emptied into the duodenum within 2 to 4 hours after you eat a meal. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with a high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, note that this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish.
IN CONTEXT
Bariatric Surgery
Bariatric surgeries are weight-loss surgeries that are typically done when an individual has serious weight-related health issues or when diet and exercise have not been sufficient to achieve weight loss. There are several different types of procedures that involve modifying the gastrointestinal system to reduce the amount of food that can be eaten and/or limiting how much of the digested food can be absorbed (Mayo Clinic, 2022).
Gastric banding is an example of a bariatric surgery in which a small pouch is created in the stomach. This reduces the size of the stomach which can be used for digestion. Because the stomach is smaller, an individual who had this surgery will feel full after consuming less food.
Gastric Banding - Gastric banding surgery creates a small pouch of stomach, reducing the size of the stomach that can be used for digestion.
terms to know
Mixing Wave
A unique type of peristalsis that occurs in the stomach.
Gastric Emptying
The process by which mixing waves gradually cause the release of chyme into the duodenum.
summary
In this lesson, you learned about how the physiology of the stomach during digestion. First, you explored the phases of gastric secretion, and how the mucosal barrier protects the stomach from gastric enzymes. Finally, you reviewed the digestive functions of the stomach, and how it is involved in most digestive activities. Specifically, you learned about the stomach’s roles in mechanical digestion and chemical digestion.
