How Does The Esophagus Attached To The Stomach That Is Different Then Most Animals?

23.iv The Stomach

Learning Objectives

By the cease of this department, you will be able to:

Draw the functional beefcake of the stomach
Identify the iv main types of secreting cells in gastric glands, and their important products
Explain why the tummy does not digest itself
Depict the mechanical and chemical digestion of food entering the tum
Draw any absorption that happens in the stomach

Although a minimal corporeality of digestion occurs in the oral cavity, chemical digestion really gets underway in the stomach, primarily as the initial site of poly peptide digestion. An expansion of the alimentary canal that lies immediately inferior to the esophagus, the stomach links the esophagus to the first part of the small intestine (the duodenum) and is relatively fixed in place at its esophageal and duodenal ends. In between, however, information technology can be a highly active structure, contracting and continually changing position and size. These contractions provide mechanical assistance to digestion. The empty stomach is but virtually the size of your fist, but tin stretch to hold as much as 4 liters of nutrient and fluid, or more than 75 times its empty volume, and so return to its resting size when empty. Although you lot might think that the size of a person's stomach is related to how much food that individual consumes, torso weight does not correlate with stomach size. Rather, when you eat greater quantities of food—such as at holiday dinner—yous stretch the stomach more than than when yous eat less.

Popular civilization tends to refer to the stomach every bit the location where all digestion takes place. Of course, this is non true. An important function of the stomach is to serve as a temporary holding chamber. Y'all tin ingest a meal far more than quickly than it tin be digested and absorbed by the small-scale intestine. Thus, the stomach holds food and parses merely small amounts into the modest intestine at a time. Foods are not processed in the gild they are eaten; rather, they are mixed together with digestive juices in the breadbasket until they are converted into chyme, which is released into the small intestine.

As you volition see in the sections that follow, the stomach plays several of import roles in chemical digestion, including the continued digestion of carbohydrates until salivary amylase is inactivated past stomach acid, and the initial digestion of proteins and triglycerides. Piffling if whatever assimilation occurs in the stomach, with the exception of lipid soluble substances such equally alcohol and aspirin.

Construction

There are four main regions in the stomach: the cardia, fundus, body, and pylorus (Figure 23.4.ane). The cardia (or cardiac region) is the betoken where the esophagus connects to the tum and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the chief office of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the torso of the stomach. The narrower finish is called the pyloric culvert, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls tum emptying. In the absence of nutrient, the stomach deflates inward, and its mucosa and submucosa autumn into large folds called rugae.

This image shows a cross-section of the stomach, and the major parts: the cardia, fundus, body and pylorus are labeled. — **Figure 23.4.ane – Stomach:** The tum has four major regions: the cardia, fundus, body, and pylorus. The addition of an inner oblique smooth muscle layer gives the muscularis the ability to vigorously churn and mix food.

The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the bottom omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.

Histology

The wall of the stomach is fabricated of the aforementioned four layers as most of the rest of the gastrointestinal tract, but with adaptations to the mucosa and muscularis for the unique functions of this organ. In add-on to the typical round and longitudinal smooth musculus layers, the muscularis has an inner oblique polish musculus layer (Figure 23.4.ii). As a event, in addition to moving food through the canal, the stomach can vigorously churn nutrient, mechanically breaking information technology downward into smaller particles.

This diagram shows the histological cross-section of the stomach. The left panel shows the stomach and the center panel shows a magnified view of a small region including the epithelium and the gastric glands. The right panel shows a further magnification of the mucosa and the different cell types are labeled. — **Figure 23.4.2 – Histology of the Tummy:** The stomach wall is adapted for the functions of the stomach. In the epithelium, gastric pits atomic number 82 to gastric glands that secrete gastric juice. The gastric glands (i gland is shown enlarged on the correct) contain unlike types of cells that secrete a variety of enzymes, including hydrochloride acid, which activates the protein-digesting enzyme pepsin.

The stomach mucosa'due south epithelial lining consists only of surface mucus cells, which secrete a protective coat of alkaline mucus. A vast number of gastric pits dot the surface of the epithelium, giving it the appearance of a well-used pincushion, and marking the entry to each gastric gland, which secretes a complex digestive fluid referred to as gastric juice.

Although the walls of the gastric pits are made upward primarily of mucus cells, the gastric glands are fabricated upward of different types of cells. The glands of the cardia and pylorus are composed primarily of fungus-secreting cells. Cells that make up the pyloric antrum secrete fungus and a number of hormones, including the majority of the stimulatory hormone, gastrin. The much larger glands of the fundus and torso of the stomach, the site of virtually chemical digestion, produce well-nigh of the gastric secretions. These glands are made upwards of a variety of secretory cells. These include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells.

Parietal cells—Located primarily in the eye region of the gastric glands are parietal cells, which are among the well-nigh highly differentiated of the body'south epithelial cells. These relatively large cells produce both muriatic acid (HCl) and intrinsic factor. HCl is responsible for the high acidity (pH ane.5 to three.v) of the tum contents and is needed to actuate the protein-digesting enzyme, pepsin. The acerbity too kills much of the bacteria you ingest with food and helps to denature proteins, making them more bachelor for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B₁₂ in the small intestine.

Chief cells—Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin.

Mucous neck cells—Gastric glands in the upper part of the stomach contain mucous neck cells that secrete alkaline mucus that is similary to the mucus secreted past the cells of the surface epithelium.

Enteroendocrine cells—Finally, enteroendocrine cells found in the gastric glands secrete diverse hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly past enteroendocrine G cells.

Tabular array 23.vi describes the digestive functions of of import hormones secreted by the stomach.

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Watch this animation that depicts the structure of the tum and how this construction functions in the initiation of poly peptide digestion. This view of the stomach shows the characteristic rugae. What is the office of these rugae?

Hormones Secreted by the Breadbasket (Table 23.6)
Hormone	Production site	Production stimulus	Target organ	Action
Gastrin	Stomach mucosa, mainly G cells of the pyloric antrum	Presence of peptides and amino acids in stomach	Stomach	Increases secretion by gastric glands; promotes gastric emptying
Gastrin	Stomach mucosa, mainly G cells of the pyloric antrum	Presence of peptides and amino acids in stomach	Minor intestine	Promotes abdominal muscle contraction
Gastrin	Stomach mucosa, mainly K cells of the pyloric antrum	Presence of peptides and amino acids in stomach	Ileocecal valve	Relaxes valve
Gastrin	Tummy mucosa, mainly G cells of the pyloric antrum	Presence of peptides and amino acids in tummy	Large intestine	Triggers mass movements
Ghrelin	Tummy mucosa, mainly fundus	Fasting state (levels increase just prior to meals)	Hypothalamus	Regulates food intake, primarily by stimulating hunger and satiety
Histamine	Tum mucosa	Presence of food in the stomach	Stomach	Stimulates parietal cells to release HCl
Serotonin	Tummy mucosa	Presence of nutrient in the tum	Breadbasket	Contracts stomach muscle
Somatostatin	Mucosa of breadbasket, especially pyloric antrum; besides duodenum	Presence of food in the tummy; sympathetic axon stimulation	Stomach	Restricts all gastric secretions, gastric motility, and emptying
Somatostatin	Mucosa of tum, especially pyloric antrum; besides duodenum	Presence of food in the stomach; sympathetic axon stimulation	Pancreas	Restricts pancreatic secretions
Somatostatin	Mucosa of tum, specially pyloric antrum; also duodenum	Presence of food in the breadbasket; sympathetic axon stimulation	Small intestine	Reduces intestinal absorption by reducing blood menstruation

Gastric Secretion

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 chosen the cephalic, gastric, and intestinal phases (Figure 23.4.3). However, once gastric secretion begins, all three phases can occur simultaneously.

The cephalic phase (reflex phase) of gastric secretion, which is relatively brief, takes place before nutrient enters the stomach. The aroma, sense of taste, sight, or thought of food triggers this phase. For instance, when you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your encephalon, which returns signals that increment gastric secretion to prepare your stomach for digestion. This enhanced secretion is a conditioned reflex, meaning information technology occurs only if you lot like or want a particular food. Depression and loss of appetite tin can suppress the cephalic reflex.

The gastric phase of secretion lasts 3 to 4 hours, and is set up in move past local neural and hormonal mechanisms triggered past the entry of food into the stomach. For case, when your sushi reaches the tummy, it creates distention that activates the stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which so provokes increased secretion of gastric juice. Partially digested proteins, caffeine, and ascension pH stimulate the release of gastrin from enteroendocrine Grand cells, which in plough induces parietal cells to increase their production of HCl, which is needed to create an acidic environs for the conversion of pepsinogen to pepsin, and protein digestion. Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, information technology should be noted that the breadbasket does have a natural means of fugitive excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the tummy 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 office in regulating the stomach and its emptying. When partially digested nutrient fills the duodenum, abdominal mucosal cells release a hormone chosen abdominal (enteric) gastrin, which further excites gastric juice secretion. This stimulatory activity is brief, notwithstanding, because when the intestine distends with chyme, the enterogastric reflex inhibits secretion. Ane of the furnishings of this reflex is to shut the pyloric sphincter, which blocks additional chyme from entering the duodenum. In addition to the enterogastric reflex, several hormones such as cholecystokinin (CCK) and secretin are released by the enteroendocrine cells of the duodenum when fat, acidic, or carbohydrate rich chyme enters the duodenum. CCK and secretin enter the blood and travel to the tum inhibiting the production of HCl and pepsin equally well equally inhibiting gastric movement allowing time for the duodenum to break down the chyme.

The Mucosal Barrier

The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice. Gastric enzymes that tin assimilate protein can also digest the stomach itself. The breadbasket is protected from cocky-digestion by the mucosal bulwark. This barrier has several components. First, the stomach wall is covered past a thick blanket of bicarbonate-rich mucus. This fungus forms a physical bulwark, and its bicarbonate ions neutralize acid. 2d, the epithelial cells of the stomach'due south 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 supercede damaged epithelial mucosal cells, when the epithelial cells are shed. In fact, the surface epithelium of the tum is completely replaced every 3 to 6 days.

Homeostatic Imbalances – Ulcers: When the Mucosal Barrier Breaks Down

As effective as the mucosal barrier is, it is not a "neglect-safe" machinery. Sometimes, gastric juice eats away at the superficial lining of the stomach mucosa, creating erosions, which generally heal on their own. Deeper and larger erosions are called ulcers.

Why does the mucosal barrier break downward? A number of factors can interfere with its ability to protect the stomach lining. The bulk 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 every bit "burning" hurting and indigestion. When ulcers are caused past NSAID use, switching to other classes of hurting relievers allows healing. When caused by H. pylori infection, antibiotics are effective.

A potential complexity of ulcers is perforation: Perforated ulcers create a hole in the stomach wall, resulting in peritonitis (inflammation of the peritoneum). These ulcers must be repaired surgically.

Digestive Functions of the Stomach

The stomach participates in virtually all the digestive activities with the exception of ingestion and defecation. Although well-nigh all absorption takes identify in the small intestine, the tum does absorb some nonpolar substances, such as booze and aspirin.

Mechanical Digestion

Within a few moments after nutrient afterwards enters your stomach, mixing waves begin to occur at intervals of approximately twenty 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 trunk 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 thirty mL (1 fluid ounce) of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, just non fully, closed pyloric sphincter. In a procedure called gastric emptying, rhythmic mixing waves force about three mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater corporeality of chyme at in one case would overwhelm the chapters of the small intestine to handle it. The rest of the chyme is pushed back into the body of the tummy, 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 boosted chyme from being released by the stomach before the duodenum is ready to process it.

Chemical Digestion

The fundus plays an important role, considering it stores both undigested food and gases that are released during the procedure of chemical digestion. Food may sit down in the fundus of the stomach for a while before being mixed with the chyme. While the nutrient is in the fundus, the digestive activities of salivary amylase continue until the nutrient begins mixing with the acidic chyme. Ultimately, mixing waves contain this food with the chyme, the acidity of which inactivates salivary amylase and activates lingual lipase. Lingual lipase then begins breaking down triglycerides into gratis fatty acids, and mono- and diglycerides.

The breakup of protein begins in the stomach through the actions of HCl and the enzyme pepsin. During infancy, gastric glands also produce rennin, an enzyme that helps digest milk protein.

Its numerous digestive functions notwithstanding, there is only ane tummy office necessary to life: the production of intrinsic factor. The abdominal absorption of vitamin B₁₂, which is necessary for both the production of mature red claret cells and normal neurological functioning, cannot occur without intrinsic cistron. People who undergo total gastrectomy (stomach removal)—for life-threatening tummy cancer, for instance—can survive with minimal digestive dysfunction if they receive vitamin B₁₂ injections.

The contents of the breadbasket are completely emptied into the duodenum within 2 to 4 hours after you eat a meal. Different types of food take different amounts of fourth dimension to process. Foods heavy in carbohydrates empty fastest, followed past high-protein foods. Meals with a high triglyceride content remain in the tummy the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for vi 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 showtime to stop.

Affiliate Review

The tum participates in all digestive activities except ingestion and defecation. It vigorously churns food. It secretes gastric juices that break down food and absorbs certain drugs, including aspirin and some booze. The stomach begins the digestion of protein and continues the digestion of carbohydrates and fats. Information technology stores food as an acidic liquid called chyme, and releases it gradually into the minor intestine through the pyloric sphincter.

Interactive Link Questions

Watch this blitheness that depicts the construction of the stomach and how this structure functions in the initiation of protein digestion. This view of the stomach shows the characteristic rugae. What is the function of these rugae?

Answers may vary.

Review Questions

Critical Thinking Questions

one. Explain how the breadbasket is protected from self-digestion and why this is necessary.

2. Describe unique anatomical features that enable the tummy to perform digestive functions.

Glossary

body: mid-portion of the stomach

cardia: (also, cardiac region) part of the breadbasket surrounding the cardiac orifice (esophageal hiatus)

cephalic stage: (as well, reflex phase) initial stage of gastric secretion that occurs before food enters the stomach

chief prison cell: gastric gland cell that secretes pepsinogen

enteroendocrine cell: gastric gland cell that releases hormones

fundus: dome-shaped region of the stomach to a higher place and to the left of the cardia

Thou cell: gastrin-secreting enteroendocrine jail cell

gastric elimination: process by which mixing waves gradually crusade the release of chyme into the duodenum

gastric gland: gland in the tum mucosal epithelium that produces gastric juice

gastric phase: phase of gastric secretion that begins when food enters the stomach

gastric pit: narrow channel formed by the epithelial lining of the stomach mucosa

gastrin: peptide hormone that stimulates secretion of muriatic acid and gut motility

hydrochloric acid (HCl): digestive acid secreted by parietal cells in the stomach

intrinsic cistron: glycoprotein required for vitamin B₁₂ assimilation in the small intestine

intestinal phase: phase of gastric secretion that begins when chyme enters the intestine

mixing wave: unique type of peristalsis that occurs in the stomach

mucosal bulwark: protective barrier that prevents gastric juice from destroying the tummy itself

mucous neck cell: gastric gland prison cell that secretes a uniquely acidic mucus

parietal prison cell: gastric gland cell that secretes muriatic acid and intrinsic factor

pepsinogen: inactive class of pepsin

pyloric antrum: wider, more superior office of the pylorus

pyloric canal: narrow, more inferior part of the pylorus

pyloric sphincter: sphincter that controls stomach emptying

pylorus: lower, funnel-shaped part of the tummy that is continuous with the duodenum

ruga: fold of gastrointestinal tract mucosa and submucosa in the empty breadbasket and other organs

stomach: alimentary canal organ that contributes to chemical and mechanical digestion of nutrient from the esophagus before releasing it, as chyme, to the pocket-sized intestine

Solutions

Answers for Critical Thinking Questions

The mucosal bulwark protects the stomach from self-digestion. It includes a thick blanket of bicarbonate-rich fungus; the mucus is physically protective, and bicarbonate neutralizes gastric acid. Epithelial cells meet at tight junctions, which block gastric juice from penetrating the underlying tissue layers, and stem cells quickly replace sloughed off epithelial mucosal cells.
The stomach has an additional inner oblique smooth muscle layer that helps the muscularis churn and mix nutrient. The epithelium includes gastric glands that secrete gastric fluid. The gastric fluid consists mainly of mucous, HCl, and the enzyme pepsin released every bit pepsinogen.

Source: https://open.oregonstate.education/aandp/chapter/23-4-the-stomach/

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