Stomach

Stomach

The stomach is a pouch-like organ shaped like the ­letter “J.” It hangs inferior to the diaphragm, in the upper left abdominal cavity, and is partially obscured by the liver and diaphragm. The stomach is approximately­ 15–25 cm or 6–10 inches in length, although its volume and diameter change based on how much food is contained. When empty, its volume is about 50 mL, and it has a cross-­sectional diameter just a little larger than the large intestine. The organ collapses inward when empty. Its capacity is approx-imately 4 liters or 1 gallon, and its inner lining con-sists of thick folds of mucosal and submucosal layers called rugae. When the stomach is distended,­ these folds disappear, and it can extend downward almost to the pelvis. The stomach is very movable in its mid-dle regions but fixed at either end. The stomach mixes food from the esophagus with gastric juice, begins protein digestion and limited absorption, and moves food into the small intestine (FIGURE 24-8). It is divided into the cardiac, fundic, body, and pyloric regions:

■■ Cardiac region: A small area near the esophageal opening. This portion of the stomach that attaches to the esophagus is called the cardia or cardial part. It surrounds the cardial orifice, which is where food enters from the esophagus.

■■ Fundic region: Balloons superior to the cardiac portion and acts as a temporary storage area. It is also called the fundus and is dome-shaped. The fundus is located beneath the diaphragm. It bulges superolaterally to the cardia.

■■ Body region: The dilated, main portion of the stomach. It is the large middle portion, commonly called the body, and is continuous inferiorly with the pyloric region.

■■ Pyloric region: Narrower than the rest of the stomach, this funnel-shaped region begins with a wider and superior pyloric antrum, becoming the pyloric canal as it nears the small intestine. This terminates at the pylorus. At its end, the muscular wall thickens to form a powerful, circu-lar pyloric sphincter, which acts as a valve con-trolling gastric emptying. The pylorus is actually continuous with the duodenum.

The greater curvature of the stomach is its con-vex lateral surface. The stomach’s lesser curvature is its concave medial surface. Two mesenteries extend from these curvatures. Known as omenta, they help tie the stomach to the body wall and other digestive organs. The two omenta are individually named the lesser omentum and the greater omentum.

The lesser omentum links the lesser curvature of the stomach to the liver. It is continuous with the visceral peritoneum that covers the stomach. The greater omen-tum is draped inferiorly from the greater curvature, cov-ering the small intestine’s coiled structure. It continues dorsally and superiorly to wrap the spleen and trans-verse large intestine. Then, it blends with the mesocolon, which is a dorsal mesentery securing the large intestine to the parietal peritoneum of the ­posterior abdominal wall. The mesentery proper is a thick sheet that ­suspends all the small intestine, except for the first 25 cm or 10 inches. It allows movement yet provides stability.

The autonomic nervous system serves the stom-ach. Thoracic splanchnic nerves relay sympathetic fibers through the celiac plexus. The vagus nerve sup-plies parasympathetic fibers. The gastric and splenic branches of the splenic trunk provide the stomach’s arterial supply. Its corresponding veins are part of the hepatic portal system. They eventually drain into the hepatic portal vein.

Microscopic Anatomy of the Stomach

Like most of the alimentary canal, the wall of the stom-ach contains four tunics. However, its muscularis and mucosa have specialized functions. The muscularis externa has an incomplete, innermost, smooth muscle fibril layer, which runs obliquely. This is in addition to the usual longitudinal and circular layers of smooth muscle of the rest of the alimentary canal.

The stomach mucosa has a simple columnar epi-thelium made up only of mucous cells. A two-layer coat of cloudy alkaline mucus protects the stomach. The top layer of mucus is viscous and insoluble. It keeps the second layer trapped beneath it, which consists of rich amounts of bicarbonate. The stomach’s inner ­lining of mucous membranes is thick, with many small gastric pits at the ends of tubular gastric glands. These glands have three main types of secretory cells: mucous, chief, and parietal cells. Mucous cells lie in the necks of the glands near the gastric pit openings; chief and parietal cells are in the deeper areas. The gastric pits have walls that are mostly formed by mucous cells. Together, the three types of secretory cells produce gastric juice. Mucus is secreted primarily from the cells in the glands of the cardia and pylorus. In the pyloric antrum, the cells produce mucus as well as several hormones, which include most of the gastrin, a stimulatory hormone. Gastric juice denatures proteins and inactivates most enzymes found in food. Gastric juice components are summarized in TABLE 24-2.

Types of Gland Cells

The glands of the fundus and body are much larger than in other areas and produce most of the secre-tions from the stomach. Most digestion occurs in these areas. The glands here contain many different secretory cells, which include mucous neck, parietal, chief, and enteroendocrine cells. The mucous neck cells are located in the neck and more basal regions of the stomach glands. They produce a thinner, more soluble mucus than the mucous cells of the surface epithelium. This acidic mucus is not fully understood in its function but helps to prevent the stomach from digesting itself.

Parietal Cells

The parietal cells are found mostly in the more apical region of stomach glands and are scattered among the chief cells. The parietal cells secrete both ­hydrochloric acid (HCl) and the glycoprotein known as intrinsic factor. HCl causes the contents of the stomach to be very acidic, with a pH between 1.5 and 3.5, a condition needed so pepsin can be activated effectively. HCl, and the environment it creates, is able to denature pro-teins, break down plant food cell walls, break down connective tissues from meats, and kill most bacteria ingested with foods and is of significant importance for digestion. For vitamin B₁₂ to be absorbed in the small intestine, intrinsic factor is required. Negative feedback is used to regulate the concentration of HCl in the stomach (FIGURE 24-9).

Chief Cells

Chief cells are most abundant near the base of a gas-tric gland. Cuboidal chief cells secrete pepsinogen, an inactive proenzyme that is converted by the acid in the gastric lumen to pepsin, an active proteolytic or protein-digesting enzyme. Stimulation of these cells results in the first-released pepsinogen molecules to be activated by HCl in the apical region of the gland. When pepsin becomes present, it additionally cata-lyzes pepsinogen conversion into more pepsin. This activation requires a small fragment of peptide from pepsinogen. When the fragment is removed, the pep-sinogen changes shape and its active site is exposed. This is a positive feedback process that is unlimited, except for the amount of available pepsinogen. The chief cells also secrete fat-digesting enzymes known as lipases, which are believed to handle nearly 15% of all lipolysis in the gastrointestinal tract.

Enteroendocrine Cells

Enteroendocrine cells release many chemical mes-sengers into the interstitial fluid of the lamina propria. They are usually found deep inside gastric glands. Some act locally as paracrines such as serotonin and histamine. Somatostatin and other messengers act locally as paracrines and also as hormones, which diffuse into blood capillaries, influencing several digestive system target organs. Gastrin is one of the hormones vital for regulating stomach secretion and motility. TABLE 24- 3 explains the hormones and para-crines involved in digestion.

Mucosal Barrier

The mucosal barrier is responsible for the stomach’s self-protection against corrosive and acidic gastric juices. Protein-digesting enzymes in this juice are able to actually digest the stomach. For example, the hydrogen ion concentration of the stomach can be up to 100,000 times of the same concentration that is in the blood. The mucosal barrier is created by the fol-lowing three factors:

■■ Mucus: A thick coating, built up on the stomach walls, which is rich in bicarbonate.

■■ Tight junctions: The mucosa’s epithelial cells are joined tightly together, preventing gastric juice from leaking into tissue layers that lie underneath .

■■ Cell replacement: Damaged epithelial mucosal cells are shed, then quickly replaced as undiffer-entiated stem cells divide; these cells lie in areas where gastric pits join gastric glands. The surface epithelial mucosal cells are totally renewed every three to six days. However, deeper and better-­ protected glandular cells inside the gastric glands live for a much longer period of time.

Digestion in the Stomach

The stomach not only holds ingested foods, but also physically and chemically degrades them, which produces chyme. Chyme is then moved from the stomach to the small intestine. The primary type of enzymatic breakdown in the stomach is pro-tein digestion. HCl denatures dietary proteins so enzymes can digest them. The amino acid chains in dietary proteins are unfolded, so digestive enzymes become more effective.

Pepsin is the most important protein-­digesting enzyme produced by the gastric mucosa, but in infants, rennin is another stomach enzyme that is needed. Rennin converts milk protein or casein into a curd-like substance. Lingual lipase from salivary glands also helps to digest certain triglycerides in the stomach, until the lipase also becomes digested. Lipid-soluble substances, such as aspirin or alcohol, easily­ pass through the mucosa of the stomach into the blood. Both of these substances are prone to caus-ing gastric bleeding and should not be used if a gastric ulcer is present.

However, the secretion of intrinsic factor is actually the only stomach function that is essential for life. Intrinsic factor is vital for the intestine to absorb vitamin B₁₂, which is required for production of mature erythrocytes. Without this, pernicious ane-mia develops. Even after a complete surgical removal of the stomach, known as a total gastrectomy, injected vitamin B₁₂ minimizes digestive problems.

1. List and name the various portions of the stomach.

2. What are pepsin and pepsinogen and the substances needed for their production?

3. Which types of stomach epithelial cells secrete pepsin and pepsinogen?

4. Explain how the mucosal barrier of the stomach is created. 

Gastric Secretion

Gastric secretion is controlled by neural and hor-monal mechanisms. Normally, about 3 liters of gas-tric juice are released by the gastric mucosa every day. This acidic mixture is so strong it can dissolve metal objects. Nervous control of gastric secretion is pro-vided by long and short nerve reflexes. Long reflexes are mediated by the vagus nerve and short reflexes are local, enteric reflexes. Nearly all stomach glands expe-rience increased secretory action when the stomach is stimulated by the vagus nerve. Secretory activity is reduced by activation of the sympathetic nerves.

Gastrin is the primary hormonal controller of gastric secretion. It stimulates secretion of both HCl and enzymes. Small intestine hormones also help to control gastric secretion and are primarily gastrin antagonists. The three phases of gastric secretion are the cephalic, gastric, and intestinal phases. For all these, the stomach acts as the effector site. Once begun, one or all phases may occur simultaneously.

Cephalic (Reflex) Phase

The cephalic (reflex) phase is the first to occur, hap-pening before food enters the stomach. It lasts for a few minutes and is triggered by the smell, sight, taste, or even thought of food. This phase prepares the stom-ach for digestion via the vagus nerve.

Gastric Phase

When food enters the stomach, the gastric phase is started by local neural and hormonal processes. The gastric phase lasts for three to four hours. About two-thirds of the gastric juice that is released is provided by this phase. Protein- rich foods in the stomach cause the pH to usually rise because proteins buffer hydro-gen ions. The higher pH stimulates gastrin secretion, then the release of HCl. This provides the required amount of acid needed to digest the proteins. Higher amounts of proteins in food cause more gastric juice and HCl to be released. During digestion, the gastric contents eventually become more acidic. This inhibits the gastrin­-secreting cells once again, a negative feed-back mechanism that maintains desired pH needed for gastric enzymes to work. Besides gastrin, ace-tylcholine and histamine also stimulate this phase. When just one of these three chemicals binds to parietal cells, secretion of HCl is minimal. When all three of them bind, however, HCl secretion is exten-sive. Histamine is most critical in this regard, and therefore antihistamines such as cimetidine are used to treat gastric ulcers caused by hyperacidity. This is because they bind to and block the histamine recep-tors of the parietal cells.

Intestinal Phase

There are both stimulatory and inhibitory compo-nents of the intestinal phase. As partly digested foods fill the duodenum of the small intestine, the excitatory or stimulatory actions begin. The intesti-nal mucosal cells are stimulated to release intestinal or enteric gastrin. This hormone keeps the gastric glands continually secreting. However, the effect is short as the intestine is distended with chyme that has many fats, hydrogen ions, partially digested proteins, and various irritants. The enterogastric reflex then triggers the inhibitory actions.

The enterogastric reflex is made up of three actions: inhibition of the vagal nuclei in the medulla oblongata, inhibition of local reflexes, and activation of sympathetic fibers, causing tightening of the pyloric sphincter. This tightening prevents additional entry of food into the small intestine. Several intestinal hor-mones called enterogastrones are then released, which include secretin, cholecystokinin, and vasoactive intestinal peptide (VIP). They inhibit gastric secretion while the stomach is highly active and play various other roles.

1. Explain the most important substances required for normal digestion.

2. What are the three phases of gastric secretion?

3. Describe how eating a large meal, in comparison with a smaller one, affects stomach emptying.