29.2 Introduction to the Esophagus and Stomach

Esophagus

The esophagus is a muscular tube at the base of the pharynx behind the trachea (Figure 29.4). The esophagus is approximately 10 inches long and extends to the stomach. Its primary function is to move food and fluids from the mouth to the stomach for digestion. The upper esophageal sphincter and the lower esophageal sphincter are two structures within the esophagus that control the movement of food and liquid. The sphincters are ringlike bands of smooth muscle fibers that function as valves to protect the body by propelling food and fluids through the esophagus and into the stomach. These neuromuscular sphincters open and close in response to autonomic nervous system stimulation and control.

The upper esophageal sphincter, also known as the oropharyngeal sphincter, is located between the pharynx and the mouth. A properly functioning upper esophageal sphincter is vital to prevent aspiration of food and liquids into the airways. Another structural membrane that prevents aspiration is the epiglottis. The epiglottis is a sheetlike flap that closes over the trachea to prevent food and fluids from entering the lungs (aspiration).

The lower esophageal sphincter is located where the esophagus meets the stomach. It is commonly referred to as the gastroesophageal sphincter because of its location between the esophagus and the stomach. It is also sometimes referred to as the cardiac sphincter because it is located close to the heart.

If either of the esophageal sphincters is not functioning properly, ingested food, fluids, and acidic stomach contents may move backward into the esophagus, causing irritation or damage to the tissues. Common problems with the esophagus include GI inflammation, esophagitis, gastroesophageal reflux disease (GERD), esophageal varices, and hiatal hernia. Structural defects in the esophagus may also cause problems such as obstruction. Esophageal dysfunction often interferes with nutrition and at times may cause varying degrees of discomfort. Medications are often used to alleviate uncomfortable symptoms and prevent further symptoms and potential complications.

Figure 29.4 The upper esophageal sphincter controls the movement of food from the pharynx to the esophagus. The lower esophageal sphincter controls the movement of food from the esophagus to the stomach. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Stomach

The stomach plays an essential role in mechanical and chemical digestion yet is dependent on many different body structures, functions, and processes (Figure 29.5). Located in the upper part of the abdomen, the stomach has four parts: the cardia (the opening from the esophagus into the stomach), the fundus (the expanded upper portion), the body (the central and largest portion), and the pylorus (the lowest portion).

Figure 29.5 The stomach 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. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The regulation of food and fluids in the stomach is controlled by autonomic neuromuscular innervation of two sphincters: the lower esophageal sphincter (gastroesophageal sphincter) and the pyloric sphincter. The pyloric sphincter separates the stomach from the first part of the small intestine, the duodenum. Much like the upper and lower esophageal sphincters, it is also a ringlike band of smooth muscle. The pyloric sphincter opens and closes under neuromuscular automaticity to control the passage of partially digested food and stomach contents into the small intestine. A malfunctioning pyloric sphincter may lead to delayed gastric emptying, known as gastroparesis. In another type of dysfunction, the pyloric sphincter empties the gastric contents too quickly into the small intestine, causing bloating and discomfort.

The digestive process involves three phases: the cephalic phase, the gastric phase, and the intestinal phase. The cephalic phase received its name because it begins with the brain. Food is ingested in the mouth, but the experience of seeing, smelling, and tasting food involves the brain. The neurologic sensory stimulation of ingesting food and fluids stimulates the vagus nerve, the secretion of acetylcholine, and motor responses of the parasympathetic nervous system. The production of gastric acid is essential to aid digestion, absorption, and metabolism. It is regulated through neuronal and endocrine pathways (Engevik et al., 2020). Histamine, gastrin (released from G cells), and acetylcholine are substances necessary for the parietal cells to produce and release hydrochloric (HCl) acid. The stomach produces 1500–2000 mL of HCl acid daily, and an adequate amount of it is necessary for vitamin B₁₂ to be extracted from food sources. In addition, intrinsic factor, which is crucial for vitamin B₁₂ absorption, is produced and released by the parietal cells. The entrance of food and fluids into the stomach initiates the second phase of digestion, the gastric phase. As indicated by its name, the gastric phase takes place in the stomach.

Within the parietal cells, hydrogen–potassium (H⁺-K⁺) ATPase enzymes comprise the proton pump that generates and secretes HCl acid in response to acetylcholine, histamine, or gastrin binding to receptors on the parietal cells. This action increases the hydrogen ion (H⁺) concentration, creating an acidic environment in the stomach that supports digestion by breaking down food, primarily carbohydrates and protein. Hydrochloric acid is also important in promoting the absorption of minerals. The acidic environment further aids digestion by controlling harmful microorganisms such as bacteria or viruses in the stomach.

The stomach acts as a basin for digestion. Stomach muscles and rugae stretch and contract in response to the volume of food in the stomach. When stretched, the rugae provide increased surface area for enhanced nutrient absorption. The upper muscles relax to let food and fluids enter the stomach; the lower muscles churn and mix food with digestive juices to form chyme and start the breakdown of proteins. Absorption of nutrients is minimal at this point, but protein digestion is further enhanced when the chief cells secrete pepsinogen, which is converted to pepsin. An acidic environment with a pH of 1.5–2.0 is needed for pepsin activation. The HCl acid lowers and maintains the acidic pH of the stomach.

One of the main ways the stomach keeps itself from becoming irritated by the HCl acid is by producing mucus via the goblet cells. However, the protective structure of the mucosal membrane may become eroded when exposed to excessive HCl acid and pepsin, especially if these substances permeate the epithelium. This condition is commonly referred to as gastritis and may lead to serious problems as the stomach lining becomes irritated and inflamed. Gastritis can be painful and impair digestion and may cause life-threatening complications, such as bleeding or perforation.

Gastric mucosal cells counteract the HCl acid by secreting prostaglandin E₂, a strong, lipid-rich molecule that stimulates gastric mucus and pancreatic bicarbonate. Prostaglandins ensure adequate submucosal perfusion. When prostaglandin E₂ is not sufficient to prevent gastritis, clients may need medications such as antacids to reduce or eliminate symptoms. Because prostaglandins play an essential role in enhancing protective features of the stomach, some medications that inhibit prostaglandin synthesis, such as ibuprofen, can injure the stomach. Nurses should instruct clients that taking too much ibuprofen can be harmful to the stomach and kidneys and can lead to bleeding due to altered platelet aggregation.

Prostaglandin E₂ also has a role in GI motility as chyme is peristaltically pushed toward the pyloric sphincter and into the duodenum for the final and third phase of digestion—the intestinal phase, which begins in the small intestine.