27.1 Gastrointestinal Tract Structures and Functions

Mouth

The mouth is the beginning of the GI tract. It is the orifice into which food is introduced into the GI system. The mouth consists of the lips, tongue, palates (soft and hard), gums, and three of the accessory organs of the GI system: teeth, tongue, and salivary glands. The tongue is a large muscle attached to the floor of the mouth by the frenulum linguae.

Pharynx

The pharynx is a funnel-shaped organ; its upper end is wider, and its lower end is narrower. This muscular organ begins in the head behind the nasal cavity, runs below the oral cavity, and then merges with the larynx and esophagus. The walls of the pharynx are made of both circular and longitudinal muscles. The pharynx is considered part of both the GI and respiratory systems. In regard to the GI system, constriction of the circular muscles of the pharynx helps push food to the esophagus and at the same time prevent air from being swallowed. The longitudinal muscle fibers lift the walls of the pharynx during swallowing.

Esophagus

The esophagus is a muscular tube situated behind the trachea and in front of the spine. It begins at the pharynx and goes through the diaphragm to the stomach. Both ends of the esophagus are closed off by sphincters. The sphincters open to allow food to pass through and then close to prevent regurgitation. Peristalsis (contraction of the muscular esophageal walls) moves the food down the esophagus.

Stomach

The stomach, a muscular organ located in the upper abdomen just below the rib cage, plays a pivotal role in the digestive process. Shaped like a hollow pouch, the stomach can vary in size and capacity depending on factors such as age, diet, and individual anatomy. On average, the adult human stomach can hold approximately 1 quart (946 mL) of food when fully distended (Frothingham, 2018).

Anatomically, the stomach is divided into four main regions: cardia, fundus, body, and pylorus. The cardia is the uppermost part of the stomach closest to the esophagus, where food enters. The fundus is the rounded portion of the stomach that expands as the stomach fills with food. The stomach body is the central region of the stomach where most of the digestion occurs; the pylorus is the lower portion of the stomach that connects to the small intestine (National Institute of Diabetes and Digestive and Kidney Diseases [NIDDK], 2017).

The stomach’s muscular walls are lined with several layers of smooth muscle tissue that contract and relax in coordinated rhythmic, wavelike movements known as peristalsis. This muscular activity serves to mix food with gastric juices, promoting thorough digestion. Gastric glands located in the stomach lining secrete gastric juices containing hydrochloric acid and enzymes, including pepsinogen that is converted to pepsin in the acidic environment of the stomach. Pepsin plays a crucial role in breaking down proteins into smaller peptides, initiating the process of protein digestion (NIDDK, 2017).

As food is partially digested in the stomach, it forms a semiliquid mixture of partially digested food, water, and digestive juices called chyme. The stomach’s role in bowel elimination involves both mechanical and chemical digestion, as well as the regulation of chyme passage into the small intestine. The pyloric sphincter, located at the junction between the stomach and the duodenum (the first part of the small intestine), is a muscular valve that controls the release of chyme into the small intestine (NIDDK, 2017). This regulated release ensures that the small intestine can efficiently digest and absorb nutrients while allowing the remainder of undigested food to progress through the digestive tract for eventual elimination as feces. Thus, the stomach’s intricate anatomy and physiological functions are essential for effective digestion and bowel elimination.

Small Intestine

The small intestine is a convoluted tube that stretches from the stomach to the large intestine. Despite its name, it is anything but small, measuring around 22 ft (6.7 m) long (Cleveland Clinic, 2021). In fact, the small intestine is longer than the large intestine but gets its name due to its smaller diameter compared to the large intestine. The small intestine consists of three segments: the duodenum, the jejunum, and the ileum.

The first and shortest segment of the small intestine is the duodenum. Positioned in the upper abdomen just below the stomach, the duodenum begins at the pyloric sphincter, forming a C-shaped curve. Acidic chyme from the stomach along with digestive juices from the pancreas and bile (digestive fluid produced by the liver and stored in the gallbladder) enter the duodenum via the common bile duct. This concoction neutralizes the acidic chyme and initiates further digestion, particularly of fats, proteins, and carbohydrates (NIDDK, 2017).

The jejunum, the middle portion of the small intestine, is primarily responsible for the absorption of nutrients, including carbohydrates, proteins, fats, vitamins, and minerals. The inner surface of the jejunum is lined with millions of tiny fingerlike projections called villi and microvilli that increase its surface area for efficient absorption. These intricate features serve to amplify the absorptive surface area, facilitating the efficient uptake of nutrients (e.g., carbohydrates, proteins, fats, vitamins, minerals) into the bloodstream.

The ileum is the longest part of the small intestine and connects the small intestine to the large intestine. It continues the absorption process, particularly focusing on the uptake of vitamin B₁₂, bile salts, and any remaining nutrients, as well as the reabsorption of some water and electrolytes. Together, the segments of the small intestine play a crucial role in bowel elimination by further digesting food, absorbing nutrients, and preparing waste products (e.g., undigested food, bacteria, cellular debris) for elimination through the large intestine.

Large Intestine

The large intestine consists of the cecum and colon. This vital component of the GI tract is responsible for bowel elimination. The first 6 in (15 cm) of the large intestine is the cecum, which serves as the initial connection point between the small intestine and the colon. Measuring about 5 ft (1.5 m) in length, the colon consists of four segments, each with unique functions: ascending colon, transverse colon, descending colon, and sigmoid colon (Cleveland Clinic, 2021). The ascending colon travels upward from the cecum on the right side of the abdomen. The transverse colon runs horizontally across the abdomen, connecting the ascending and descending segments of the large intestine. The descending colon descends along the left side of the abdomen, while the sigmoid colon is the final S-shaped segment that leads to the rectum. Finally, the rectum serves as the last portion of the large intestine, where feces are stored before elimination. The anus is the opening at the end of the GI tract through which feces (solid waste) are expelled from the body.

Sympathetic Nervous System

The sympathetic nervous system, a component of the autonomic nervous system, plays a significant role in the regulation of the bowel elimination process. During periods of stress or heightened arousal, the sympathetic nervous system becomes activated, triggering a cascade of physiological responses aimed at preparing the body for action, often referred to as the fight-or-flight response. In the context of bowel elimination, sympathetic activation leads to inhibition and constriction of bowel motility and sphincter tone. This can result in decreased peristalsis and slower movement of fecal matter through the intestines as well as increased constriction of the anal sphincter, making it more difficult to pass stool. In essence, the sympathetic nervous system acts to conserve energy and redirect blood flow away from the GI tract during times of stress, which can contribute to symptoms such as constipation or delayed bowel movements (Sharkey & Mawe, 2023).

Parasympathetic Nervous System

The parasympathetic nervous system, on the other hand, is often associated with the rest-and-digest response; the parasympathetic division promotes relaxation and facilitates GI function. In the context of bowel elimination, activation of the parasympathetic nervous system stimulates peristalsis, the rhythmic contractions of the intestinal muscles that propel food. water, and digestive juices forward through the digestive tract. Additionally, the parasympathetic system promotes relaxation of the anal sphincter, making it easier for stool to be expelled during defecation, or the process of eliminating feces from the body through the anus. Through its intricate network of nerves and neurotransmitters, the parasympathetic division ensures the efficient expulsion of waste from the body, contributing to regularity and effective bowel movements (Sharkey & Mawe, 2023).

Digestion

The GI tract serves a crucial role in the process of digestion, encompassing both mechanical and chemical processes. Mechanical digestion begins in the mouth with chewing, where food is broken down by the teeth into smaller particles to increase its surface area for enzymatic action. The tongue mixes the food with saliva. The food passes from the mouth through the pharynx and esophagus. This digestive process continues in the stomach, where muscular contractions churn and mix the food with gastric juices, forming chyme. Additionally, peristalsis (rhythmic contractions of the muscles lining the GI tract) propels food along its length, aiding in digestion and absorption (Ogobuiro et al., 2023).

Chemical digestion involves the action of the accessory organs of the GI system in the form of enzymes and acids secreted throughout the GI tract. For example, salivary amylase in the mouth starts the breakdown of carbohydrates into simpler sugars, while gastric acid and pepsin in the stomach break down proteins into amino acids. In the small intestine, pancreatic enzymes and liver bile further digest carbohydrates, proteins, and fats, allowing for the absorption of nutrients into the bloodstream. Overall, the digestive functions of the GI system ensure that ingested food is broken down into its basic components, which can then be absorbed and used by the body for energy, growth, and repair (Ogobuiro et al., 2023).

Absorption

The GI tract plays a crucial role in the absorption of nutrients from ingested food. After food is broken down into smaller particles through the process of digestion, the segments of the small intestine, particularly the lining of the jejunum and ileum, are primarily responsible for absorbing these nutrients into the bloodstream. A specialized cell called an enterocyte line the walls of the small intestine and is equipped with microvilli, tiny projections that greatly increase the surface area available for absorption. This extensive surface area allows for efficient absorption of nutrients, such as carbohydrates, proteins, fats, vitamins, and minerals.

The absorption process involves movement of nutrients across the intestinal lining into the bloodstream. This process has three components: active transport, facilitated diffusion, and passive diffusion. Energy is required for active transport, which involves the movement of nutrients against their concentration gradient, assisted by specific carrier proteins. The process of facilitated diffusion also relies on carrier proteins but does not require energy expenditure and moves nutrients down their concentration gradient. The process of passive diffusion occurs when nutrients move across the intestinal lining from an area of higher concentration to an area of lower concentration without the need for carrier proteins or energy.

After being absorbed, nutrients are transported via the bloodstream to various cells and tissues throughout the body, where they are used for energy production, growth, repair, and maintenance of bodily functions. Any unabsorbed nutrients, along with water and electrolytes, continue through the GI tract to the large intestine, where further absorption of water and electrolytes occurs before the remaining waste products are eliminated from the body.

Elimination

The GI tract’s role in elimination involves a series of coordinated actions to remove waste products from the body. As digestion progresses through the stomach and small intestine, nutrients and water are absorbed into the bloodstream, leaving behind undigested food particles, bacteria, and waste material. This residue, along with digestive secretions, enters the large intestine or colon.

In the colon, the main focus shifts to the absorption of water and electrolytes from the waste material. As the waste travels through the colon, water is reabsorbed, gradually transforming the material into solid feces, also known as stools or bowel movements, which are the solid waste products of digestion that are expelled from the body through bowel elimination. This process is crucial for maintaining proper hydration and electrolyte balance in the body.

Simultaneously, the muscles of the colon perform peristaltic contractions, moving the fecal material toward the rectum. The rectum acts as a storage chamber for feces, capable of sensing its volume and signaling the body when it is time for defecation. This sensory feedback triggers the urge to have a bowel movement (Nall, 2018).

When the rectum becomes adequately distended and the individual decides to defecate, the internal anal sphincter relaxes reflexively, allowing feces to enter the anal canal. At the same time, conscious control over the external anal sphincter allows for the voluntary initiation and regulation of defecation (Nall, 2018). Finally, the feces are expelled through the anus during defecation, completing the process of elimination. This coordinated interplay of muscular contractions, sensory feedback, and sphincter control ensures effective waste removal while maintaining bowel continence and preventing involuntary leakage.

In some cases, individuals may undergo surgical procedures that alter the natural route of bowel elimination. One such intervention is the creation of a stoma, a surgically created opening in the abdomen that allows waste (fecal matter) to exit the body when the natural route of elimination through the rectum is no longer feasible. This procedure may be necessary as a result of conditions such as bowel obstructions, inflammatory bowel disease (IBD), or colorectal cancer.

Typically, a stoma extends beyond the skin’s surface, displaying a moist, pink to red appearance and a rounded shape, devoid of nerve sensations (Figure 27.3). There are two types of ostomy procedures in which a stoma is created related to bowel elimination: colostomy and ileostomy. A colostomy involves forming a stoma using a section of the colon (large intestine), where a portion of the intestine is brought out through the abdominal wall and connected to the skin. This procedure diverts regular fecal matter away from the anus and through the stoma. Conversely, an ileostomy is created from the ileum (small intestine), similarly brought out through the abdominal wall to establish a stoma. Through an ostomy, waste products are diverted from the intestines and collected in a pouch attached to the abdomen. An ostomy may be permanent or temporary, depending on the reason for the surgery. Although this alteration in bowel function can significantly affect an individual’s daily life and self-image, proper care and management of the ostomy can help maintain comfort and quality of life.

Figure 27.3 A colostomy stoma, encircled by a wafer, is the section of the colostomy pouch that adheres to the skin. (credit: modification of “Ostomy wafer being worn by an ileostomy patient” by Eric Polsinelli (VeganOstomy)/Wikimedia Commons, CC BY 4.0)