29.1 Introduction to the Gastrointestinal System and Oral Cavity

Gastrointestinal System

The digestive system consists of the gastrointestinal (GI) tract and accessory organs. The GI tract is also known as the alimentary canal. Together with the accessory organs, the GI tract provides nutrition and hydration to the body through the processes of ingestion, digestion, absorption, and metabolism. A healthy GI system with functional digestive processes is essential to break down and absorb nutrients from food and liquids to provide energy and growth to the human body. The ability to assimilate nutrients and hydration is crucial to cellular maintenance and repair. The final phase of digestion is to eliminate waste material not used by the body.

Figure 29.2 All digestive organs play integral roles in the life-sustaining process of digestion. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The GI tract and accessory organs form one continuous structure from the mouth to the anus (Figure 29.2). Ingestion is the process of taking in food, generally by the mouth. Digestion is a systematic process that breaks down food into its simplest form to promote absorption and moves this food and fluids through the GI tract. The complex digestive process starts with the mastication process as a person uses their teeth to bite and chew food. This is referred to as the mechanical phase of digestion. In the mouth, the food mixes with saliva from the salivary glands. Muscles in the pharynx allow the mixed food to be swallowed as a bolus. The bolus continues to travel through the GI tract, passing through the esophagus into the stomach. The stomach is the basic organ of digestion. The stomach further mixes the bolus and blends it with acidic gastric juices and pepsin to digest protein. This process produces chyme. At this point, the liver, pancreas, and gallbladder further aid the digestive process by adding bile and bicarbonate before the chyme is released into the small intestine. These substances help protect the small intestine from being destroyed by the acidic chyme. The digestive enzymes and hydrochloric (HCl) acid are part of the chemical phase of digestion, which continues as the food travels to the small intestine. Nutrients are absorbed into the blood or lymph in the small intestine. Water, along with some vitamins and minerals, is absorbed in the large intestine. Material not assimilated and absorbed results in waste products that leave the body through the anus. Table 29.1 provides an overview of the organs of the GI system and their functions.

The average length of the adult human GI tract is about 30 feet from the mouth to the anus. Under normal conditions, nutrients are introduced into the human body through the mouth as food and fluids. As food and fluids move through the GI tract, they are transformed into useful nutrients to be assimilated into the body’s cells for survival and homeostasis. The breakdown and assimilation depend on the large surface area of the GI tract, which has traditionally been described as the size of a tennis court. However, recent literature notes that the surface area is much smaller. According to Rao and Johncy (2022), using geometric comparisons to approximate the intestinal surface area is inappropriate.

Multiple factors play a role in GI functions and health. Structural and nutritional deficiencies can alter the function of tissues and organs. Intestinal transit time, or how quickly food and fluids move through the GI tract, may also impact GI functions and health. Intestinal transit time varies based on age. Transit time is slower in newborns and gradually increases, starting at about 4 months of age. The transit time slows again in older adults due to an aging GI system with decreased GI peristalsis (muscular contractions and relaxations that propel food through the GI tract), motility, and intestinal blood flow. The aging GI system, coupled with medications that may alter absorption of food and fluids, can cause the most common GI complaints: constipation and diarrhea. Although these are some of the most frequent GI concerns in older adults, these uncomfortable conditions can affect people of any age.

Oral Cavity

The oral cavity is the beginning of the digestive tract (Figure 29.3). The oral cavity refers to the structures of the mouth: the upper and lower lips, teeth, gingiva (gums), hard and soft palate, uvula, retromolar trigone, tonsils, floor of the mouth, tongue, salivary glands, and buccal mucosa. The teeth are vital for breaking up food (mastication) for digestion and absorption. Mastication is the essential first step in digestion. The tongue is a strong muscle that facilitates mastication by mixing food and helping to form it into a bolus, which is the first step in swallowing. The tongue is also involved in the sensation of taste through taste buds. Taste buds are specialized cells that communicate the sense of taste (sweet, salty, sour, bitter, and umami) to the brain.

Salivary glands located in and around the mouth have many functions for oral health and digestion. The three types of salivary glands—sublingual, submandibular, and parotid—produce saliva to keep the oral cavity moist and lubricated to promote digestion and optimal oral health. In an adult, the salivary glands produce up to 2 L of saliva every day (Cleveland Clinic, n.d.). The presence of food increases saliva secretion. This is a parasympathetic response mediated by two cranial nerves, the facial nerve (cranial nerve VII) and the glossopharyngeal nerve (cranial nerve IX). Saliva helps protect the teeth from cavity-causing bacteria as well as maintain the pH balance in the mouth, but the most appreciated role of saliva is that of digestive aid. It helps moisten food to facilitate the swallowing of chewed food. Saliva contains an enzyme, amylase, to help break down foods, particularly complex carbohydrates (starches) into simple and more easily digested sugars. The enzymes in saliva kick-start the digestive process in addition to helping food travel through the esophagus to the stomach for further digestion.

Any disorder of the mouth or oral cavity may disrupt the digestive process. Common disorders such as “cold sores” from a herpes simplex virus (HSV), canker sores, mucositis, and candida infections may affect the oral cavity. These can be painful conditions. A thorough nursing history is essential to determine the source of any pain in order to alleviate it and ensure the client’s ability to eat. The nursing health history should include questions about oral health and dental practices, home-based treatments, and the use of both over-the-counter and prescription medications. A thorough physical examination of the oral cavity is equally important because careful inspection of the oral cavity may uncover issues that can impair biting and chewing. Nurses should teach clients that good oral hygiene is essential for this first step in digestion.

Figure 29.3 The mouth includes the lips, tongue, hard and soft palate, gums, and teeth. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)