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Pharmacology for Nurses

29.1 Introduction to the Gastrointestinal System and Oral Cavity

Pharmacology for Nurses29.1 Introduction to the Gastrointestinal System and Oral Cavity

Learning Outcomes

By the end of this section, you should be able to:

  • 29.1.1 Describe the structure and function of the gastrointestinal system and oral cavity.
  • 29.1.2 Discuss common conditions that affect 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.

A diagram shows the entire digestive system, from the mouth to the anus. The digestive system begins with the mouth, tongue, and salivatory glands, which include the parotid, sublingual, and submandibular glands. Next is the pharynx, which connects to the esophagus. From there, is the stomach, which is surrounded by the liver, gallbladder, spleen, and pancreas. The stomach and liver are large organs, while the spleen and pancreas are smaller, and the gallbladder is the smallest. The next part of the digestive system is the small intestines, which consists of the duodenum, jejunum, and ileum. After that is the large intestine which consists of the transverse colon, ascending colon, descending colon, cecum, and sigmoid colon. The small colon appears as a smooth tube that is folded up on itself many times, while the large colon is a partitioned tube that bends around the small intestine, enclosing it on three sides. The final parts of the digestive system are the rectum, anal canal, and anus.
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.

Major Organs of the GI System Function
Mouth/oral cavity When food enters the mouth for nourishment, processes of sensory stimulation, mechanical processing, and lubrication begin. The salivary glands secrete amylase and ptyalin to begin to break down starches. Lipase is secreted by the salivary glands to break down fats.
Pharynx Commonly called the throat, the pharynx is a passageway for food to the esophagus.
Esophagus The esophagus empties food materials into the stomach through peristalsis during swallowing. It secretes large amounts of mucus, which helps lubricate food.
Stomach 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. Minimal absorption of nutrients occurs in the stomach; rather, it secretes proteases such as pepsin (breaks down protein into amino acids), amylase (breaks down carbohydrates into sugars), and lipase (breaks down fats into glycerol and fatty acids).
Small intestine (duodenum, jejunum, ileum) The small intestine is responsible for the absorption of 90%–95% of food with the aid of the pancreatic enzymes (amylase, trypsin, and lipase) and enzymes secreted in the intestine (maltase, sucrase, lactase, and peptidase).
Large intestine The large intestine receives unabsorbed and undigested food material (waste products), absorbs water and electrolytes, and forms feces.
Anus The anus is responsible for defecation.
Accessory Organs of the GI System Function
Teeth The teeth bite and chew (masticate) food as the first step in digestion.
Tongue The tongue is a muscular organ that helps with mastication and assists in swallowing the food bolus.
Salivary glands Located in the mouth are three pairs of salivary glands (parotid, sublingual, submandibular) that secrete saliva (consisting of water, amylase, lipase, electrolytes, mucus, immunoglobins, and amino acids) and digestive enzymes. The main role of saliva is to lubricate the mouth, moisten the food bolus, and start the digestive process.
Pancreas The pancreas produces enzymes to digest proteins (trypsin and chymotrypsin), carbohydrates (amylase), and fats (lipase).
Liver The liver makes and secretes bile (which includes cholesterol, bile acids, and bilirubin). Bile helps emulsify fats into fatty acids, aiding lipid absorption.
Gallbladder The gallbladder stores and concentrates bile.
Table 29.1 Gastrointestinal System Organs

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.

A diagram shows the different structures of the mouth. Starting at the top, the mouth consists of the superior lip. The superior labial frenulum is a narrow strip that connects the lip to the gums. Next are the upper teeth, which surround the hard and soft palate, the palatoglossal arch, fauces, and palatopharyngeal arch. At the back of the mouth is the palatine tonsil and uvula. At the bottom of the mouth is the tongue, opening duct of submandibular gland, the lower teeth, gums, inferior labial frenulum, and inferior lip. The teeth, tongue, palates, and gums are in the oral vestibule. Starting with the teeth at the bottom middle of the mouth and moving along one side towards the back, there are two incisors, one canine, two premolars, and three molars. These teeth are the same on the other side and on the top of the mouth.
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)

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