The Alimentary Canal: Digestion and Absorption

The Alimentary Canal

Digestion and Absorption

Foods are composed of large chemical molecules that cannot be used unless they are broken down to an absorbable form. The digestive system is designed to (a) ingest foods; (b) digest or break down complex molecules into simple, soluble materials that can be absorbed; and (c) eliminate unused residues. Only energy-providing macronutrients (carbohydrate, protein, and fat) must be digested for absorption. Most vitamins, minerals, alcohol, and water can be absorbed as eaten.

The gastrointestinal tract can also be used to deliver complex chemical substances, such as oral medications. Medications frequently affect or can be affected by foods, modifying absorption, metabolism, or excretion of either the food or the drug. They may also affect nutritional status by altering taste or salivary flow; both of these conditions influence the amount and types of foods consumed. Dental hygienists need to become familiar with normal gastrointestinal processes because disturbances in the gastrointestinal tract may affect nutritional status and oral health of patients.

Physiology of the Gastrointestinal Tract

The digestive system includes the alimentary canal and several accessory organs (Fig. 3-1). The alimentary canal is a tubular structure approximately 30 feet long (five times the height of an average man). The alimentary canal, extending from the mouth to the anus, comprises all the body parts through which food passes. It includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The small intestine is comprised of the duodenum, jejunum, and ileum; the large intestine includes the cecum, colon, and rectum. Accessory organs—the salivary glands, liver, gallbladder, and pancreas—provide secretions essential for digestion and absorption. Digestion involves two basic types of action on food: (a) mechanical and (b) chemical. Mechanical actions include chewing and peristalsis, which break up and mix foods, permitting better blending of foodstuffs with digestive secretions. Chemical actions involve salivary enzymes and digestive juices, reducing foodstuffs to absorbable molecules.

Chemical Action

A large molecule can be split into smaller ones that are water-soluble and can be used by cells; this process is called hydrolysis. The hydrolysis of energy nutrients requires water. The following are basic hydrolysis reactions in food digestion:

< ?xml:namespace prefix = "mml" />Protein+H2Oamino acids


Fat+H2Ofatty acids+glycerol




These reactions depend on enzymes. Enzymes are complex proteins that enable metabolic reactions to proceed at a faster rate without being exhausted themselves. In protein hydrolysis, the substrate for the enzyme is protein, and amino acids are basic end products. An enzyme forms a temporary chemical compound with the substrate. When the reaction is completed, the complex separates, releasing new chemical compounds and the enzyme.

Because the enzyme is reused, only small amounts are needed. Enzymes function comparable to keys: they are very specific and function on only one substrate, similar to a key fitting a particular lock, as shown in Chapter 2, Figure 2-10. The name for some enzymes is derived from the name of the substrate, with the suffix -ase (e.g., lactase is the enzyme produced to catalyze the breakdown of lactose).

Mechanical Action

The wall of the gastrointestinal tract is similar from the esophagus to the rectum (Fig. 3-2, A). A layer of muscles encircles the tube, allowing the diameter of the tube to expand and contract. Food particles are broken up and mixed by the churning action. The outer fibers of the muscular coat (longitudinal muscle) run lengthwise and are responsible for peristalsis, involuntary rhythmic waves of contraction traveling the length of the alimentary tract.

Doorlike mechanisms between the digestive segments, called valves or sphincter muscles, are designed to (a) retain food in each segment until completion of the mechanical actions and digestive juices, (b) allow measured amounts of food to pass into the next segment, and (c) prevent food from “backing up” into the preceding area. Regulation of these valves is complex, involving muscular function and different pressures on each side of the valve.

Oral Cavity

Taste and Smell

Generally, food choices are influenced by three sensory perceptions: sight, smell, and taste. Gustatory (taste) sensations evoke pronounced feelings of pleasure or aversion; in the United States, taste is the primary determinant of food choices. The presentation of food, its color and aroma, may be the basis for acceptance or rejection. Food flavors are derived from characteristics of substances ingested, including taste, aroma, texture, temperature, and irritating properties. Approximately 75% of flavor is derived from odors.

The mouth, or oral cavity, plays an important role in the digestive system. It is the “port of entry” where receptors for sense of taste, or taste buds, are located. A taste bud consists of 30 to 100 cells embedded in the surrounding epithelium, termed papillae. Taste papillae appear on the tongue as little red dots, or raised bumps, and are most numerous on the dorsal epithelium. These cells replace themselves every 3 to 10 days; disease, drugs, nutritional status, radiation, and age can affect them. As food is chewed, gustatory receptors come into contact with chemicals dissolved in saliva.

Nerve cells carry messages to the brain, which interprets flavors as sweet, sour, salty, or bitter. These four basic tastes reflect specific constituents of food. Taste buds for all four sensations are located throughout the mouth, but specific kinds of buds are concentrated in certain (but overlapping) areas. Taste buds also are found on the soft palate, epiglottis, larynx, and posterior wall of the pharynx (Fig. 3-3). Taste and smell are essential for maintaining sufficient intake to meet physiological needs.

Food stimulates taste buds, and aromas stimulate olfactory nerves, receptors for smell. In contrast to the four basic tastes, an almost unlimited number of unique odors can be detected. No tactile sensation indicates the origin of odor sensations. Food-related aromas may be confused with taste sensations, and taste disorders often result from problems in smell rather than taste. Prevalence of olfactory impairment is high in older adults with the problem increasing with age. This is most likely the reason an elderly patient will state that food “just doesn’t taste good.” Exactly how and why taste preferences shift remains unclear, but preferences are known to change significantly with aging.

Loss of smell, or anosmia, results in limited capacity to detect flavor of food and beverages. Ability to smell food being prepared and eaten influences food selection. Smell is also a protective mechanism; odors are used to help determine whether foods are harmful or spoiled. Upper respiratory infections, nasal or sinus problems, neurological disorders, endocrine abnormalities, aging, or head trauma may cause anosmia. A common cold often impairs a person’s sense of smell, causing loss of appetite and limiting the ability to “taste” and enjoy food. The rate of the continuous renewal process undergone by olfactory receptor cells is depressed in malnutrition and by some antibiotics. Some of these disorders are self-limited; however, chemosensory losses from chemotherapy, upper respiratory infection, and aging may be irreversible.1,2

Dysgeusia is persistent, abnormal distortion of taste, including sweet, sour, bitter, salty, or metallic tastes. Dysgeusia without identifiable taste stimuli is called phantom taste. Dysgeusia may be caused by a previous viral upper respiratory infection, head trauma, a neurological or psychiatric disorder, a systemic condition (a disease or disorder that affects the whole body), xerostomia (dry mouth from inadequate salivary secretion), a severe nutritional deficiency, an invasive dental procedure resulting in nerve damage, an oral bacterial or fungal infection, or burning mouth syndrome,3 or it may have an iatrogenic causation. Iatrogenic refers to an adverse condition resulting from medical treatment, e.g., medications, irradiation, surgery. These conditions may also cause hypogeusia or loss of taste, and hypergeusia or heightened taste acuity. Dysgeusia may also result from breathing through the mouth. The dental hygienist is frequently the first healthcare provider to detect a patient’s taste disorder. Hyperkeratinization of the epithelium causing blockage of taste buds and affecting dietary intake may be observed during an oral examination.

Gustatory and olfactory disorders, whether caused by disease or drugs, are not mere inconveniences or neurotic symptoms. They affect food choices and dietary habits. A poor appetite, also called anorexia, may occur when medications cause loss of taste acuity. Taste stimulants affect salivary and pancreatic secretions, gastric contractions, and intestinal motility. Therefore, gustatory disorders can also affect digestion.

Because gustatory and olfactory disorders can result in deterioration of a patient’s general condition or nutritional status, these abnormalities must always be considered in dental and nutritional care. Potentially adverse compensatory habits may develop (e.g., decreased sweet or salty perceptions may result in excessive usage of sweets or salts, which may be potentially harmful, especially for patients with diabetes or hypertension). Also, addition of sugar can increase the incidence of caries. Persistent taste distortions can lead to inadequate caloric intake, resulting in unintentional weight loss or malnutrition.


Adequate saliva flow is essential for oral health which includes maintenance of soft tissues in the oral cavity, including taste buds. Saliva, secreted by salivary glands, is essential in taste sensations, functioning to (a) lubricate oral tissues for chewing, swallowing, and digestion; (b) remove debris and microorganisms; (c) provide antibacterial action; (d) neutralize, dilute, and buffer bacterial acids; (e) remineralize (restoration or renewal of calcium, phosphates, and other minerals to areas that have been damaged by incipient caries, abrasion, or erosion); (f) prevent plaque accumulation; (g) facilitate taste; and (h) promote ease of speech. An average of 1 to 2 mL/min of this complex fluid helps maintain integrity of teeth against physical, chemical, and microbial insults.

Saliva is supersaturated with calcium phosphates that allow demineralized areas of hydroxyapatite in enamel to be remineralized. Demineralization occurs when calcium, phosphate, and other minerals are lost from tooth enamel, causing tooth enamel to dissolve. This occurs because of acids produced by fermentable carbohydrates combining with acidogenic bacteria (see Chapter 18); it is not caused by insufficient calcium. Genetic variations of saliva, especially amylase, affect food preferences and intake by influencing oral sensory properties of food.4

Acidic, sour, or bitter tastes stimulate salivary flow. Saliva production is also stimulated by consumption of tasty foods and gum chewing. An increase in oral clearance rate decreases risk of caries formation (for more information, see Chapter 18). Saliva blends with food particles to moisten foods so they are more easily manipulated and prepared for swallowing.

Some chemical action or hydrolysis of nutrients begins in the mouth. Table 3-1 shows the functions of the different constituents in saliva. Because food is normally in the mouth briefly, ptyalin, or salivary amylase, initiates starch digestion. If a carbohydrate food, such as a cracker, is chewed and held in the mouth for a few seconds, it will begin to taste sweet, indicating that some starch is being hydrolyzed to dextrin and maltose.

Table 3-1

Digestive functions of saliva

Saliva Component Classification Function
Mucin Glycoprotein Lubricates food for easier passage and protects the lining of the gastrointestinal tract
Ptyalin (amylase) Enzyme Initiates hydrolysis of complex carbohydrates to simple sugars
Lysozyme (antibody) Enzyme Breaks down cell walls of some ingested bacteria

Dry mouth from inadequate salivary flow, also called xerostomia, leads to diminished gustatory function (see Chapter 20 for additional details). Xerostomia may result in frequent oral ulcerations, increased sensitivity of the tongue to spices and flavors, and increased risk of dental caries. Many drugs, including diuretics, cause xerostomia. Diuretics, prescribed to help the body eliminate fluids, also cause a decrease in salivary flow. Increasing fluid intake to 8 to 10 cups daily is important to compensate for these losses.


Teeth play a major role in digestion by crushing and grinding food into smaller pieces, a process known as mastication. In contrast to bone, neither tooth enamel nor dentin can be repaired or replaced in significant amounts by any natural process. Only small amounts of enamel and dentin are repaired or replaced through enamel remineralization and through secondary dentin deposition around the pulp chamber of the tooth (Fig. 3-4). Mineral deposition and resorption affect the bone that supports the dentition. This supporting bone, known as alveolar bone, is primarily trabecular bone (bony spikes forming a meshwork of spaces) and cancellous bone (bone within the spaces created by the network of trabecular bone, which appears spongy and contains bone marrow in small hollows). Negative calcium balance increases susceptibility to resorption and bone loss in the alveolar process

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Feb 12, 2015 | Posted by in Dental Hygiene | Comments Off on The Alimentary Canal: Digestion and Absorption
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